seeds || germination ecology of seeds with physical dormancy

Chapter 6

Germination Ecology of Seeds withPhysical Dormancy

PURPOSE

Natural openings that permit entry of water into seeds

with physical dormancy (PY) have been described in 18

plant families (Table 3.14). The “unplugging” of these

openings in the seed or fruit coat is not a random or hap-

hazard event, and timing of germination of seeds with PY

is environmentally controlled in nature. Depending on

species and habitat, various environmental factors cause

seeds to become permeable during a certain time(s) of the

year; thus, timing of germination is predictable.

In this chapter, examples of germination phenologies

of seeds with PY will be presented, and the role of drying

in the development and maintenance of PY will be dis-

cussed. Much information on the germination require-

ments of seeds (after PY is broken) comes from studies in

which various laboratory techniques have been used to

make seeds permeable. Thus, methods for artificially

breaking PY, as well as germination requirements for

seeds after they become permeable, will be surveyed. The

role of various environmental factors in the breaking of

PY will be examined. Lastly, attention will be given to

how physiological dormancy is broken in those seeds

with both physical and physiological dormancy.

Throughout this chapter, we have used the word

“lens” to refer to the water-gap on seeds of species

belonging to the Fabaceae. However, in many cases if the

reader consults the references we cite, the word “stro-

phiole” will be found. The strophiole now is considered

to be an outgrowth on the seed, i.e., a seed appendage

(e.g., Grear and Dengler, 1976; Werker, 1997) and is not

the appropriate word for the specialized area (lens) near

the hilum that frequently functions as the water-gap (see

Lersten et al., 1992).

GERMINATION PHENOLOGY

When seeds with an impermeable seed (or fruit) coat are

sown on soil and exposed to natural seasonal temperature

changes, germination may be spread over a number of

years, depending on the species (Table 6.1). Thus, many

species whose seeds have PY have the potential to form

long-lived seed reserves; however, a discussion of this

topic will be delayed until Chapter 7.

Although seeds of a species may germinate over a

period of years, the germination season each year is about

the same (Figure 6.1). The germination period in some

species is restricted to a few weeks. For example, seeds

of Melilotus alba germinate in winter and early spring

(Figure 6.1), and it is not unusual to find healthy seed-

lings with radicles 1�2 cm in length lying on soil that is

frozen solid. In many species, a few seeds germinate

throughout the growing season, but there is a peak of ger-

mination at some specific time of the year (Brenchley and

Warington, 1930; Roberts and Boddrell, 1985b,c). For

example, seeds of Geranium carolinianum may germinate

to low percentages in spring and summer, but they exhibit

a strong peak of germination in autumn (Figure 6.1). In

still other species, seeds germinate throughout the grow-

ing season. For example, seeds of Napaea dioica

(Figure 6.1) germinate from late winter through autumn;

however, much of the germination takes place in spring.

Germination of this species is sporadic during summer,

often occurring after periods of unusually high

temperatures.

The examples of germination phenology of seeds

with PY shown in Figure 6.1 are for species growing in

a temperate climate with hot, moist summers and cold,

wet winters. Seeds of many species with PY growing in

subtropical/tropical regions with annual wet and dry

seasons germinate at the beginning of the wet season

(Bhardwaj and Prabhakar, 1990; Nazrul-Islam and

Hoque, 1990; Papavassiliou et al., 1994; Wagner and

Spira, 1994), but those of a few species germinate at

the beginning of the dry season (Lonsdale and

Abrecht, 1989).

INTERNAL MOISTURE CONDITIONSOF SEEDS

If seeds (or fruits) that are capable of developing imper-

meable coats are collected at the time of embryo

145C.C. Baskin and J.M. Baskin: Seeds, Second Edition. DOI: http://dx.doi.org/10.1016/B978-0-12-416677-6.00006-8

© 2014 Elsevier Inc. All rights reserved.

http://dx.doi.org/10.1016/B978-0-12-416677-6.00006-8

maturity but before any drying occurs, many of them

will germinate (Chapter 2). However, the number of

days from anthesis until seeds reach physiological matu-

rity and maximum mass (i.e., when they first would ger-

minate if removed from the plant and placed on a moist

surface) is 12�16 in Sida spinosa (Egley, 1976), 21 in

Malva parviflora (Michael et al., 2007), 82 in Acacia

auriculiformis (Pukittayacamee and Hellum, 1988), 230

in Albizia lebbeck (Uniyal and Nautiyal, 1995a), 54 in

Lotus ornithopodioides and 47 in Scorpiurus muricatus

(Gresta et al., 2011). In Trifolium ambiguum, seeds

reached mass maturity at 33�36 days after pollination

(DAP), gained the ability to germinate between 14�47

DAP and could tolerate drying (not die) at 30�40 DAP

(Hay et al., 2010). Seeds of Ipomoea lacunosa reached

physiological maturity 22 DAP, could germinate at 24

DAP and became water-impermeable at 37�30 DAP;

the hilum fissure was the last place on the seed from

which water was lost (Jayasuriya et al., 2007b). Seeds of

Sicyos angulatus reached physiological maturity at 20

DAP, excised embryos could germinate at 20�36 DAP,

and seeds became water-impermeable at 32�36 DAP;

the hilum was the last site of water loss (Qu et al.,

2010). Seeds of Geranium carolinianum reached

germinability, physiological maturity and became

water-impermeable 9, 14 and 20 DAP, respectively

(Figure 6.2).

Impermeability of coats develops as seeds dry (Hyde

et al., 1959), and moisture content (MC) of seeds at the

time they become impermeable ranges from 4.0 to 21%

(Table 6.2). The amount of water lost from seeds depends

on the RH and temperature during the time of drying. Thus,

if RH is relatively low and temperatures are high more

seeds will become water-impermeable, as for example in

some varieties of soy beans (Baciu-Miclaus, 1970).

However, under these drying conditions the proportion of

TABLE 6.1 Germination of seeds with physical

dormancy sown on soil in a nontemperature-

controlled greenhouse in Lexington, Kentucky (USA).

germination was considered to be completed (and

monitoring was stopped) if no additional seeds

germinated after 1 yr (Baskin and Baskin, 1998a,

unpubl.).

Species Year

Sown

Year of Last

Germination

No. of Years over

which Seeds

Germinated

Abutilontheophrasti

1989 2009 20

Aeschynomenevirginica

1993 1997 4

Anoda cristata 1990 2005 15

Astragaluscanadensis

1989 1994 5

A. distortus 1989 2013 24

A. tennesseensis 1989 2003 14

Baptisia australis 1969 1977 8

B. bracteata 1993 1998 5

Callirhoe bushii 1988 1995 7

Cardiospermahalicacabum

1990 1995 5

Cassia fasciculata 1992 1997 5

Dalea foliosa 1993 2006 13

D. gattingeri 1969 1977 8

Desmanthusillinoensis

1989 2010 21

Desmodiumpaniculatum

1988 1992 4

Dioclea multiflora 1990 1994 4

Evolvulusnuttallianus

1982 1989 7

Geraniumcarolinianum

1988 1995 7

Hibiscus palustris 1990 2007 17

Iliamna corei 1989 2012 23

I. remota 1986 2007 21

Ipomoea lacunosa 1990 1999 9

I. purpurea 1989 2001 12

Lespedeza capitata 1986 2005 19

L. leptostachya 1986 1989 3

L. violacea 1988 1995 7

Medicago lupulina 1989 1995 6

Melilotus alba 1987 1992 5

M. officinalis 1989 1993 4

Napaea diocia 1986 2004 18

Orbexilumonobrychis

1987 1993 6

Oxytropislambertii

1993 1995 2

Pediomelumsubacaule

1988 2011 23

Rhus aromatica 1989 1995 6

Senna marilandica 1989 1997 8

Sesbania exaltata 1992 2007 15

(Continued )

TABLE 6.1 (Continued)

Species Year

Sown

Year of Last

Germination

No. of Years over

which Seeds

Germinated

Sicyos angulatus 1989 1998 9

2000 2006 6

Sidahermaphrodita

1984 1990 6

S. spinosa 1989 1998 9

Stylismahumistrata

1990 2010 20

Trifoliumcampestre

1993 1999 6

T. stoloniferum 1986 1997 11

146 Seeds

seeds with PY in the seed crop may remain the same in

some species, but the PY is more difficult to break than the

PY that develops under mesic conditions (Bolingue et al.,

2010). If rainfall is high during the natural time of seed dry-

ing in the field, seeds may germinate prior to dispersal. For

example, observations of germinated seeds in pods of

Tephrosia purpurea (Shra and Sen, 1984) probably mean

that seeds were drying at a very slow rate. If seeds with per-

meable coats are stored at high RH, they do not develop

impermeable seed coats (Barrett-Lennard and Gladstones,

1964). Thus, the rate and degree of development of seed

coat impermeability is controlled by atmospheric moisture

(Quinlivan, 1971). However, other factors such as stage of

seed development or maturity when drying starts (Aitken,

1939; Samarah et al., 2003; Samarah, 2005, 2007) and

genetics (Lebedeff, 1943; Bolingue et al., 2010) help deter-

mine the proportion of seeds in a given seed crop that devel-

ops impermeable coats.

MC of seeds with impermeable coats has implications

for germination ecology because it is related to rate of

loss of PY. Jones (1928) found that most seeds of Vicia

villosa held at 25�C and 75�80% RH became permeable

if their MC was above 14%. Seeds of Lupinus digitatus

(Gladstones, 1958), L. varius (Quinlivan, 1968a), L.

angustifolius and L. luteus (Quinlivan, 1970) with an MC

of 6�10% were impermeable to water, and they did not

become permeable when placed on a moist substrate.

However, seeds of these species with an MC of 11�15%

eventually imbibed water when they were placed on

a moist substrate. Some seeds of Trifolium pratense,

T. hybridum and Melilotus alba with an MC of 16.6, 16.2

and 16.2%, respectively, became permeable at both 32

and 81% RH (Nakamura, 1962). Storage of T. repens

seeds with an MC of 22.4% at 32 and 81% RH resulted

in a 30 and 20% increase in impermeable seeds,

respectively. Seeds of T. repens var. latum with an

MC of 20.7% exhibited a 10% decrease in impermeable

seeds at 81% RH but a 35% increase in imperme-

able seeds at 32% RH. Further, when seeds of

Vigna unguiculata with an initial MC ranging from

4.6�12.6% were stored at 70% RH at 25�C for 3 wk,

germination increased significantly in those with an

initial MC of 9.2�12.6% (Murphy et al., 1986).

Whereas seeds of Senna bispinosa with an MC of 9.6%

incubated at 20, 25, 30 and 35�C for 198 days

germinated to 3�7%, those with an MC of 14.4% germi-

nated to 100% at all temperatures (Graaf and Van

Staden, 1987).

Although measurements made in the early 1900s

showed that seeds with PY continued to lose water after

they were harvested, no one knew exactly how this hap-

pened. Hyde (1954) found that MC of Trifolium repens,

T. pratense and Lupinus arboreus (all are members of the

Papilionoideae, a subfamily of the Fabaceae) seeds at the

time of embryo maturity was 150% of the dry mass, and

it declined to about 25% by water loss from the palisade

epidermis. When seed MC drops below 25%, the hilum

(Figure 6.3) acts as a hygroscopic valve. The hilum in the

Papilionoideae has a layer of heavily thickened cells,

called the counter palisade, on each side of the hilum fis-

sure. When the RH of the surrounding air becomes lower

than any RH previously experienced by the seed, the

counter palisade cells dry and shrink, and the hilum fis-

sure opens. When the fissure is open, water vapor diffuses

from the seed. As RH of the surrounding air increases,

cells in the counter palisade swell causing the fissure to

close, and water vapor no longer diffuses out of the seed.

Thus, MC of the seed decreases when there is a further

decrease in RH of the air, but it never increases. Hyde

(1954) found that seeds could absorb water vapor from

Melilotus

Geranium

Napaea

100

80

% o

f tot

al a

nnua

lge

rmin

atio

n

60

40

20

0J F M A M J J A S O N D J F M A M J J A S O N D J F M A M J J A S O N D

WiAuSuSpWiAuSuSpWiAuSuSpWi

FIGURE 6.1 Germination phenology of seeds of Geranium carolinianum, Melilotus alba and Napaea dioica sown on soil in a nontemperature-

controlled greenhouse in Lexington, Kentucky (USA). Wi, winter; Sp, spring; Su, summer; Au, autumn.

147Chapter | 6 Germination Ecology of Seeds with Physical Dormancy

the air if RH was increased so slowly that the hilum did

not close. Over a period of 71 days, he raised the RH

from 45 to 70%, and seed MC increased from about 10 to

13%. Such a slow rise in RH probably seldom, if ever,

occurs in nature.

According to Rolston (1978), the other two subfami-

lies of legumes (Caesalpinioideae and Mimosoideae) gen-

erally do not have a counter palisade in the hilum. Thus,

an understanding of seed drying in these subfamilies will

require information on the structure and function of the

hilum. Also, Gunn (1981) suggested that the pleurogram

is involved in seed drying of Caesalpinioideae and

Mimosoideae, but De-Paula and Oliveira (2012) did not

know of any evidence to support this idea. One reason so

much is known about impermeable seed coats in the

Papilionoideae is that many of the important pasture

legumes (e.g., Lupinus spp., Trifolium spp, Medicago spp.

and Stylosanthes spp.) in Australia belong to this subfam-

ily (Quinlivan, 1971).

When seed coats become permeable under natural

conditions, imbibition of water usually occurs through an

unplugged natural opening. However, water may be

imbibed across the whole seed coat, depending on MC of

the seeds. For example, seeds of Lupinus varius at MCs

of 12�14% imbibe water over the surface of the seed,

whereas those with an MC of 8.5% or less will imbibe

FIGURE 6.2 Seed development in Geranium carolinianum (mean 6 SE). (a) Seed length and width. (b) Moisture content and dry matter accumula-

tion. (c) Percentage of fresh intact seeds that imbibed. (d) Germination of fresh intact seeds. (e) Germination of isolated embryos. Different letters

indicate significant differences between values (P, 0.05). From Gama-Arachchige et al. (2011), with permission.

148 Seeds

water only after the lens has been rendered permeable

(Quinlivan, 1968a). MC may influence the germination

phenology of L. varius seeds in the summer-dry, winter-

moist pasture ecosystems of Australia. Seeds with high

MC can imbibe water following summer showers and

thus germinate sporadically over several months, while

those with low moisture percentages cannot germinate

until after high fluctuating summer temperatures make

TABLE 6.2 Examples of species in which seed coat

impermeability increases as seed moisture content

decreases and moisture content of seeds at the time

they became impermeable.

Species % Seed

Moisture

References

Abelmoschusesculentus

ca. 10 Demir, 1997

Acaciaauriculiformis

�a Pukittayacamee & Hellum,1988

A. senegal � Kaul & Manohar, 1966;Danthu et al., 1992

A. suaveolens � Auld, 1986a

Albizia lebbek � Uniyal & Nautiyal, 1995a

Astragalus sinicus � Nakamura, 1962

Bixa orellana 12.2 Yogeesha et al., 2005

Cajanus cajan 4 Khattra & Singh, 1992

Cassia acutifolia � Bhatia et al., 1977

C. angustifolia � Bhatia et al., 1977

Convolvulusarvensis

13 Swan, 1980

Crotalaria spectabilis 11 Egley, 1979

Cuscuta campestris � Hutchison & Ashton, 1979

Delonix regia ca. 13 Chaves & Kageyama, 1981

Enterolobiumcontortisiliquum

13.1 Borges et al., 1980


11 Gama-Arachchige et al.,2011

Gossypium hirsutum 9�13 Patil & Andrews, 1985

Gymnocladus dioica 5�8 Raleigh, 1930

Ipomoea lacunosa 13 Jayasuriya et al., 2007b

I. turbinata 8.5 Chandler et al., 1977

Lathyrus maritimus � Dinnis & Jordan, 1939

Lotusornithopodioides

7 Gresta et al., 2011

Lupinus arboreus 14 Hyde, 1954

L. digitatus ,9 Gladstones, 1958

L. varius ,8.5 Quinlivan, 1968a

Malva parviflora 20 Michael et al., 2007

Medicago lupulina � Sidhu & Cavers, 1977

Melilotus alba � Helgeson, 1932

Merremia aegyptia ,33�$ 2.0 Sharma & Sen, 1974

M. dissecta ,42�$ 4.3 Sharma & Sen, 1974

Ornithopuscompressus

7�9 Revell et al., 1999

Peltophorumpterocarpum

15 Mai-Hong et al., 2003

Rhus aromatica ca. 15�16 Li et al., 1999a

R. glabra ca. 9�10 Li et al., 1999a

Scorpiurusmuricatusvar. subvillosus

8 Gresta et al., 2011

Sesbania bispinosa 9.6 Graaff & Van Staden, 1987

Sicyos angulatus 14.6 Qu et al., 2010

Sida spinosa 20 Egley, 1976

Sophoraalopecuroides

10 Hu et al., 2009b

Strophostyleshelvola

,21 Hutton & Porter, 1937

(Continued )


Species % Seed

Moisture

References

Tephrosia hamiltonii � Bhardwaj & Prabbakar,1990

Trifolium pratense 14 Hyde, 1954

T. repens 10 Hyde, 1950

Vicia villosa ,14 Jones, 1928b

Vigna unguiculata ca. 10% Lush et al., 1980; Lush &Evans, 1980

aData not given, but the relationship between seed drying anddevelopment of physical dormancy is mentioned/discussed.

FIGURE 6.3 (a) Transmedian section of the hilum and adjacent tissues

in a seed of Lupinus arboreus. (b) Transmedian section of the hilum in a

seed of Lupinus arboreus. p, parenchyma; cp, counter-palisade, hf, hilum

fissure; sc, sclerenchyma; c, cuticle; pe, palisade epidermis; tb, tracheid

bar; l, light line; s, stellate cells. From Hyde (1954), with permission.


the lens permeable. Consequently, these seeds with low

MC germinate over a very short period of time following

rains in autumn (Quinlivan, 1968a).

After seeds with PY become permeable, they generally

either germinate or rot; however, there may be some

exceptions. Hagon and Ballard (1970) artificially made the

lens on seeds of Trifolium subterraneum permeable

(by shaking seeds in a glass bottle) and then subjected

the seeds to varying levels of RH. The lens closed after 12

days at 0 and 5% RH, but it remained open at $ 20% RH.

The minimum number of days for the lens to close at 0

and 5% RH was not determined. These data imply that

under prolonged, very low RH conditions, the lens would

close; consequently, permeable seeds would become

impermeable again. However, drying conditions of the

length and severity used by Hagon and Ballard are

unlikely to occur in nature.

ARTIFICIAL SOFTENING OF WATER-IMPERMEABLE SEED (OR FRUIT) COAT

Horticulturalists, foresters, farmers, gardeners and

researchers have developed a number of techniques for

quickly making physically dormant seeds permeable.

These methods include mechanical scarification, concen-

trated sulfuric acid scarification, enzymes, organic sol-

vents, percussion, high atmospheric pressures, hot water,

dry heat, radiation, dry storage, ultrasound and low

temperatures. Treatments such as mechanical and acid

scarification will almost always make a seed permeable to

water, but the success of other methods varies with the

species and treatment intensity and duration. Studies

involving artificial methods of breaking PY have contrib-

uted greatly to our understanding of (1) how the lens and

other natural openings work, (2) effects of various envi-

ronmental factors such as drying, heating and freezing on

the loss of PY and (3) the rate and path of water entry into

seeds that have become permeable.

Mechanical Scarification

It should be noted that although seeds are made water-

permeable by various methods such as wet heat, dry heat

and acid scarification (see below), the most effective

method to make seeds permeable is to mechanically scar-

ify them, i.e., make a small hole in them (Hanna, 1973;

Oladiran, 1986; Shaukat and Burhan, 2000; Baes et al.,

2001; Tigabu and Oden, 2001; Robles and Castro, 2002;

Martyn et al., 2003; Uzun and Aydin, 2004; Baeza and

Vallejo, 2006; Patane and Gresta, 2006; Perez-Garcia and

Gonzalez-Benito, 2006; Silveira and Fernades, 2006;

Clifton-Cardoso et al., 2008; Wang and Hanson, 2008;

Perez-Garcia, 2009; Pereira and Ferreira, 2010; Wang

et al., 2011e).

A file, sand paper, knife, razor blade, scalpel or needle

can be used to make a small hole in the seed coat, i.e.,

mechanically scarify them, and water enters through this

opening. Also, seeds can be scarified by rubbing them

with sand paper (Sahai and Pal, 1995; Shaukat and

Burhan, 2000; Uzun and Aydin, 2004; Cruz and Carvalho,

2006; Clifton-Cardoso et al., 2008), or seeds may be held

against an electric emery, i.e., a sander, (Cruz et al.,

2001b). However, mechanical scarification can be a very

time-consuming method of making seeds permeable to

water, especially if large numbers of scarified seeds are

required. Thus, machines that roll, rub or blow seeds

against an abrasive surface such as glass splinters or sand

paper in some kind of a container have been built (Porter,

1949; Townsend and McGinnies, 1972a; Dignart et al.,

2005). Cavanagh (1987) points out, however, that

while these machines work fine for small, thin-coated

seeds like those of Trifolium subterraneum, they may

not work well for thick-coated seeds like those of

Acacia spp.

Mechanical scarification of Acacia nilotica seeds has

been accomplished by burning a hole in each seed with a

hot (450�C) soldering iron (Gosling et al., 1995a,b). The

hot iron was applied until “a quiet popping” sound was

heard, which indicated that the seed coat had been pene-

trated. A hot-wire seed scarifier has been devised that

allows up to 50 seeds to be scarified at a time (Masamba,

1994). The scarifying apparatus has a hot wire in a groove

in a fire-resistant board. At the beginning of a treatment,

50 seeds are pushed into the groove, and thus on top of

the hot wire, with a ruler. At the end of a specific period

of time for scarification, the board is tipped thereby

removing all seeds from the hot wire at the same time.

Below we will discuss a variety of methods that have

been used to break PY. The success of these methods var-

ies with the species, and in some cases the treatment may

kill the seeds. Thus, if nongerminated seeds remain after

the germination test, it is important to determine if the

seeds are still viable and capable of germinating. A quick,

easy way to determined seed viability/germinability is to

mechanically scarify them and see if they germinate

(Wang and Hanson, 2008).

Acid Scarification

Seeds (or fruits) usually are soaked in concentrated

H2SO4 and then washed several times to remove the acid.

[Remember add acid to water; never add water to acid.]

Trials have to be run to determine the appropriate period

of time for the acid to break through the seed coat but not

damage the embryo (e.g., Jin et al., 2006); this varies

with the species (Table 6.3). However, the actual time

required to make the seed coat permeable can vary with

the individual seed, i.e., some seeds in the population

150 Seeds

TABLE 6.3 Examples of species in which physical

dormancy has been broken by sulfuric acid, and

duration of treatment for maximum germination.

Species Time

(min) in

H2SO4

References

Abutilontheophrasti

15 Steinbauer & Grigsby, 1959

Acacia alba 30 Teketay, 1996a

A. albida 10, 20 Halliday & Nakao, 1984;Bebawi & Mohamed, 1985

A. aneura 40 Al-Mudaris et al., 1998

A. angustissima 15 Rincon-Rosales, 2003

A. auriculiforme 40, 15 Halliday & Nakao, 1984;Khasa, 1993; Ouattare &Louppe, 1993

A. cyanophylla 90 Jones, 1963

A. decurrens 10 Aveyard, 1968

A. ehrenbergiana 20 Bebawi & Mohamed, 1985

A. erioloba 20 Materechera & Materechera,2001

A. farnesiana 90, 40,120

Scifres, 1974; Rana & Nautiyal,1989; Al-Mudaris et al., 1998

A. mangium 15 Halliday & Nakao, 1984

A. nubica 25 Halliday & Nakao, 1984

A. pennatula 30 Halliday & Nakao, 1984

A. salicina 20 Aveyard, 1968

A. senegal 14 Palma et al., 1995a

A. seyal 90 Argaw et al., 1999

A. sieberiana 120 Teketay, 1996a

A. spirocarpa 20 Bebawi & Mohamed, 1985

A. tortilis 90 Argaw et al., 1999

Acosmiumnitens

20 Souza & Silva, 1998

Acrocarpusfraxinifolius

8, 20 Souza & Silva, 1998; Gupta &Bhardwaj, 2005

Adansoniagregorii

1440 Turner & Dixon, 2009

Albiziagummifera

20 Tigabu & Oden, 2001

A. julibrissin 120 Merou et al., 2002, 2011

A. lebbek 5 Agboola et al., 2005

A. procera 30 Das & Saha, 1999

Ambromaaugusta

20 Dutta et al., 2002

Apeibamembranacea

2, 10 Acuna & Garwood, 1987

Aspalathuslinearis

120 Kelly & Van Staden, 1985

Astragalus cicer 20 Miklas et al., 1987

Atylosia albicans 30 Rao et al., 1985a

A. cajanifolia 30 Rao et al., 1985a

A. lineata 30 Rao et al., 1985a

A. platycarpa 30 Rao et al., 1985a

A. scarabaeoides 30 Rao et al., 1985a

A. sericea 30 Rao et al., 1985a

A. volubilis 30 Rao et al., 1985a

(Continued )


Species Time

(min) in

H2SO4

References

Baptisia australis 40 Boyle & Hladun, 2005

B. tinctoria 50 Voss et al., 1994

Bauhiniarufescens

30 Ouattare & Louppe, 1993

B. ungulata 20 Souza & Silva, 1998

Caesalpiniaparaguariensis

5 Noir et al., 2004

C. spinosa 30 Teketay, 1996a

Calicotomeintermedia

60 Buhk & Hensen, 2006

Calystegiasoldanella

180 Ko et al., 2004

Canavaliamaritima

60 Ayala-Herrada et al., 2010

Cassia fistula 5�11, 90,10

Randhawa et al., 1986;Babeley & Kandya, 1988;Bhattacharya & Saha, 1990

C. moschata 30 Souza & Silva, 1998

C. siamea 5 Agboola et al., 2005

C. sieberiana 45 Todd-Bockarie et al., 1993

Chamaecristachoriophylla

3 Gomes et al., 2001

C. desvauxii 3 Gomes et al., 2001

C. mucronata 3 Gomes et al., 2001

C. rotundifolia 3 Gomes et al., 2001

C. venulosa 3 Gomes et al., 2001

Chordospartiumstevensonii

10 Conner & Conner, 1988b

Cistus albidus 60 Buhk & Hensen, 2006

C. clusii 60 Buhk & Hensen, 2006

C. salvifolius 60 Buhk & Hensen, 2006

Clitoria ternatea 30�40 Mullick & Chatterji, 1967

Colutea armena 30 Olmez et al., 2008

Convolvulusarvensis

45�60 Brown & Porter, 1942

C. lanuginosus 60 Buhk & Hensen, 2006

Corchorusfascicularis

12�15 Kak et al., 2009

C. pseudo-olitorious

12�15 Kak et al., 2009

C. tridens 10 Emongor et al., 2004

C. trilocularis 12�15 Kak et al., 2009

Crotalariamedicaginea

30 Bohra & Sen, 1974

Cuscutacampestris

20 Lados, 1998

C. indecora 30 Allred & Tingey, 1964

C. trifollii 20 Lados, 1998

Delonix regia 60 Teketay, 1996a

Dialiumguineense

6 Todd-Bockarie &Duryea, 1993

Dichrostachyscinerea

30, 120 Ouattare &Louppe, 1993;Argaw et al., 1999

(Continued )



Species Time

(min) in

H2SO4

References

Discariatoumatou

20 Keogh & Bannister, 1994

Entadaabyssinica

60 Teketay, 1996a

E. polystachya 20 Souza & Silva, 1998

Enterolobiumcyclocarpum

30, 15 Brahmam, 1996; Halliday &Nakao, 1984; Varela &Lizardo, 2010

E. schomburgkii 5 Souza & Varela, 1989

Erodium botrys 1 Young et al., 1975b

Erythrina burana 60 Teketay, 1994

E. caffra 120 Small et al., 1977

E. lysistemon 60 Teketay, 1996a

Fumanaericoides


F. laevipes 60 Buhk & Hensen, 2006

F. thymifolia 60 Buhk & Hensen, 2006

Galegaofficinalis

10�40 Oldham & Ransom, 2009

Glycine soya 30 Xu et al., 2009

Glycyrrhizaglabra

5 Gupta et al., 1997

Guazumaulmifolia

2, 10 Acuna & Garwood, 1987

Gymnocladusdioicus

120 Liu et al., 1981

Halimiumatriplicifolium


Hibiscus elatus 60 Brito, 1985

H. trionum 20 Everson, 1949

Hippocrepisciliata


Hymenaeacourbaril

15 Brahmam, 1996

Indigoferaglandulosa

10�15 Ghane et al., 2010

I. hirsuta 20 Ayala-Herrada et al., 2010

I. linifolia 10 Rao & Reddy, 1981

Ipomoeacrassicaulis

720 Misra, 1963

I. hederacea 60 Gomes et al., 1978

Lathyruslatifolius

5 Koike & Inoue, 1997

L. littoralis 30 Lemmon et al., 1943

L. martimus 20 Lemmon et al., 1943

Leucaenaleucocephala

30, 60 Daguma et al., 1988; Teketay,1996a

Lupinusangustifolius

180 Burns, 1959

L. sulphureus 20 Wilson et al., 2003

L. varius 720�960 Karaguzel et al., 2004

Medicagoarabica

60 Ruiz & Devesa, 1998

M. ciliaris 60 Ruiz & Devesa, 1998

M. coronata 90 Ruiz & Devesa, 1998

(Continued )


Species Time

(min) in

H2SO4

References

M. doliata 60 Ruiz & Devesa, 1998

M. italica 60 Ruiz & Devesa, 1998

M. minima 60 Ruiz & Devesa, 1998

M. orbiculatus 60 Ruiz & Devesa, 1998

M. polymorpha 60, 15�20 Ruiz & Devesa, 1998; Martin& de la Cuadra, 2004

M. praecox 90 Ruiz & Devesa, 1998

M. rigida 60 Ruiz & Devesa, 1998

M. truncatula 60 Ruiz & Devesa, 1998

Mimosatenuiflora

15 Camargo-Ricalde & Grether,1998

Nelumbo lutea 300 Jones, 1928a

Onobrychismelanotricha

2 Majidi & Barati, 2011

O. sintenisii 1 Majidi & Barati, 2011

O. viciifolia 1 Majidi & Barati, 2011

Ononisornithopodioides


Ormosia arborea 15 Marques et al., 2004a

O. coarctata 15 Souza & Silva, 1998

O. flava 10 Souza & Silva, 1998

O. smithii 10 Souza & Silva, 1998


30 Fu et al., 1996

O. pinnatus 30 Fu et al., 1996

Parkia auriculata 15 Coutinho & Struffaldi, 1971

P. biglobosa 90 Ouattare & Louppe, 1993

P. nitida 20 Cruz et al., 2001a

Parkinsoniaaculeata

45 Everitt, 1983b

Pelargoniumzonale

4, 8 Bachthaler, 1985

Peltophorumdubium

20, 15 Perez et al., 2001; Zhanget al., 2004b

Piliostigmathonningii

60 Ouattare & Louppe, 1993

Prosopis farcta 25 Dafni & Negbi, 1978

P. ferox 3 Baes et al., 2002

P. juliflora 15 Shiferaw et al., 2004; Zareet al., 2011

P. koelziana 15 Zare et al., 2011

P. pubescens 15 Vilela & Ravetta, 2001

P. tamarugo 20 Halliday & Nakao, 1984

P. velutina 15 Vilela & Ravetta, 2001

Psoraleacorylifolia

20 Mitter et al., 1993

Rhus aromatica 60 Li et al., 1999f

R. glabra 120 Farmer et al., 1982; Li et al.,1999f

R. trilobata 60 Li et al., 1999f

R. virens 60 Li et al., 1999f

Robiniapseudoacacia

30 Bhardwaj et al., 1996

Sennabicapsularis

60 Teketay, 1996b

(Continued )

152 Seeds

(or seed lot) require longer periods of acid scarification

than others to become water permeable (Baskin and

Baskin, 1997; Oldham and Ransom, 2009).

Since seeds are immersed in H2SO4, both the seed

coat and the plugged natural opening are subject to being

destroyed. Lumens of macrosclereids in the seed coat of

Coronilla varia were exposed by acid treatment (Brant

et al., 1971). However, Tran and Cavanagh (1984) noted

that Brant et al. (1971) did not check to see if water could

pass through the lower side of the macrosclereids. Acid

treatment of Rhus ovata fruits for 3 hr caused areas

around the micropyle and hilum to be destroyed (Stone

and Juhren, 1951). Acid scarification (conc. H2SO4 for

1 hr) made fruits of R. aromatica water-permeable but not

those of R. glabra (Li et al., 1999c). Anatomical studies

revealed that sulfuric acid removed both the brachyscler-

eids and osteosclereids (but not the macrosclereids) at the

carpellary micropyle of water-permeable R. aromatic

fruits, but only the brachysclereids were removed at the

carpellary micropyle of the water-impermeable R. glabra

fruits.

Acid scarification for 3 hr partially destroyed the

counter palisade cells of the hilum in Lupinus angustifolia

(Burns, 1959) seeds. Consequently, the hilum could not

close when L. angustifolia seeds were placed on a moist

surface, and water entered through it. If acid-treated seeds

of this species subsequently were redried and then placed

on a wet surface, they took up water through the lens.

The lens was the major region affected when seeds of

Astragalus cicer were acid scarified for 20 min (Miklas

et al., 1987). Acid dissolved the cuticle over the lens as

well as portions of the subtending Malpighian cells, leav-

ing a small circular cavity with a large groove in the bot-

tom. Consequently, most of the water entered A. cicer

seeds through the lens. Acid scarification first made seeds

of Vigna oblongifolia (Hu et al., 2009a) and Sophora alo-

pecuroides (Hu et al., 2008) permeable at the hilum, and

after a period of slow imbibition of water through the

hilum there was rapid imbibition via the lens. However,

acid scarification made seeds of Sesbania sesban water

permeable only through the lens. A 30- to 90-min period

of acid scarification destroyed a plug-like structure in the

bottom of the micropylar depression in seeds of

Convolvulus lanatus, C. negevensis and C. secundus, and

this is where water entered (Koller and Cohen, 1959).

Enzymes

A few attempts have been made to use enzymes to over-

come PY, but they usually do not work very well. After

24 hr of soaking in hemicellulase and pectinase, 52 and

46%, respectively, of the seeds of Coronilla varia were

permeable; 32% of the control seeds were permeable

(Brant et al., 1971). The site of water entry was not

determined.

Organic Solvents

Barton (1947) found that seeds of Cassia, Cercidium,

Gleditsia, Gymnocladus, Cercis (all members of the

Caesalpinioideae) and Acacia greggii (Mimosoideae)

became permeable after they were soaked in absolute

ethyl alcohol, but those of Papilionoideae did not. Ether

caused “a large percentage” of Prosopis juliflora

(Mimosoideae) seeds to germinate (Crocker, 1909), a

very low percentage of the impermeable fruits of

Nelumbo lutea to imbibe water (Shaw, 1929) and a 52%

(above the control) increase in germination of Discaria

toumatou (Rhamnaceae) seeds (Keogh and Bannister,

1994). Small but significant percentages of Coronilla

varia seeds became permeable after soaking in acetone


Species Time

(min) in

H2SO4

References

S. didymobotrya 60 Teketay, 1996b

S. marilandica 60 Nan, 1992; Baskin et al.,1998e

S. multiglandulosa 60 Teketay, 1996b

S. obtusifolia 30 Nan, 1992; Baskin et al.,1998e

S. occidentalis 60, 15 Teketay, 1996b; Delachiave& de Pinho, 2003a,b

S. septemtrionalis 60 Teketay, 1996b

Sesbaniadrummondii

240 Eastin, 1984

S. rostrata 40 Sheelavantar et al., 1993

Sophora davidii 30 Shao et al., 2010

S. flavescens 45 Voronkova & Kholina, 2003

S. secundiflora 120, 10,60

Ruter & Ingram, 1991; Wang,1991

S. rostrata 30 Kumar & Lal, 1999

Tamarindusindica

15 Halliday & Nakao, 1984

Thespesiapopulnea

10 Gupta et al., 2004

Trigonellacorniculata

10 Pandita et al., 1999

Ulex europaeus 180 Sixtus et al., 2003a

U. parviflorus 10 Baeza & Vallejo, 2006

Vignamembranacea

9�15 Wang et al., 2007d

V. oblongifolia 9 Wang et al., 2007d

V. racemosa 9 Wang et al., 2007d

V. schimperi 12 Wang et al., 2007d

V. umbellata 2 Tomer & Singh, 1993

V. vexillata 6�9 Wang et al., 2007d


and petroleum ether (Brant et al., 1971). Soaking in ethyl

alcohol and acetone stimulated 10�20% germination in

seeds of Acacia nilotica and 25�60% in those of

A. torilis; the point of water entry was the hilum (Brown

and de Van Booysen, 1969). Seeds of Sphaeralcea gros-

sulariaefolia germinated to 0 and 69% after soaking in

diethyl dioxide (dioxan) for 0 and 4 hr, respectively (Page

et al., 1966).

Other solvents, including acetone, carbon tetrachlo-

ride, chloroform, ethylene chloride and ethyl alcohol,

resulted in 0�14% germination of Prosopis stephaniana

seeds; the water control germinated to 6% (Khudairi,

1956). In an atmosphere of butylamine and at a constant

temperature of 20�C, seed permeability of Trifolium sub-

terraneum was 40% higher than that of the control

(Fairbrother, 1991). Ethyl alcohol and ether removed the

plug over the chalaza aperture in Gossypium hirsutum

seeds and allowed water to enter (Christiansen and

Moore, 1959).

Percussion

Hamly (1932) observed that the Malpighian cells in the

lens of an impermeable Melilotus officinalis seed

appeared to be in a strained condition. That is, the cells

were bent. However, the Malpighian cells in the lens of a

permeable seed were split apart, and they were only

slightly bent. Hamly (1932) surmised that a physical blow

on the lens would release the tension and cause the cells

to pull apart. Consequently, he put some seeds in a glass

bottle and shook them at a rate of three times per second

for several minutes, for a calculated total of 3600 blows

on each seed. He was correct. Impaction (later to be

called percussion) caused 92% of the seeds to become

permeable; only 0.5% of the seeds in the control germi-

nated. Percussion now has been shown to induce perme-

ability in seeds of a number of species (Table 6.4).

With the use of osmic acid, which turns black when it

comes into contact with cytoplasm, Hamly (1932) showed

that water enters permeable seeds of M. officinalis

through the lens. Ballard (1973) also demonstrated that

water enters Medicago truncatula seeds at the lens

(Figure 6.4). Since the seed coat and lens are parts of an

integrated system, cutting, filing, piercing, squeezing or

striking the seed coat at some point away from the lens

caused the lens on Medicago scutellata, Stylosanthes

humilis, Trifolium hirtum and T. subterraneum seeds to

become permeable to water (Ballard, 1976).

Stubsgaard (1986) developed a “seed gun” that pro-

pels seeds against a concrete wall at speeds of 11�15

m/sec. A seed-gun treatment increased germination of

Acacia tortilis subsp. raddiana, A. seyal var. seyal, A. sie-

beriana and Faidherbia albida seeds but decreased ger-

mination of Parkinsonia aculeata seeds (Mahjoub, 1993).

High Atmospheric Pressures

Davies (1926, 1928a,b) put Medicago sativa and

Melilotus alba seeds in a container of water inside a

TABLE 6.4 Examples of species with physical

dormancy whose seeds become permeable after

being shaken in a glass bottle (percussion).

Species References

Acacia greggii Barton, 1947

Amorpha fruticosa Hutton & Porter, 1937

Aspalathus linearis Kelly & Van Staden, 1987

Cladrastis lutea Barton, 1947

C. amurensis Barton, 1947

Glycine usuriensis Porter, 1949

Lathyrus odoratus Porter, 1949

Lespedeza capitata Hutton & Porter, 1937

L. virginica Hutton & Porter, 1937

Medicago truncatula Ballard, 1973

Melilotus alba Barton, 1947

M. officinalis Hamly, 1932

Parkinsonia microphylla Barton, 1947

Pisum sativum Porter, 1949

Prosopis velutina Barton, 1947

Robinia neomexicana Khadduri et al., 2003

R. pseudo-acacia Barton, 1947

Trifolium subterraneum Hagon & Ballard, 1970

FIGURE 6.4 Progression of stages in the imbibition of a single seed of

Medicago truncatula made permeable by percussion. Water entered at

the lens, and its pattern of movement can be seen in a seed placed in a

0.003 M Fe11 solution, which stains imbibed palisade cells black.

Redrawn from Ballard (1973), with permission.

154 Seeds

sealed chamber and then applied hydrostatic pressures of

50.7 and 202.6 MPa. A pressure of 202.6 MPa was more

effective than 50.7 MPa in breaking PY, and at

202.6 MPa 1 and 10 min were optimal for subsequent ger-

mination of M. sativa and M. alba seeds, respectively.

Further, pressure was more effective when applied at

room temperatures (18 6 2�C) than at 0�C.Seeds of Cladrastis lutea showed an increase in germi-

nation (i.e., more seeds became permeable) with an

increase in pressure from 6.7 to 206.7 MPa, but

$310.1 MPa resulted in decreased germination. A pres-

sure of 68.9 MPa applied for 10, 1 and 1 min at 0, 25 and

50�C, respectively, gave 100% germination. The seed coat

broke in the region of the hilum, and this is where water

entered (Rivera et al., 1937). Seeds of Gymnocladus dioi-

ca germinated to 90% after being exposed to 6.9 MPa of

pressure for 1 min; pressures lower than 6.9 MPa were

ineffective and those higher than 6.9 MPa injured the

embryos (Rivera et al., 1937).

Wet Heat

Immersion in hot water causes impermeable seeds of a

number of species to become permeable (Table 6.5). In

these studies, seeds are placed in a cloth bag or something

like a tea strainer and then dipped into hot water for the

required period of time. Seeds usually are removed from

the water and allowed to cool to room temperature, but

sometimes they are plunged into cold water (Hopkinson

and English, 2004; Agboola et al., 2005). Many (83%)

seeds of Sophora moorcroftiana were made permeable by

giving them seven cycles of wet heat (90�C water for

30 sec) and ice water (2 min); the lens was the site of

water entry (Baskin et al., 2007a). Maximum breaking of

PY for seeds of Trifolium reflexum (93%) occurred when

seeds were soaked in cold water (temperature not given)

for 12 hr and then into boiling water for 60 sec (McNair,

1917). Another way to give seeds a wet heat treatment is

to place them in a beaker, fill the beaker with boiling

water and allow the seeds and water to cool for some spe-

cific period of time, sometimes overnight (Aveyard,

1968; Gupta and Thapliyal, 1974; Oakes, 1984; Olvera

and West, 1985; Khasa, 1993; Al-Mudaris et al., 1998;

Costa et al., 2010). Also, the beaker holding the seeds

can be filled with water that is less than 100�C, e.g., 80�C(Barbosa et al., 2004a), and allowed to cool.

Even a 5 sec exposure to boiling water can kill some

seeds with PY (Mandal, 1994), but those of many species

can tolerate 5 sec or more of such treatment and remain

viable (Table 6.5). The period of time that seeds can be

kept in hot water before they are killed decreases with an

increase in treatment time and/or temperature. For exam-

ple, treatment times of up to 600 sec at 80�C resulted

in high germination percentages of Acacia falcata,


dormancy of the seeds is broken by wet heat

treatments. d, days; hr, hours; min, minutes;

s, seconds.

Species Temp

(�C)Duration References

Abutilonindicum

70 10 min Gupta et al., 2001

A. pauciflorum 80 2 min Galindez et al., 2010

A. theophrasti 70 1 hr Horowitz &Taylorson, 1984

Acacia albida 100 5 s Teketay, 1996a

A. celastrifolia 100 1 min Bell & Williams, 1998

A. cyanophylla 100 5 mina Aveyard, 1968

A. decurrens 100 15 mina Aveyard, 1968

A. drummondiisubsp.candolleana

100 1 min Bell & Williams, 1998

A. extensa 100 1 min Bell & Williams, 1998

A. falcata 100 5 s Clemens et al., 1977

80 600 s Clemens et al., 1977

A. lebbeck 100 5 s Teketay, 1996a

A. longifolia 80 200 min Clements et al., 1977

A. mangium 100 1 min Smiderle et al., 2005

A. mearnsii 100 5 min Gupta & Thapliyal,1974

A. melanoxylon 100 2�5 s Burrows et al., 2009

85 1 min Culshaw et al., 2002

A. nilotica 100 0 min Brown & de VanBooysen, 1969


A. pendula 100 15 mina Aveyard, 1968

A. salicina 100 33 s Rehman et al., 1999

A. saligna 100 20 s Bell & Williams, 1998

A. sieberiana 100 45 s Teketay, 1996a

A. sophorae 100 15 mina Aveyard, 1968

A. suaveolens 80 200 s Clemens et al., 1977

A. terminalis 100 30 s Clemens et al., 1977

A. tortilis 100 10 min Brown & de VanBooysen, 1969

A. urophylla 100 1 min Bell & Williams, 1998

Albiziajulibrissin

40 4 hr Merou et al., 2002

A. lebbek 100 30 s Agboola et al., 2005

Apeibamembranacea

62�70 10 min Acuna & Garwood,1987

A. tibourbou 80 2 min Daws et al., 2006b

Astragalushamosus

80 10 min Patane & Gresta, 2006

Bauhiniaracemosa

80 2 hr Prasad & Nautiyal,1996a

Cassia aspera 90 4 min Martin et al., 1975

C. nictitans 70, 80 4 min Martin & Cushwa,1966; Martin et al.,1975

C. siamea 100 30 s Perez-Garcia &Escudero, 1997;Agboola et al., 2005

(Continued )



Species Temp


Ceanothus spp. 100 5�20 s Quick & Quick, 1961

Centrosemapubescens

100 4 min Omokanye &Onifade, 1993

Ceratoniasiliqua

40 48 hr Martins-Loucao et al.,1996

Chamaecytisusproliferus

100 6�8 min Reghunath et al., 1993

Cistuspopulifolius

80 30 s Perez-Garcia &Escudero, 1997

Corchorusolitorius

80 5 s Oladiran, 1986;Velempini et al.,2003

5�15 min

Coronilla varia 100 30 s Brant et al., 1971

Crotolariasericea

65 10 min Saha & Takahashi,1981

Cupinusarboreus


Cuscutaaustralis

100 10 s Jayasuriya et al.,2008a

Cytisusreverchonii

100 �b Herranz et al., 1998

C. striatus 100 �b Herranz et al., 1998

Daviesiacordata

100 30 s Bell & Williams, 1998

Desmanthusleptophyllus

100 5 s Hopkins & English,2004

D. pubescens 100 5 s Hopkins & English,2004

D. virgatus 100 5 s Hopkins & English,2004


88 1 min Van Staden et al., 1994

100 10 s Idu & Omonhinmin,1999

Dodonaeapetiolaris

100 2�5 min Turner et al., 2009

D. viscosae 75, 100 4, 8 min3 s

Kladiano & CamachoMorfin, 1994;Baskin et al., 2004d

Entadaabyssinica

100 5 s Teketay, 1996a

Enterolobiumcyclocarpum

94, 100,100

1 min30 sc

Vasquez-Yanes &Perez-Garcia, 1977;Brahmam, 1996

Erythrina brucei 100 5 s Teketay, 1994

Gompholobiummarginatum


Gossypiumhirsutum

80 1 min Christiansen & Moore,1959

Guazumaulmifolia

62�70 2, 10 min Acuna & Garwood,1987

Hardenbergiaviolacea

100 1 min Jhurree et al., 1998

Heliocarpusdonnell-smithii

60, 80 1 min Vazquez-Yanes, 1981

Hovea trisperma 100 1 min Bell & Williams, 1998

Hymenaeacourbaril

40 2 d Guimaraes et al., 1995

100 30 sc Brahmam, 1996

(Continued )


Species Temp


Iliamna corei 100 5 s Baskin & Baskin,1997

Indigoferaaustralis

80 3 min Jhurree et al., 1998

I. linifolia 56 360 min Rao & Reddy, 1981

Kennediarubicunda

80 1 m Jhurree et al., 1998

Lespedezacyrtobotrya

70 0.5�3 min Iwata, 1966


100 15 s Teketay, 1996a

100 2�5 s Oakes, 1984

80 2�5 min Oakes, 1984

80 30 s Gosling et al., 1995b

80 5 min Amodu et al., 2000

Lupinusarboreus


Ochromalagopus

100 15 s Vazquez-Yanes, 1974

80 �b Barbosa et al., 2004a

Parkinsoniaaculeata


35 14 d van Klinken & Flack,2005

Phaseolusmungo

100 7 min Rao & Mukherjee,1978

Prosopisjuliflora


10 min Agrawal, 1996a

Rhus laurina 85 1 min Culshaw et al., 2002

Robinia hispida 100 1 min Wilson, 1944

R. pseudoacacia 100 1 min Wilson, 1944


Sennaobtusifolia

100 5 s Baskin et al., 1998e

Sesbaniapunicea

98 30 s Graaff & Van Staden,1983

S. sesban 80 8 min Wang & Hanson,2008

Stylobasiumspathulatum

100 30 s Baskin et al., 2006

Stylosanthesmacrocephala

100 1 s Silva & Felippe, 1986

S. scabra 75 5 min Gilbert & Shaw, 1979

Tamarindusindica

100 60 min Muhammad & Amusa,2003

Templetoniaretusa


Trifoliumreflexum

100 60 s McNair, 1917

Tylosemaesculentum

100 2�4 mind Travlos et al., 2007b

Ulex parviflorus 60 30 min Ballini, 1992

80 5 min Ballini, 1992

aSeeds were covered with boiling water and allowed to stand for15 min.bSeeds were placed in a container that was filled with boiling water,after which the water was allowed to cool to room temperature.cSeeds were dipped into boiling water for 30 sec and then soaked inwater at room temperature for 6 hr.dDry heat at 100 to 150�C for 5 min also increased germination.

156 Seeds

A. terminalis and A. suaveolens seeds (Figure 6.5).

However, treatment times of 200, 100 or 30 sec at 100�Cfor seeds of A. falcata, A. terminalis and A. suaveolens,

respectively, decreased germination, possibly indicating

that embryos had been damaged.

Boiling water caused the palisade layer in seeds of

Acacia spp. to soften and separate from the underlying

mesophyll; consequently, cracks in the seed coat occurred

at nonlocalized areas, allowing water to enter at many

sites (Brown and de Van Booysen, 1969). Boiling water

treatments also caused cracks to develop in seed coats of

Sesbania punicea (Graaff and Van Staden, 1983), and

they caused macrosclereids in the palisade layer of

Coronilla varia seeds to separate from each other (Brant

et al., 1971). The lens erupted or was uplifted in seeds of

Albizia lophantha (Dell, 1980), Acacia kempeana (Hanna,

1984) and Acacia melanoxylon (Burrows et al., 2009)

dipped in boiling water for a few seconds. In A. melanox-

ylon hot water caused the macrosclereids of the lens on

some seeds to split apart along their full length (Burrows

et al., 2009). Heat-treated seeds of A. kempeana did not

imbibe water if the uplifted lens was covered with petro-

leum jelly to block entry of water (Hanna, 1984). Hot

water broke the “seal” between the chalazal plug and pal-

isade layer of cells in Gossypium hirsutus seeds, and

water entered through the chalazal region (Christiansen

and Moore, 1959). Water at 65�C caused the lens of

Crotalaria sericea seeds to turn golden brown in color,

resulting in a significant increase in percentage of seeds

that imbibed and germinated (Saha and Takahashi, 1981).

Dipping seeds of Dichrostachys cinerea into boiling

water for 30 sec or 1 min made them permeable, but the

high temperature killed the embryo (Van Staden et al.,

1994). However, dipping seeds of this species into water

at 88�C for 1 min resulted in 80% germination. None of

the D. cinerea seeds kept wet at an alternating tempera-

ture regime of 30/15�C for 6 or 18 wk germinated, but 18

and 58% of the seeds incubated at 50/15�C for 6 and

38 wk, respectively, did so (Van Staden et al., 1994).

Dry Heat

The most common method for giving impermeable seeds

a dry-heat treatment is to place them in an oven at the

specified temperature. Dry heat is effective in breaking

PY in seeds of a number of species (Table 6.6), but it

100

Ger

min

atio

n (%

)

Ger

min

atio

n (%

)

(a)80˚

100˚

60˚

80

60

40

20

0 30 100

Immersion time (sec)

200 400 600

100 (b)

80˚

100˚

60˚

80

60

40

20

0 30 100


200 400 600

Ger

min

atio

n (%

)100 (c)

80˚

100˚

60˚

80

60

40

20

0 30 100


200 400 600

FIGURE 6.5 Final germination percentages of seeds of three species of Acacia given hot water treatments at various temperatures for 0 to 600 sec.

(a) A. falcata; (b) A. terminalis; (c) A. suaveolens. From Clemens et al. (1977), with permission.


does not work for all species. The appropriate dry-heat

treatment for a given species is determined by experimen-

tation to find the combination of temperature and length

of time of treatment that results in high germination per-

centages but not in death of the embryo. In general, there


dormancy of the seeds is broken by dry heat

treatments. hr, hours; min, minutes; s, seconds.

Species Temp.


Abelmoschusesculentus

50 48 hr Demir, 2001

Acacia ligulata 100 30 min Letnic et al., 2000

A. longifolia 140 1 min Mott et al., 1982

A. nilotica 120 10 min Brown & de VanBooysen, 1969

A. saligna 100 30 min Jeffrey et al., 1988

Atylosiaalbicans

80 24 hr Rao et al., 1985a

A. lineata 80 24 hr Rao et al., 1985a

A. platycarpa 80 96 hr Rao et al., 1985a

A. scarabaeoides 80 72 hr Rao et al., 1985a

A. sericea 80 16 hr Rao et al., 1985a

A. volubilis 80 16 hr Rao et al., 1985a

Caesalpiniadecapetala

80 15 min Teketay, 1996a

Cassia nictitans 70�80 4 min Martin et al., 1975

Cistus albidus 90, 100 9, 5 min Trabaud & Oustric,1989b; Escuderoet al., 1997b

C. clusii 90, 100 5 min Herranz et al., 1999;Nadal et al., 2002

C. creticus 100 1, 5 min Tilki, 2008

C. crispus 120 5 min Herranz et al., 1999

C. incanus 100 15 min Thanos &Georghiou, 1988

C. ladanifer 100 1 min Valbuena et al.,1992

100 30 min Corral et al., 1990;Perez-Garcia,1997

C. laurifolius 100 5 s Valbuena et al.,1992

100 5 min Tilki, 2008

C. monspeliensis 150 1 min Trabaud & Oustric,1989b

C. populifolius 100 5 min Nadal et al., 2002

90 10 min Herranz et al., 1999

C. salvifolius 110 9 min Trabaud & Oustric,1989b

100 5 min Nadal et al., 2002

Cytisusscoparius

65 2 min Bossard, 1993

Dodonaeaviscosa

80 60 min Baskin et al., 2004d

Entadalysistemon


Halimiumatriplicifolium

120 5 min Herranz et al., 1999

H. halimifolium 100,150 15, 1 min Thanos et al., 1992;Herranz et al.,1999

H. pilosum 100 15 min Thanos et al., 1992

H. viscosum 90 5 min Herranz et al., 1999

(Continued )


Species Temp.


Iliamna corei 80 60 min Baskin & Baskin,1997

Lespedezacyrtobotrya

90 4 min Martin et al., 1975

L. daurica 90 4 min Martin et al., 1975

L. hedysaroides 90 4 min Martin et al., 1975

L. japonica 90 4 min Martin et al., 1975

L. tomentosa 90 4 min Martin et al., 1975



Medicagosativa

104 4 min Rincker, 1954

Mimosa pudica 150 5 min Chauhan &Johnson, 2009c

Ochromalagopus

95 5 min Vazquez-Yanes,1974

Podalyriacalyptrata

60 5 min Jeffrey et al., 1988

100 1 min Jeffrey et al., 1988

Puerariaphaseoloides

50 4 hr Wycherley, 1960

Pultenaeaselaginoides

100 4 min Letnic et al., 2000

Rhus javanica 65�70 30�120 min Washitani, 1988

Sida grewioides 90 24 hr Chawan, 1971

S. rhombifolia 90 12 hr Chawan, 1971

S. spinosa 90 12 hr Chawan, 1971

S. veronicaefolia 70 24 h Chawan, 1971

Stylosantheshamata

85�95 1 hr Mott & McKeon,1979

75, 95 12 hr Gilbert & Shaw,1979

S. humilis 85�95 1 hr Mott & McKeon,1979

S. scabra 85�95 1 hr Mott & McKeon,1979

S. viscosa 85�95 1 hr Mott & McKeon,1979

Telinemonspessulana

80�120 10 min Garcia et al., 2010

Tephrosiaappolina

120 3 d Narang &Bhardwaja, 1974

Trifoliumpratense

104 4 min Rincker, 1954

Tuberaria lignoa 100 15 min Thanos et al., 1992

Ulex europaeus 80 5 min Pereiras et al., 1985

U. gallii 80 2 min Gutierrez, 1994

U. minor 60 24 hr Hossaert-Palauqui,1980

158 Seeds

is a decrease in length of time required to break PY with

an increase in temperature (Figure 6.6). Only a few min-

utes of exposure to temperatures of $100�C are required

to break PY in seeds of most species, but those of

Tephrosia appolina had to be heated at 120�C for 3 days

(Narang and Bhardwaja, 1974).

Another way of subjecting seeds to dry heat is to

allow them to slide (Lunden and Kinch, 1957) or bounce

(Mott et al., 1982) down a heated inclined plane. Also,

Mott (1979) placed seeds of Stylosanthes spp. on a tray

heated to various temperatures (125, 145, 200 and 260�C)and shook the tray vertically so that seeds bounced 3 cm

off the hot surface for 15, 30 or 60 seconds. Infra-red

radiation, radio waves and microwaves also are methods

of giving seeds a dry-heat treatment (see below).

Dry heat caused seeds coats of Tephrosia appolina

(Narang and Bhardwaja, 1974) and Acacia spp. seeds

(Brown and de Van Booysen, 1969) to develop cracks

and the palisade layer of the lens on seeds of Acacia spp.

to split (Brown and de Van Booysen, 1969). Cavanagh

(1987) expressed doubts that cracks induced by dry heat

were deep enough to allow water to reach the embryo, and

he suggested that heat treatments promote germination of

legume seeds because they made the lens permeable to

water. An alternating temperature regime of 40/20�Ccaused seeds of Astragalus sinicus to become permeable,

and the lens was opaque in permeable seeds and transpar-

ent in impermeable ones (Ueki and Suetsugu, 1958).

Dry Storage

In some species, part (or all) of the seeds with PY stored

dry at room temperatures for several months (or years)

become permeable (e.g., Egley, 1976; Silva and Felippe,

1986; Morrison et al., 1992; Tomer and Singh, 1993;

Norton et al., 2002; Chauhan et al., 2006b; Gresta et al.,

2007a; Galindez et al., 2010). Cavanagh (1987) con-

cluded that seeds of some legumes, especially those

belonging to the Papilionoideae, became permeable dur-

ing dry storage because cells in the lens broke, allowing

water to enter. However, seeds of various Australian

legumes belonging to the tribes Bossiaeeae and

Phaseoleae (Papilionoideae) permeable during dry stor-

age, and water entry was not restricted to the lens. The

mechanism of increase in seed coat permeability in these

two tribes during dry storage has not been identified, but

it may be related to the fact that seed coats were only

75% as thick as those of species in tribes Mirbelieae,

Acacieae (Mimosoideae) that failed to become permeable

during dry storage (Morrison et al., 1992).

The temperature during dry storage may have an

effect on whether seeds become water-permeable.

Percentage of Medicago sativa seeds with PY did not

decrease during 64 mo of dry storage at 20�C, but it diddecrease for seeds stored dry at 35�C if they were in plas-

tic but not in cloth bags (Acharya et al., 1999). Whereas

19% of freshly collected, round seeds of Lotus cornicula-

tus var. japonicus germinated at 20�C, 65% of those

stored dry at 3�C for 12 mo germinated at 20�C; seedsstored dry at 10, 15, 20, 25, 30 and 35�C germinated to

#10% (Kondo, 1993). All freshly collected seeds of

Ipomoea purpurea were water impermeable and did not

germinate at 15, 25 or 35�C. After 6 mo dry storage at

15, 25 and 35�C, seeds incubated at 15�C germinated to

0, 80 and 80%, respectively, and those incubated at 25�Cto 3, 76 and 95%, respectively. Regardless of storage tem-

perature, germination at 35�C apparently only reached a

maximum of 3% (Brechu-Franco et al., 2000). Dry stor-

age at room conditions (18�22�C, ca. 40% RH) for

12 mo resulted in 88% of Vicia sativa seeds becoming

water permeable (Van Assche and Vandelook, 2010).

Seeds of Prosopis juliflora collected from trees growing

on the coast of the Gulf of Oman in the United Arab

Emirates in autumn and in winter germinated to higher

percentages after 8 mo dry storage at room (206 3�C)temperature than when freshly collected. Although seeds

collected in spring germinated to higher percentages after

100 60 min.806040200

100 30 min.80604020

Ger

min

atio

n (%

)

0

100 15 min.80604020

0

100 1 min.806040200

50 60 70 80 90

Temperatures (°C)

100 110 120 130 140

FIGURE 6.6 Effect of dry heat at various temperatures for 1, 15, 30 or

60 min on dormancy break/germination of Iliamna corei seeds. From

Baskin and Baskin (1997), with permission.


8 mo storage than freshly collected (in spring) seeds, ger-

mination was less than for stored autumn and winter

seeds, especially the winter seeds (El-Keblawy and

Al-Rawai, 2006).

Long-term dry storage at, or near, room temperatures

may promote the breaking of PY. The percentage of seeds

with water-impermeable seed coats in fresh seeds of 27

species of Geraniaceae ranged from 20�100. After 5 yr

of dry storage in paper bags at 20�C, 100% of seeds of

five species (Geranium canariense, G. dissectum, G. pyr-

enaicum, G. pusillum and Erodium cicutarium) were per-

meable. Five yr of dry storage resulted in an increase

(e.g., Pelargonium australe, P. inquinans and P. zonale),

decrease (e.g., G. macrorhizum, G. robertianum and

P. mollicomum) or no change (e.g., G. phaeum and

P. vitifolium) in percentage of seeds with impermeable

seed coats (Meisert, 2002). Only 8% of seeds of Sophora

microphylla3 S. godleyi from New Zealand stored dry

for 24 yr were viable, whereas about 92% of freshly col-

lected seeds were viable. Although most of the 24-yr-old

seeds were dead, they were still water-impermeable

(Fountain et al., 2002).

Radiation

Infrared and gas-plasma radiation, radio frequencies and

ultrahigh radio frequencies (microwaves) have been used

to break PY. Much of the work has been done in an effort

to find an easy and efficient way to make impermeable

seeds of economically important members of the

Papilionoideae (e.g., Medicago spp., Trifolium spp.,

Stylosanthes spp.) permeable to water. All of these treat-

ments are with electrically generated radiation, and they

cause seed temperatures to increase.

Infrared radiation supplied by a 250-watt infrared bulb

produced a temperature of 104�C on the surface of

Medicago sativa seeds. After 1.5 min of exposure, germi-

nation was increased 47%; after 5 min most of the seeds

were dead (Rincker, 1954). A Philips and an Osram infra-

red lamp with a 90- and 250-watt bulb, respectively, pro-

duced a temperature of 50 and 43�C, respectively, at theposition of test seeds (Wycherley, 1960). Regardless of

the type of lamp, germination of Flemingia congesta and

Pueraria phaseoloides seeds increased by about 30 and

50% (over the controls), respectively. Maximum germina-

tion of F. congesta seeds occurred after a 15-min and 1-h

exposure to the Philips and Osram lamps, respectively,

and maximum germination of P. phaseoloides seeds was

after a 4-hr exposure to both types of lamps. With a 500-

watt infrared quartz lamp, 110 and 130 volts for 1.41 and

1.01 sec, respectively, caused 65�100% of the seeds of

Trifolium pratense and of seven varieties of Medicago

sativa to become permeable (Works, 1964). Germination

of Stylosanthes seabrana seeds increased from 3 to 44 %

after exposure to microwave energy of 980 W g21 min21

(Anand et al., 2011).

Gas-plasma radiation (glow discharge) caused seeds

of Narragansett, Ranger and Alaskan cultivars of

Medicago sativa to become permeable, with treatments

being more effective if seed MC was low (Pettibone,

1965). Infrared radiation, radio frequency at 39 MHz and

gas-plasma radiation were about equally effective in

increasing permeability in seeds of three cultivars of

Medicago sativa (Nelson et al., 1964).

Radio frequencies also increase the temperature of

seeds because seeds are poor conductors of electrical

charges (i.e., they are dielectric substances). Seeds absorb

some of the energy when they are subjected to alternating

electromagnetic fields, and this causes heating. The

amount of temperature rise depends on several factors,

especially seed MC (Ballard et al., 1976). In an early

study, germination of seeds of Medicago sativa placed

between two electrodes and exposed to alternating elec-

tromagnetic fields at 27 MHz for 25 to 30 sec increased

by 10�36%; optimum seed temperature for breaking PY

was 56�C (Eglitis and Johnson, 1957).

Medicago sativa seeds subjected to radio frequencies

of 39 MHz in a dielectric oven (i.e., exposed to an alter-

nating electromagnetic field) reached temperatures of

66 to 78�C, and germination increased from 42�80%

(controls) to 65�88%, depending on the cultivar (Nelson

et al., 1968). Radio frequencies of 5, 10 and 39 MHz

in a dielectric oven increased temperatures of M. sativa

seeds to 71�77�C, and germination ranged from

60�95%, regardless of radio wave frequency; controls

germinated to 46�53%, depending on the cultivar

(Nelson and Wolf, 1964). A hot-air oven, dielectric oven

at a radio frequency of 39 MHz and a microwave oven

at 2450 MHz were equally effective in overcoming PY

of M. sativa seeds, if seed temperature reached 66�88�C(Stetson and Nelson, 1972). Additional studies on M.

sativa seeds at normal moisture contents showed that

radio frequency treatments break PY without killing the

embryo, when temperatures rise to 70�80�C (Nelson

et al., 1977). Seeds of Medicago sativa, M. scutellata,

M. truncatula, Stylosanthes humilis, Trifolium hirtum and

T. subterraneum subjected to radio frequencies of 39 and

2450 MHz reached temperatures of 60�80�C before they

became permeable. At about 80�C, however, seed viabil-

ity began to decline. In seeds that reached 60�80�C,water entered through the lens. As seed temperature

increased to 120�130�C, water also entered through

random cracks that developed in the seed coat (Ballard

et al., 1976).

During exposure of Acacia longifolia and A. sophorae

seeds to 2450 MHz in a microwave oven seed tempera-

ture reached 93�96�C. The lens was raised, golden in

color and permeable to water, and then 74�90% of the

seeds germinated (Tran, 1979). However, species as well

160 Seeds

as length of microwaving treatment may influence the

degree of loss of PY. After 130 sec of microwaving, the

lens was raised and golden in color on 85 of 87 (97%)

Acacia longifolia seeds and 83 of the 85 seeds imbibed

water. After 110 sec of microwaving, A. sophorae seeds

had raised, golden lenses, but only 56 of the 68 seeds

imbibed water (Tran and Cavanagh, 1980). Seeds of

Lathryus latifolius subjected to 900 watts of microwave

radiation germinated to 96%, whereas those in the control

germinated to 80% (Barnes, 1995).

Ultrasound

Seeds of various species have been subjected to sonica-

tion (e.g., Weinberger and Burton, 1981), but little is

known about the effects of this treatment on breaking of

PY. However, sonication increased germination of

Medicago sativa (Kolokol’tseva and Profof’ev, 1974) and

Cassia holosericea (Faruqi et al., 1974) seeds.

Low Temperatures

PY of several species has been broken by freezing at

very low temperatures (Table 6.7): 2 l95.8�C (liquid

nitrogen), 2185�C (liquid oxygen), 2190�C (liquid air)

and 290�C (solid carbon dioxide). Seeds of Trifolium

repens and Lotus corniculatus placed in liquid nitrogen

and oxygen for 1, 2, 5, 10, 20 and 60 min germinated as

well after 5 min of freezing as they did after the longer

treatments (Eynard, 1958). Seeds of Trifolium hybridum,

T. pratense and M. sativa placed in liquid nitrogen or

liquid for 10 min germinated to about the same percen-

tages as those treated for 60 min (Eynard, 1960).

Regardless of the number of times seeds of Coronilla

varia were plunged into liquid nitrogen (2195.8�C) orhow long they stayed there, germination increased from

about 40 to 80% (Brant et al., 1971). However, four dips

of 30 sec each into liquid nitrogen were more effective

in decreasing impermeability of Melilotus alba seeds

than one dip of 1 or 5 min (Barton, 1947). A single 1-

min dip of freshly harvested Medicago sativa seeds into

liquid nitrogen increased germination from 19�27 to

94�98%, depending on the cultivar (Acharya et al.,

1999). Although treatment with liquid nitrogen made

the small-sized seeds of M. sativa water permeable, it

had no effect on the medium-sized seeds of Sesbania

sesban and damaged the large-sized seeds of Sophora

alopecuroides (Hu et al., 2009a).

Alternately dipping into boiling water and liquid

nitrogen broke PY in seeds of Cytisus scoparius, but the

heating/freezing treatment was more effective if seeds

were dipped into the boiling water before they were fro-

zen (Abdallah et al., 1989). Slow cooling and warming

before and after being plunged into the liquid nitrogen,

respectively, were as effective in breaking dormancy of

Trifolium arvense seeds as rapid cooling and warming

(Pritchard et al., 1988). These authors found channels

through the layer of Malpighian cells in the lens of

T. arvense seeds that had been frozen at 2195.8�C; there-fore, water entered through the lens. Cracks also devel-

oped in the seed coats of seeds that had been frozen, but

TABLE 6.7 Species in which physical dormancy is

broken by freezing of the seeds.

Species Temp.

(�C)References

Apeiba tibourbor 2196 Salomao, 2002

Astragalusmongholicus

220,222 Shibata et al., 1995; Shibata &Hatakeyama, 1995

Bowdichiavirgilioides

2196 Salomao, 2002

Coronilla varia 2196 Brant et al., 1971

Lotus corniculata 2185,2196

Eynard, 1958

Medicago marina 220a Scippa et al., 2011

M. orbicularis 2196 Patane & Gresta, 2006

M. polymorpha 218-2196,25b

Martin & de la Cuadra, 2004;Khaef et al., 2011

M. sativa 280 Busse, 1930

2185,2196

Eynard, 1960

2210 Gonzalez-Benito et al., 2003

M. scutellata 25b Khaef et al., 2011

Melilotus alba 2196 Barton, 1947

Mimosasomnians

2196 Salomao, 2002

Oxytropischankaensis

2196 Kholina & Voronkova, 2001,2012

O. kamtschatica 2196 Kholina & Voronkova, 2012

O. ochotensis 2196 Kholina & Voronkova, 2012

O. retusa 2196 Kholina & Voronkova, 2012

O. revoluta 2196 Kholina & Voronkova, 2012

Pterodonemarginatus

2196 Salomao, 2002

Sophoraflavescens

2196 Kholina & Voronkova, 2012

Stryphnodendronpolyphyllum

2196 Salomao, 2002

Trifolium arvense 2196 Pritchard et al., 1988

T. hybridum 2185,2196

Eynard, 1960

T. lupinaster 2196 Kholina & Voronkova, 2001,2012

T. pratense 2185,2196

Eynard, 1960

T. repens 2185,2196

Eynard, 1958

T. subterraneum 218 -2196

Martin & de la Cuadra, 2004

Vicia subrotunda 2196 Kholina & Voronkova, 2012

aFrozen at220�C for 60 days.bFrozen at25�C for 10 days.


water did not enter through them. Germination of

Astragalus cicer seeds increased with an increase in num-

ber of times (up to 15) seeds were frozen (10 min in liq-

uid nitrogen) and thawed (steam bath at 100�C for

10 min); the percentage of seeds with PY decreased from

49 to 9 after 15 cycles (Acharya et al., 1993).

Seeds of various endemic legumes from eastern

Russia were cryopreserved in liquid nitrogen at 2196�Cfor 1 to 60 days and then tested for germination at 18�C.However, both the control (nonfrozen) seeds and those

stored in liquid nitrogen were treated with 95�96%

H2SO4 prior to the germination test. Germination in the

scarified “control” compared to scarified cryopreserved

seeds varied depending on the species: Hedysarum

austrokurilense, 85 and 72%, respectively; Oxytropis

chamkaensis, 2 and 72%, respectively; O. kamtschatica,

65 and 90%, respectively; O. retusa, 5 and 38%,

respectively; and Vicia subrotunda, 5 and 55%, respec-

tively (Voronkova and Kholina, 2010a). Thus, in four

of the five species germination was promoted by low

temperature storage when seeds subsequently were acid

scarified.

Some seedlings from seeds frozen in liquid nitrogen

are abnormal because the petiole is broken resulting in a

detachment of the cotyledons (Pritchard et al., 1988;

Wiesner et al., 1994). However, cotyledon detachment

was reduced if seeds of Medicago sativa were mechani-

cally scarified prior to being frozen with liquid nitrogen

(Wiesner et al., 1994).

Seeds of Convolvulus arvensis stored on dry filter

paper in Petri dishes in the dark at 5�C for 0, 21 and 42

days germinated to 10, 55 and 85%, respectively, in dark-

ness at 20�C. Scanning electron microscopy showed that

the palisade sclerenchyma layer in nonchilled seeds was

tightly packed and the parenchyma cell region below the

palisade layer had few, if any, pores. In contrast, seeds

chilled at 5�C for 42 days exhibited a breakdown of cells

in both the palisade layer and the subtending parenchyma

layer (Jordan and Jordan, 1982). It is hard to explain how

“cell digestion” occurred in dry seeds at 5�C!

GERMINATION REQUIREMENTS OFPERMEABLE SEEDS

Although embryos in some seeds with PY also have non-

deep physiological dormancy (see “Seeds with Physical

and Physiological Dormancy” on p. 181), most seeds with

PY have nondormant embryos. Thus, the purpose of this

section is to discuss germination requirements of the latter

type of seeds after PY has been broken.

Imbibition

Seeds that become permeable by either artificial or natu-

ral means must imbibe water before they can germinate,

and maximum strength (enthalpy) of water sorption

depends on seed MC. Scarified seeds of Acacia bivenosa,

at an MC of 4�5%, had an enthalpy of 224.6 kJ, but

after seeds began to imbibe and had an MC of 10�15%,

enthalpy was 25 kJ (Merritt et al., 2003b).

Imbibition rates of permeable seeds usually increase

with an increase in temperature (Brown and Worley,

1912; van Klinken and Flack, 2005), but in seeds of the

Santorini cultivar of Ornithopus compressus, rate of imbi-

bition decreased with an increase in temperature (Taylor,

2004). In mechanically scarified seeds, imbibition usually

is completed in less than 1 day at about room temperature

(20�25�C). However, size of the seed may play a role in

time required for imbibition to occur. Imbibition curves

for small seeds, such as those of Dalea foliosa (seed5 ca.

2.0 mg), begin to plateau 3 hr after scarified seeds are

placed on a wet substrate, and imbibition is completed by

9 hr (Baskin and Baskin, 1998d). On the other hand, the

imbibition curves for large seeds, e.g., Acacia longifolia

(13.33 mg), A. mytrifolia (9.62 mg), A. sauveolens

(32.44 mg) and A. terminalis (39.94 mg) (Moles and

Westoby, 2003), began to plateau in 5�10 hr, and imbibi-

tion was completed in 7�15 hr, depending on the species

(Figure 6.7). Increase in mass of A. aroma (brown seeds)

and A. caven seeds did not exceed 20% until after 8 hr of

imbibition, but after 24 hr of imbibition mass had

increased 117 and 68%, respectively (Funes and Venier,

2006).

Within a population of seeds, there can be great varia-

tion in the amount of time required for seeds to become

permeable and for them to imbibe after PY is broken.

175

150

125

100

Fres

h m

ass

incr

ease

(%

)

75A. terminalisA. falcataA. suaveolensA. longifoliaA. myrtifolia

Time (hours)

50

25

0 5 10 15 20 25 30

FIGURE 6.7 Imbibition curves for mechanically scarified seeds of five

species of Acacia. One hundred percent of the scarified seeds imbibed,

but only 0�5% of those in the nonscarified controls (not shown on

graph) did so. From Clemens et al. (1977), with permission.

162 Seeds

Seeds of Colubrina oppositifolia were incubated on wet

filter paper at room temperature, and each was weighed

daily (Figure 6.8). Seeds became permeable in ,1 to

20 days. The first seed to imbibe had increased in mass

by 110% after 1 day but did not germinate until day 7.

The seed that imbibed last had a 5% increase in mass on

day 20 but did not germinate until day 27.

Seeds that have been made permeable by causing the

natural water-gap to open may imbibe water slower and

germinate slower than those that have been mechanically

scarified. Although mechanically scarified seeds of

Leucaena leucocephala and those dipped in 100�C water

for 7 sec, 90�C water for 3 sec, 80�C for 30 sec and 70�Cfor 4 min germinated to 65�85% at 20�C, the mean ger-

mination time of heat-treated seeds was less than that of

scarified seeds (Gosling et al., 1995a). On the other hand,

whereas seeds of Dodonaea viscosa dipped in boiling

water for 5 sec required 3�11 days to imbibe and 7�14

days to germinate, mechanically scarified seeds required

1 day to imbibe and 2�6 days to germinate (Figure 6.9).

The relatively slow rate of imbibition in seeds with an

open water-gap (boiling water treatment) vs. rapid imbibi-

tion in those that were mechanically scarified suggests

that in nature the water-gap serves as a “rain gauge” pre-

venting germination until there is a relatively long period

of moist soil (see Thanos and Georghiou, 1988).

After PY is broken, seeds in nature can imbibe when

there is available water in the soil. In Mediterranean cli-

mates, ND seeds would imbibe when the first rains fall in

autumn. If the first rains are small and/or followed by a

rainless period, the seedlings that emerged in response to

the first rain may die. Later, additional rains fall and the

soil will remain more or less continuously moist. Seeds

that germinate after the first rain and then die are said to

have responded to a “false break” in the dry weather. For

some economically important winter annual legumes such

as Trifolium subterraneum in Western Australia, false

breaks can be a serious problem, resulting in poor stand

establishment in about 2 out of every 3 yr (Chapman and

Asseng, 2001). In fact, Taylor et al. (1991) modeled the

5

6

78

83

3 5

6

21

4412

9

910

11

1111

12

220

200

180

160

Incr

ease

in m

ass

(%)

140

120

100

80

60

40

20

00 2 4 6 8 10 12

Time (days)14 16 18 20 22 24 26 28

FIGURE 6.8 Percent increase in mass of 12 individual seeds (numbered 1 to 12) of Colubrina oppositifolia incubated on moist filter paper at ambi-

ent room conditions (ca. 24�C and white fluorescent light for 10�12 hr per day). The last point on each curve was recorded the day (or other time

interval) before germination (radicle emergence) for a particular seed. From Baskin et al. (2007b), with permission.

100

80

See

d im

bibe

d or

ger

min

ated

(%

)

60

40

20

00 2 4 6 8 10

= Mechanical

= Boiled 5 sec

Time (days)

12 14

I G I G

FIGURE 6.9 Rate of imbibition of water (I) and germination (G) of

Dodonaea viscosa seeds that were dipped in boiling water for 5 sec to

open the water-gap and for those that were mechanically scarified. Bars

indicate SEs that were $5%. From Baskin et al. (2004d), with

permission.


effect of having T. subterraneum cultivars with different

proportions of water-impermeable seeds at the end of the

germination season on the long-term persistence of the

taxon at a site. The model indicated that a cultivar in

which only about 40% of the seeds became water-

permeable the first summer would greatly improve estab-

lishment of T. subterraneum in the pasture phase of the

normal rotation between crop/wheat and pasture/sheep in

the farming system of southwestern Australia.

Temperature

Temperature requirements for germination of permeable

seeds in light have been determined for a number of spe-

cies. Seeds germinate over a wide range of temperatures,

but the maximum and minimum temperatures vary with the

species. The low temperature that inhibits seed germination

varies with the species: Acacia aroma, A. caven, 15/5�C(Funes and Venier, 2006); Adansonia gregorii, 15�C(Turner and Dixon, 2009); Canna indica, 5�C (Filho et al.,

2011); Cassia obtusifolia, 15�C (Creel et al., 1968); C. tora,

15�C (Nazrul-Islam and Hoque, 1990); Convolvulus spp.,

5�7�C (Koller and Cohen, 1959); Cuscuta campestris, 7�C(Allred and Tingey, 1964); Desmanthus velutinus, 15/5�C(Haferkamp et al., 1984); Erythrina burana, 10�C (Teketay,

1994); Geranium carolinianum, 5�7�C (Washitani,

1985a); Helianthemum squamatum, 5�C (Escudero et al.,

1997b); Indigofera astragalina, I. senegalensis, 20�C (Sy

et al., 2001); Parkinsonia aculeata, #20�C (van Klinken

and Flack, 2005); Prosopis farcta, 10�C (Dafni and Negbi,

1978); Sesbania drummondii, 10�C (Eastin, 1984); Vicia

graminea, 1�C (Labouriau, 1970) and Zornia reticulata,

5�C (Felippe, 1984). On the other hand, permeable seeds of

some species can germinate at low temperatures: Acacia

furcatispina, 15/5�C (Funes and Venier, 2006); Erodium

botrys, 2/2 and 2/5�C (Young et al., 1975b); Ipomoea cras-

sicaulis, 10�C (Misra, 1963); Malva pusilla, 5�C(Blackshaw, 1990); Rhus glabra and R. copallina, 15/5�C(Farmer et al., 1982a); Ulex europaeus, 4.2�C (Ivens,

1983); and Vicia graminea, 2�C (Labouriau, 1970).

The maximum temperature for germination of

permeable seeds may be high, e.g., Canna indica, 40�C(Filho et al., 2011); Prosopis farcta, 40�C (Dafni and

Negbi, 1978); Sesbania drummondii, 40�C (Eastin,

1984); Stylosanthes humilis, 40�C (McKeon, 1985);

Erodium botrys, 40/30�C (Young et al., 1975b); and

Parkia auriculata, 42�C (Coutinho and Struffaldi, 1971).

Some mechanically scarified seeds of Aeschynomene

americana, Calopogonium mucunoides, Centrosema

pubescens, Desmodium heterocarpon, D. intortum,

Macroptilium atropurpureum, Stylosanthes guianensis,

S. hamata and S. viscosa germinated at 46�C, but thoseof all these species, except A. americana, germinated to

higher percentages at either 30 or 38�C than at 46�C(Gomes and Kretschmer, 1978).

The optimum temperature for germination of perme-

able seeds varies with the species: Acacia tortilis,

21�23�C (Loth et al., 2005); Adansonia gregorii, 30 or

35�C (Turner and Dixon, 2009); Caragana licentiana,

20�C (Zhao et al., 2005); C. opulens, 15�C (Zhao et al.,

2005); Cassia obtusifolia, 30�C (Sy et al., 2001); C. occi-

dentalis, 35�C (Sy et al., 2001); Coronilla juncea, 10�C(Robles et al., 2002); Cryptandra arbutiflora, 18/7�C(Turner et al., 2005); Cuscuta campestris, 28�C (Lados,

1998); C. trifolii, 22�C (Lados, 1998); Cytisus scoparius,

20/15�C (Harrington, 2009); Erodium cicutarium, 5�15�C(Blackshaw, 1992); Halimium atriplicifolia, 20�C (Perez-

Garcia and Gonzalez-Benito, 2005); H. halimifolium,

H. ocymoides, H. umbellatum, 15�C (Perez-Garcia and

Gonzalez-Benito, 2005); Indigofera senegalensis, 35�C(Sy et al., 2001); Ipomoea lacunosa, 20�25�C (Oliveira

and Norsworthy, 2006); Lespedeza floribunda, 25�C(Zheng et al., 2007a); Scorpiurus subvillosus, 20�25�C(Gresta et al., 2007a); Sida glaziovii, 25�C (Cardoso,

1992); Siegfriedia darwinioides, 18/7�C (Turner et al.,

2005); Spyridium globosum, 18/7�C (Turner et al., 2005);

Swainsona sejuncta, 25�C (Martyn et al., 2003);

Trymalium ledifolium, 18/7�C (Turner et al., 2005); and

Vicia graminea, 20, 26�C (Labouriau, 1970). The opti-

mum germination temperature for some species tested

only in darkness are: Anthyllis hermanniae, 15�C; A. vul-neraria, 15�C; Convolvulus elegantissimus, 10�C; and

Fumana thymifolia, 15�C (Doussi and Thanos, 1993).

In general, alternating vs. constant temperature regimes

do not have much effect on germination of PY-seeds made

water-permeable (e.g., McDonald, 2002), but there are

exceptions. Increasing the amplitude of the daily tempera-

ture fluctuation to 8�C decreased the rate of germination of

Chamaecrista rotundifolia, Desmanthus virgatus and

Stylosanthes scabra seeds; however, it had no effect on

germination rate of Glycine latifolia, Lablab purpureus or

Macroptilium atropurpureum seeds (McDonald, 2002). In

contrast, germination percentage was higher for Medicago

sativa seeds incubated at an alternating temperature regime

of 28/14�C than at a constant temperature of 20�C (Hall

et al., 1998). Seeds of Caragana korshinskii also germi-

nated to higher percentages at alternating than at constant

temperatures (Zheng et al., 2004c).

Light and Dark

Frequently, seeds are tested only in light, thus there is no

way to know how well they would have germinated in

darkness. Permeable seeds of many species, including

Acacia aneura (Preece, 1971), A. farnesiana (Scifres,

1974), Apeiba membranacea (Acuna and Garwood,

1987), Burkea africana (Zida et al., 2005), Caragana

164 Seeds

korshinskii (Zheng et al., 2004c), Calliandra fasciculata

(Oliveira et al., 2005), Chordospartium stevensonii

(Conner and Conner, 1988b), Detarium microcarpum,

Entada africana (Zida et al., 2005), Guazuma ulmifolia

(Acuna and Garwood, 1987), Helianthemum vesicarium,

H. ventosum (Gutterman and Agami, 1987), Merremia

aegyptica (Sharma and Sen, 1975), Malva parviflora

(Chauhan et al., 2006b), Mimosa invisa (Chauhan and

Johnson, 2008h) and Ulex europaeus (Ivens, 1983), ger-

minated equally well in light and darkness.

Seeds of Cassia occidentalis (Norsworthy and

Oliveira, 2005) and Dichrostachys cinerea (Bell and Van

Staden, 1993) germinated to higher percentages in light

than in darkness. Fresh seeds of Prosopis juliflora germi-

nated equally well in light and in darkness at 25�C, but at15 and at 40�C, they germinated to higher percentages in

light than in darkness. After an 8-mo period of storage,

seeds germinated equally well in light and in darkness at

40�C but to a higher percentage in light than in darkness

at 15�C and equally well in light and in darkness at 25�C(El-Keblawy and Al-Rawai, 2006).

Seeds of Zornia reticulata germinated (1) only in

darkness at 10�C, (2) to a higher percentage in darkness

than in light at 15�C, (3) faster in darkness than in light

at 20, 25 and 30�C and (4) equally well in darkness and

light at 35�C (Felippe, 1984). Seeds of Cryptandra arbu-

tifolia and Trymalium ledifolium made permeable by a

5-min hot (88�92�C) water treatment germinated

equally well (about 80%) in light and in darkness at 18/

7�C. However, at 26/13�C, seeds of C. arbutifolia germi-

nated to 0 and 40% in light and dark, respectively, and

those of T. ledifolium to 10 and 50%, respectively

(Turner et al., 2005).

Seeds of Sida spinosa germinated to 100% in both

light and darkness at 25/15, 30/15, 35/20 and 40/25�C,but they germinated to 8 and 42% in light at 15/6 and 20/

10�C, respectively, and to 95 and 97% in darkness at the

two temperatures, respectively (Baskin and Baskin,

1984f). Seeds of S. glaziovii germinated equally well in

light and dark at 30/25�C, to a higher percentage in dark

than in light at 25/20 and 30/20�C and to a higher per-

centage in light than in dark at 30/15 and 35/20�C(Cardoso, 1992).

Seeds of Sida grewioides germinated to about 70% in

continuous darkness and in far-red light but to only about

38% in red light (Chawan and Sen, 1973a), while those

of S. spinosa (Chawan and Sen, 1973a) and Merremia

aegyptica (Sharma and Sen, 1975) germinated to

75�100% in darkness and in far-red and red light. Seeds

of Cistus albidus and C. monspeliensis germinated to

higher percentages (and faster rates) at a red/far-red ratio

of 1.1 than at 0.7 (Roy and Sonie, 1992). Permeable seeds

of Ipomoea lacunosa germinated to 73 and 40% in a

greenhouse in natural light and in darkness, respectively

(Oliveira and Norsworthy, 2006), but germination was

not stimulated by red light nor inhibited by far-red light

(Norsworthy and Oliveria, 2007a).

Moisture Stress

After seeds with PY become water-permeable, their sensi-

tivity to water stress ranges from high to relatively low,

depending on the species. Little, or no, germination of seeds

of Cassia occidentalis occurred at 20.75 MPa (Norsworthy

and Oliveira, 2005), Ipomoea lacunosa at 21.0 MPa

(Oliveira and Norsworthy, 2006), Malva parviflora at

20.6 MPa (Chauhan et al., 2006b), Mimosa invisa at

21.2 MPa (Chauhan and Johnson, 2008h); Peltophorum

dubium at21.4 MPa (Zhang et al., 2004b), Pueraria lobata

at 20.8 MPa (Susko et al., 1999), Senna spectabilis at

20.7 MPa (Jeller and Perez, 2001) or Trigonella coerulea

at21.0 MPa (Akhalkatsi and Losch, 2001). Germination of

Trifolium repens seeds under water stress (2 0.3 MPa) was

reduced by a 3-hr exposure to various kinds of light; seeds

exposed to blue, far-red and red light and no light germi-

nated to about 40, 35, 25 and 50%, respectively

(Niedzwiedz-Siegien and Lewak, 1992).

Some germination (10% or more) is possible at

increased water stress: Acacia farnesiana, 20.18 (Scifres,

1974); A. senegal, 21.38 MPa (Palma et al., 1995b);

A. tortilis, 21.0 MPa (Coughenour and Delting, 1986);

Cassia obtusifolia, 20.5 MPa (Daiya et al., 1980);

C. occidentalis, 20.5 (Daiya et al., 1980); Chordospartium

stevensonii, 21.5 MPa (Conner and Conner, 1988b);

Malva pusilla, 21.53 MPa (Blackshaw, 1990); Medicago

sativa, 21.0 MPa (Delaney et al., 1986); Oxytropis riparia,

22.0 MPa (Delaney et al., 1986); Sida rhombifolia,

20.8 MPa (Smith et al., 1992) and S. spinosa, 20.6 MPa

(Smith et al., 1992). Scarified seeds of Anthyllis cyti-

soides germinated to 48% at 21.12 MPa (Ibanez and

Passera, 1997).

NaCl solutions of 0.325 M (21.48 MPa) to 0.5 M

(22.27 MPa) caused a 50% reduction in germination of

Prosopis farcta seeds, depending on geographical loca-

tion in Israel where seeds were collected (Dafni and

Negbi, 1978). Seeds of Desmodium cephalotus, D. gang-

eticum, D. gyrans and D. pulchellum germinated to 50,

38, 83 and 45%, respectively, at 0.2 M NaCl

(2 0.91 MPa) [the highest concentration tested] (Datta

and Sen, 1987), and those of Stylosanthes humilis germi-

nated to about 25�55%, depending on location in Brazil

where seeds were collected, at 0.268 M (21.22 MPa)

NaCl [the highest concentration tested] (Lovato et al.,

1994). Water-permeable seeds of Aeschynomene virgini-

ca germinated to 80, 84, 41 and 5% in 0, 1.0

(20.9 MPa), 1.5 (21.2 MPa) and 2.0% (21.8 MPa)

NaCl, respectively (Baskin et al., 1998f). At 0.17 M

(20.77 MPa) NaCl, seeds of Acacia schaffneri and


Parkinsonia aculeata germinated to about 60 and 80%,

respectively (Everitt, 1983b). Permeable seeds of

Prosopis juliflora germinated (50�80%) equally well in

light and in darkness at 15�C in water control and in

NaCl concentrations of 100, 200, 300 and 400 mM. At

40�C, however, seeds germinated (40�70%) only in the

water control and in 100 mM NaCl, and more seeds ger-

minated in light than in darkness (El-Keblawy and

Al-Rawai, 2005). At 500 mM NaCl, germination of

P. juliflora seeds was reduced from 85% (in control) to

56%, and at 600 mM NaCl germination was reduced to

1.2%. However, when treated with fusicoccin, GA3,

kinetin or thiourea, seeds germinated to 19, 33, 15

and 28%, respectively, in 600 mM NaCl (El-Keblawy

et al., 2005).

Flooding

Mechanically scarified seeds of Canavalia rosea,

Chamaecrista chamaecristoides and Ipomoea pes-caprae

germinated to about 55, 22 and 40%, respectively, on

moist sand in a greenhouse, while seeds of the three spe-

cies flooded in fresh water for 15 days germinated to

#10% (Martinez et al., 2002). However, water-

permeable seeds of Lotus corniculatus and L. tenuis

flooded for 15 days germinated to about 35 and 70%,

respectively (Vignolio et al., 1995), and those of

Aeschynomene virginica flooded for 7 days germinated to

70% (Baskin et al., 2005).

pH

Permeable seeds of most species germinate over a broad

range of pH values. Seeds of Acacia schaffneri and

Parkinsonia aculeata germinated at pH 3 to 11 but not at

pH 12 (Everitt, 1983b), and those of Mimosa bimucronata

did not germinate well at pH 4 or 8 (Ferreira, 1976). The

optimum for germination of Cassia occidentalis seeds

was pH 6, but the range for germination was pH 3 to 9

(Norsworthy and Oliveira, 2005). Seeds of Senna obtusi-

folia germinated at pH 3 to 9 with the optimum at pH 6

(Norsworthy and Oliveira, 2006). Similarly, seeds of

Ipomoea lacunosa germinated at pH 3 to 10 with an opti-

mum at pH 6 to 8 (Oliveira and Norsworthy, 2006) and

those of Pueraria lobata over the range of pH 4 to 9 with

an optimum at pH 5.4 (Susko et al., 1999). Scarified

seeds of Anthyllis cytisoides germinated at pH 6.0 to 9.2

with an optimum at pH 7.0 (Ibanez and Passera, 1997).

The optimum for seeds of Cuscuta trifolii was pH 5.5

(Lados, 1998). The optimum pH range for germination of

Malva parviflora seeds was 4 to 6.3 (Chauhan et al.,

2006b), and it was 4 to 10 for Mimosa invisa (Chauhan

and Johnson, 2008h).

ENVIRONMENTAL CONTROL OFBREAKING PHYSICAL DORMANCY

If seeds with PY are placed on the soil surface in the natu-

ral environment and monitored for several weeks, or even

months, it is not uncommon for the percentage of water-

permeable seeds to increase, e.g., Chamaecrista rotundifo-

lia cv. Wynn (Jones et al., 1998a), Hedysarum carnosum,

H. coronarium, F. flexuosum (Bell et al., 2003), Trifolium

angustifolium, T. argutum, T. cherleri, T. clusii, T. clypea-

tum, T. glanduliferum, T. isthmocarpum, T. lappaceum,

T. nigrescens, T. obscurum, T. purpureum (Norman et al.,

1998) and T. subterraneum (Norman et al., 2006a). Then,

the challenge for the seed biologist is to determine what

environmental factor(s) promoted opening of the water-

gap, thereby allowing water to enter the seed. The purpose

of this section is to survey what is known about the break-

ing of PY under natural conditions. That is, what environ-

mental factors are required for the chalaza, hilum, lens,

etc. to become permeable to water?

High and High Fluctuating Temperatures

Our understanding of the importance of daily temperature

fluctuations in the breaking of PY was enhanced by

results of two studies published in 1982 actually docu-

menting environmental conditions in the habitat when PY

was broken.

(1) The percentage of impermeable seeds of

Stylosanthes humilis and S. hamata began to decline in a

northern Australian pasture in September (early spring),

when mean monthly maximum and minimum tempera-

tures were about 67 and 28�C, respectively (Figure 6.10).

The number of physically dormant seeds decreased until

December (early summer), when rains stimulated all the

permeable ones to germinate. PY was not broken during

January to August, when mean daily maximum and mini-

mum temperatures were #55 and 25�C, respectively,

(McKeon and Mott, 1982).

(2) More seeds of Heliocarpus donnell-smithii

(Tiliaceae) became permeable and germinated when

placed in the center, or near the center, of a gap (clearing)

in a rain forest in eastern Mexico than when placed at the

edge of the gap or in the adjacent forest (Figure 6.11).

Amplitude of daily temperature fluctuations was about

15�C in the center of the gap and less than 5�C at the

edge of the gap and in the forest. Maximum germination

was obtained in laboratory studies at a 15�C amplitude of

daily temperature fluctuations, when the daily high tem-

perature was between 32 and 39�C for 6 hr each day

(Vazquez-Yanes and Orozco-Segovia, 1982a). The

response of seeds of H. donnell-smithii to fluctuating tem-

peratures ensures that they germinate in gaps where solar

irradiance is high and not in the shade of mature trees

166 Seeds

where seedlings are unlikely to survive. Thus, a high fluc-

tuating temperature requirement to overcome seed dor-

mancy effectively is a means of gap detection by the

species.

A gap-detecting mechanism also is implied by the

responses of physically dormant seeds of other species.

Many seedlings of Ulex europaeus (Ivens, 1978) and

Acacia melanoxylon (Farrell and Ashton, 1978) immedi-

ately appeared at sites after mature plants of each

species were cut and removed. These observations indi-

cate that seeds of both species germinated in response to

an increase in amplitude of daily temperature fluctua-

tions. Fruits of Rhus javanica also germinated after the

forest canopy was removed mechanically, or after it was

destroyed by fire. Laboratory studies showed that

PY was broken in the fruits by brief exposures to tem-

peratures of 48�74�C, which could occur in the field

following removal of the canopy (Washitani and

Takenaka, 1986).

100(a)

June July Aug. Sep.1979 1980

Oct. Jan. Feb.


Oct. Nov. Dec.

Nov.Germinating

Dec.rainfall

Jan. Feb.


Oct. Nov. Dec.

Germinating rainfall

Jan. Feb.

80

60

40

% h

ard

seed

s20

0

60 (b)

(c)

Winter Spring Summer

Min.

Max.

Rai

nfal

l (m

m)

Rai

nfal

l (m

m)

40

20

0

60

40

20

0

S. humilisS. hamata

FIGURE 6.10 (a) Change in percentage of impermeable seeds of Stylosanthes humilis and S. hamata. (b) Mean monthly temperatures

and (c) rainfall in a northern Australian pasture, where seeds were on the soil surface. From McKeon and Mott (1982), with permission.

40

(67%) (57%)

(26%) (25%)

(a) (b)

(c) (d)

35

30

250

35

30

25

07 9 11 13 15 17 19

Hour of the day

Tem

pera

ture

(˚C

)

7 9 11 13 15 17 19

FIGURE 6.11 Soil temperatures at a depth of 2 cm (a) in center of a

gap, (b) near the center of a gap, (c) at edge of a gap and (d) in adjacent

rainforest in Veracruz, Mexico. Germination percentage of Heliocarpus

donnell-smithii seeds at each site is given in parenthesis. (K)5mean

temperatures; (x)5maximum temperatures. From Vazquez-Yanes and

Orozco-Segovia (1982a), with permission.


Dormant seeds of Erodium botrys and E. brachycarpum

were placed in nylon bags and buried 1 cm deep in a bare

site (no vegetation or litter), under litter and on a gopher

mound for 3 mo in the Central Valley of California, USA,

(Rice, 1985). Soil temperature data showed that the ampli-

tude of daily temperature fluctuations at the three sites was

36, 21 and 17�C, respectively. Subsequent tests showed thatgermination was highest for seeds that had been buried in

the bare site and lowest for those buried on the gopher

mound (Rice, 1985). Thus, seeds of the two Erodium spp.

became permeable during exposure to natural summer tem-

peratures. However, did the fluctuating temperatures cause

seeds to become permeable, or was PY broken by exposure

to the absolute maximum temperature? By using alternating

temperature regimes recorded at field sites where seeds of

Erodium spp. were buried and high constant temperatures,

Rice (1985) demonstrated that temperature fluctuations

were more important than high constant temperatures in

overcoming dormancy.

Impermeable seeds of the typical variety of

Indigofera glandulosa were placed in cotton bags on the

soil surface in the field in Ujjain, India. Only 20% of the

seeds exposed to natural temperature fluctuations during

the rainy season (June�September) and winter

(October�February) became permeable by March.

However, the percentage of permeable seeds increased

to 86�93%, depending on variety, during summer

(March�May), and germination occurred in May (Bhat,

1968). Dry storage for 1 mo at 60�C, 4 mo at 35 or

60/35�C resulted in 92, 77 and 79% germination, respec-

tively; those stored at 30�C did not become permeable.

Therefore, Bhat (1968) concluded that high temperatures

in the habitat during summer caused seeds of I. glandu-

losa to become permeable.

Quinlivan (1961) subjected impermeable seeds of

Lupinus digitatus, L. luteus, Medicago tribulus and

Trifolium subterraneum to alternating temperature

regimes of 46/15, 60/15 and 74/15�C (maximum soil sur-

face temperatures during summer in New South Wales,

Australia, on cloudy days were 43�49�C and on clear

days 71�77�C) and to constant temperatures of 15 and

60�C for 0 to 5 mo. Some seeds of the four species

became permeable at all temperatures, but the highest

germination percentages were for those exposed to 4 mo

at 60/15�C. A daily alternating temperature regime of

60/15�C was effective in breaking PY in seeds of

Ornithopus compressus, with 90% of the seeds germinat-

ing after 4 mo; 28% of the seeds in the controls germi-

nated (Barrett-Lennard and Gladstones, 1964). Seeds of

Dichrostachys cinerea incubated on wet filter paper at

50/15�C, for 18 wk germinated to 18%, whereas none of

the seeds incubated at 30/15�C germinated; after 38 wk at

50/15�C, 58% of the seeds germinated (Van Staden et al.,

1994). The number of Trifolium subterraneum seeds

remaining impermeable at the end of the summer

increased, if litter (including standing dead plants of T.

subterraneum) shaded the soil surface (Quinlivan and

Millington, 1962; Quinlivan, 1965). Shading decreased

the amplitude of the daily temperature fluctuation by as

much as 17�C, and this resulted in fewer seeds becoming

permeable. Temperature fluctuations also are reduced in

the buried seed environment. For example, Medicago

truncatula seeds (still in the fruits) buried 2 cm below

the soil surface showed only a 17% reduction in PY after

27 days, while 97% of those on the soil surface became

permeable (Kirchner and Andrew, 1971).

Quinlivan (1966) found that dormancy loss in seeds of

Trifolium subterraneum was determined by the maximum

daily temperature, if there was a minimum daily temperature

fluctuation of at least 15�C. The maximum daily temperature

required for loss of PY varies with the species:

T. subterraneum, 30�C; Lupinus varius, 60�C; T. dubium,30�C; T. hirtum, T. cherleri, T. glomeratum and T. cernuum,

40�C; Medicago truncatula, M. littoralis, M. polymorpha,

and M. scutella, 50�C (Quinlivan, 1968b) and Stylosanthes

spp., 50�C (McKeon and Brook, 1983). Loss of PY in seeds

of Mimosa pigra required a temperature fluctuation of 20�C,and it occurred at alternating temperature regimes ranging

from 30/10 to 50/30�C with 40/20�C being optimal (Dillon

and Forcella, 1985). Seeds of Phaseolus lunatus were incu-

bated on wet filter paper for 2 wk at 25�C and then exposed

to 45�C for 1 hr, after which they were moved back to 25�C,where they became water-permeable (Degreef et al., 2002).

Seeds of T. subterraneum and T. incarnatum became perme-

able during summer in the Central Valley of California,

when temperatures of 60�C were recorded in dead plant

material on the soil surface (Williams and Elliott, 1960).

Seeds of 12 tropical legumes were stored dry in incu-

bators at alternating temperature regimes of 57/23 and

70/23�C that simulated conditions in the field during

summer, and rate of loss of PY was monitored. At

monthly intervals for 7 mo, a sample of seeds from each

alternating temperature regime was tested for germination

at 24�C. Significant loss of PY occurred for seeds at both

alternating temperature regimes, but loss was fastest for

Aeschynomene americana and slowest for Desmanthus

virgatus and Indigofera schimperi (McDonald, 2000).

McDonald used the data in a model to derive the thresh-

old temperature for the breaking of PY in 11 of the spe-

cies: Aeschynomene americana, 50.0�C; Chamaecrista

rotundifolia, 39.2�48.8�C (when two temperatures are

given, it is the range of values determined for various

cultivars or accessions); Desmanthus virgatus,

43.2�49.5�C; Glycine latifolia, 54.5; G. ternatea,

49.9�50.0; Indigofera schimperi, 54.8�C; Leucaena leu-

cocephala, 50.8�51.2�C; Macroptilium atropurpureum,

41.9�44.9�C; M. bracteatum, 44.0�47.2�C; Stylosanthesscabra, 44.3�46.4�C; and S. seabrana, 43.7�C.

168 Seeds

Two Steps in Breaking Physical Dormancywith High Temperatures

Loss of PY in seeds of Trifolium subterraneum occurs in

two stages (Taylor, 1981a, 2005a). (1) The preliminary or

preconditioning phase will occur if seeds are at constant

temperatures, and the rate at which this stage is com-

pleted increases with an increase in temperature. Seeds

prevented from drying (by blocking the hilum) during the

first stage are more likely to become water-permeable in

the second stage than those that dehydrate further during

stage one. (2) The second stage (when seeds actually

become permeable) requires fluctuating temperatures for

maximum loss of dormancy, but constant temperatures of

$50�C are somewhat effective. Both stages of seed coat

softening take place simultaneously as seeds are exposed

to high daily temperature fluctuations during summer.

Taylor (1981a) suggested that thermal degradation occurs

during the first stage which results in a weakening of the

lens. In the second stage, physical expansion and contrac-

tion associated with daily increases and decreases in tem-

perature cause cells in the lens to separate.

Seeds of Ornithopus compressus were placed on the

soil surface and buried at a depth of 0.5 cm in the field in

Western Australia from December (summer) to April

(autumn), and at 2-wk intervals the number of water-

permeable seeds was determined before and after seeds

were exposed to seven daily temperature cycles of 48/

15�C (Taylor and Revell, 1999). Preconditioning (i.e.,

first step in dormancy break) occurred in the water-

impermeable seeds on both the surface and in the soil in

early summer. In the laboratory, preconditioned seeds

became permeable (second step), if they were exposed to

48/15�C. In the field, preconditioned seeds became per-

meable when daily maximum and minimum temperatures

were 45�50 and 10�20�C, respectively. In addition to

requiring alternating temperatures to become permeable,

preconditioned seeds were more likely to become perme-

able if they were in darkness than in light. In fact, precon-

ditioned seeds did not become permeable at a light level

of 5�25% (Taylor and Revell, 1999).

Winter annual species of Medicago also are a part of

the pasture portion of the farm rotation system in Western

Australia, and seeds of Medicago spp. become water-

permeable in two stages. The first stage of dormancy

break occurs in the field during summer resulting in latent

soft seeds that are still impermeable; the portion of latent

soft seeds varies with the species/variety, year and site.

Latent soft seeds became water-permeable when they

were transferred to the laboratory and subjected to four

diurnal cycles of 35/10�C, after which they germinated on

wet filter paper at 20�C (Taylor, 1996a). Thus, in the two-

stage breaking of PY in seeds of winter annuals, seeds

basically become “ready” for dormancy break to occur

during the summer, but PY is not broken until appropriate

environmental conditions occur, at which time seeds rap-

idly become water-permeable. PY was broken in seeds of

Trifolium subterraneum during summer, but it was not

broken in seeds of Medicago polymorpha and Trifolium

glomeratum until autumn (Smith et al., 1996b).

Species in families other than the Fabaceae also may

have a two-step process for the breaking of PY. Seeds of

Sida spinosa incubated on moist sand at 15/6, 20/10,

25/15, 30/15 and 35/20�C for 30, 90 or 180 days germi-

nated to higher percentages when seeds at each tempera-

ture were shifted to all temperatures higher than each

respective temperature, e.g., seeds at 15/6 shifted to

20/10, 25/15, 30/15, 35/20 and 40/25�C, germinated to

higher percentages than those kept continuously at each

temperature. Seeds germinated to 90% or more when

moved from 20/10 and 25/15�C to 35/20 and 40/25�Cand from 30/15 to 40/25�C after 180 days. Thus, while

seeds were at relatively low temperatures, they were con-

ditioned for rapid increases in permeability when shifted

to relatively high temperatures (Baskin and Baskin,

1984f). Response of conditioned seeds to an increase in

temperatures is a means of detecting a shift from burial in

the soil to the soil surface.

Seeds of Ipomoea lacunosa (Convolvulaceae) become

conditioned, i.e., become latent soft seeds or “sensitive”

during long periods of incubation on a moist substrate, and

this can occur during exposure to low winter temperatures

or high summer temperatures (Jayasuriya et al., 2008c).

After seeds become sensitive, they will become water-

permeable when exposed to high temperatures and high

moisture conditions. Sensitive seeds of I. lacunosa incu-

bated on wet sand at 35�C for $2 hr became water-

permeable, but PY was not broken when sensitive seeds

were incubated on wet sand at 15, 20, 25 or 30�C(Jayasuriya et al., 2008c).

On the other hand, seeds of Ipomoea hederacea stored

either wet or dry at high ($25�C) temperatures became

sensitive, and PY was broken when sensitive seeds were

stored dry at high temperatures (Jayasuriya et al., 2009b).

After seeds had been stored wet or dry at 35/20�C for

1 mo, they became sensitive, and then a 2-wk period of

dry storage at 35/20�C resulted in the sensitive seeds

becoming water-permeable. Seeds of Cuscuta australis

become sensitive during 2 mo dry storage at ambient lab-

oratory temperatures, after which seeds incubated on wet

sand at 35/20�C for 30 days become water-permeable

(Jayasuriya et al., 2008a).

For seeds with PY that become permeable via a two-

step process, evidence is accumulating that the first step

can be reversed. Reversal occurs when environmental

conditions are not favorable for the second step to occur.

Seeds of Ornithopus compressus made sensitive on the

soil surface or buried 0.5 cm in the soil in the field during


summer in Western Australia lost their ability to respond

to high alternating temperatures (and thus to become

water permeable) if exposed to low, e.g., 8�C, tempera-

tures before they were exposed to high alternating tem-

peratures (Taylor and Revell, 1999).

Other examples of reversal of sensitivity and non-

sensitivity to conditions that break PY have been found.

Seeds of Ipomoea lacunosa lost their sensitivity if stored

dry at either low (#5�C) or high ($30�C) temperatures, but

insensitive seeds become sensitive again if exposed to wet

conditions. After seeds become sensitive, exposure to high

RH and to temperatures $35�C will cause the water-gap to

open. After seeds become permeable, they cannot become

water-impermeable again. However, seeds can cycle

between being sensitive and nonsensitive, depending on

environmental conditions (Figure 6.12). Seeds of I. hedera-

cea also cycle between the sensitive and nonsensitive states,

with high temperatures ($25�C) making them sensitive and

low (#5�C) temperatures making them nonsensitive

(Jayasuriya et al., 2009b). Seeds of Cuscuta australis cycle

between being sensitive and nonsensitive with them becom-

ing sensitive during dry storage at room temperatures and

nonsensitive during storage at high ($35�C) or low (#5�C)temperatures (Jayasuriya et al., 2008a).

The mechanism of dormancy break in sensitive seeds

of Ipomoea lacunosa and I. hederacea has been investi-

gated. In I. lacunosa, sensitive seeds can absorb water

and water vapor through the upper part of the hilum fis-

sure, but nonsensitive seeds cannot do this (Figure 6.13).

Water vapor also is lost from sensitive seeds through the

lower part of the hilum fissure. When sensitive seeds are

incubated at high RH (1) all the microspaces under the

bulges (water-gaps) are filled with water vapor, and (2)

the hilar pad absorbs water vapor and expands, thereby

sealing water vapor inside the microspaces. Then, when

sensitive seeds (with water vapor in the microspaces

under the bulges) are exposed to high temperatures, pres-

sure builds up under the bulges. The pressure causes slits

to appear around the bulges, and the bulges are dislodged/

opened, allowing liquid water to enter the seed

(Jayasuriya et al., 2009c).

Since sensitive seeds of Ipomoea hederacea become

water-permeable under hot dry (as opposed to hot wet

conditions for I. lacumosa), we would expect a different

mechanism for the breaking of PY. As seeds of I. hedera-

cea dry at high temperatures, water is lost from seeds via

the hilar pad. If enough water is lost, the hilar pad shrinks

causing micro-openings to form around the edge of the

pad. Then, water is lost from these small openings,

thereby causing vapor pressure to decrease under the

bulges. When the pressure is released, slits form around

the bulges allowing water to enter (Jayasuriya et al.,

2009b). If seeds are exposed to high RH, however, the

hilar pad absorbs water vapor and swells. Thus, water

vapor under the bulges cannot escape, and the bulges

remain closed.

Unlike many seeds with nondeep PD (Chapter 4), those

with PY cannot undergo dormancy cycles, i.e., once the

1

0.75Seeds in soilseed bank

Seeds maturein late autumn

Seeds exposedto dryconditions

Seeds exposedto wetconditions

Seeds maturein mid autumn

Seeds maturein early autumn

0.5

Sen

sitiv

ity

0.25

0

Sep Oct Nov Dec Jan Feb Mar

Month

Apr May Jun Jul Aug Sep

FIGURE 6.12 Conceptual model showing annual seasonal changes in sensitivity of Ipomoea lacunosa seeds. When seeds are sensitive, incubation

on a wet substrate at a high ($35�C) temperature results in seeds becoming water-permeable. Modified slightly from Jayasuriya et al. (2008b), with

permission.

170 Seeds

water-gap opens, it cannot close. However, the changes in

sensitivity of seeds with PY to the environmental condi-

tions that can make seeds water permeable accomplish the

same things ecologically as an annual dormancy/nondor-

mancy cycle (Jayasuriya et al., 2009b). That is, induction

of sensitivity means the seeds with PY can respond very

quickly when appropriate dormancy-breaking conditions

occur in the environment (Jayasuriya et al., 2008c). Thus,

why do not seeds with PY remain in a constant state of

sensitivity to appropriate dormancy-breaking conditions?

(1) Lack of sensitivity during some parts of the year may

help prevent physically dormant seeds from becoming per-

meable and germinating although conditions are briefly

suitable for germination but do not remain favorable long

enough for seedling establishment. (2) In Ipomoea lacu-

mosa, vigor of sensitive seeds decreased more rapidly than

that of nonsensitive seeds subjected to various aging/vigor

tests. However, no differences in vigor were detected for

sensitive and nonsensitive seeds of I. hederacea

(Jayasuriya et al., 2009d).

Drying at High Temperatures

Seeds can be exposed to dry heat treatments in nature due

to lack of precipitation during summer and/or to a fire. In

either case, drying at high temperatures indirectly influ-

ences the timing of germination of many species. For

example, seeds of Lupinus varius require a maximum of

60�C in the daily temperature cycle to become permeable

(Quinlivan, 1968b). However, high daily temperature

fluctuations are ineffective in breaking dormancy if seed

MC is above 8.5�9% (Quinlivan, 1968a). Thus, if seeds

have a low MC or if they are on the soil surface where

drying occurs via water loss through the hilum, they will

become permeable and germinate when rains come in

autumn. If seeds are buried or covered with litter, drying

and subsequent loss of PY may be inhibited. Drying prob-

ably also is important in breaking PY in seeds of other

Papilionoideae, but these studies remain to be done.

At alternating temperatures, more seeds of Trifolium

subterraneum became permeable at high (74.1�77.6%)

FIGURE 6.13 Diagrams of the hilum area of sensitive seeds of Ipomoea lacunosa showing the effect of relative humidity (RH) on the water vapor

(WV) absorbed by the hilar pad and on germination percentage of seeds incubated at 25/15�C after different blocking treatments. B, bulge; UHF,

upper part of hilar fissure; HP, hilar pad; LHF, lower part of hilar fissure; HR, hilar ring; No paint, none of hilum painted; UP, upper part of hilum

painted; LH, lower part of hilum painted; HP, all of hilum painted, following 3 hr incubation at 35�C at 100, 50 and 0% RH; X, movement of water

vapor is blocked. Thickness of arrows indicates relative amount of water movement. From Jayasuriya et al. (2009b), with permission.


than at low (0%) RH, while at constant temperatures RH

had no effect on loss of impermeability (Fairbrother,

1991). Thus, Fairbrother concluded that rupture of the lens

eventually occurred at high RH because cell wall fibers

swelled at high and shrunk at low temperatures. High fluc-

tuating temperatures under wet conditions also promoted

loss of PY. All seeds of Leucaena pulverulenta kept on a

wet substrate became permeable after 50 days at a daily

50/30�C temperature regime, while only about 10% of

those kept dry did so (Owens et al., 1995).

Substrate drying and high temperature fluctuations

play a role in germination of Neptunia oleracea. Seeds of

this aquatic angiosperm mature in autumn in Bharatpur,

India, and are dispersed into the water. In summer (March

to mid-June) water levels recede, and seeds are exposed to

daily temperature fluctuations of 55/22�C. Seeds become

permeable during summer but do not germinate until pools

begin to fill with water at the start of the rainy season in

mid-June. In laboratory studies, seeds germinated to 100%

after being subjected to an alternating temperature regime

of 60/20�C for 2 days (Sharma et al., 1984). If the water

fails to recede, seeds are not exposed to sufficiently high

alternating temperature regimes to become permeable.

It has been suggested that germination of Abutilon

theophrasti seeds follows intensive drying that causes

cracks to develop in the seed coat. Although no proof was

given that this actually happens in the field, drying over

calcium chloride for 5 wk increased germination from 11

to 63% (LaCroix and Staniforth, 1964). Flushes of

A. theophrasti seeds germinated in a field in Michigan

(USA) following extended periods when showers of rain

did not exceed 12.5 mm (Dekker and Meggitt, 1986).

Although the results of this study suggest that drying was

important in breaking PY, the effects of high summer soil

surface temperatures were not considered.

Fire

Fire is a natural part of many ecosystems, e.g., dry sclero-

phyll woodlands (Purdie, 1977), matorral (Sweeney,

1956) and pine forests (Cushwa et al., 1968), and there is

much interest in the regeneration of various plant species

following a fire. One way in which species increase in

numbers following fire is via germination of seeds buried

in soil at the site. From lists of species germinating within

a few months following a fire, it is easy to find represen-

tatives of many families whose seeds have PY

(Table 6.8). Since these seeds germinated after the habitat

was burned, the conclusion frequently is reached that heat

from the fire made them permeable. This idea seems logi-

cal especially in view of our previous discussion on the

effects of dry heat on the breaking of PY (see Table 6.6).

Will temperatures that occur on/in soils during fires break

PY, without killing the seeds?

TABLE 6.8 Examples of species with physical dormancy

whose seeds have been observed to germinate in the

field after fire.

Species Family References

Acacia aneura Fabaceae Griffin & Friedel, 1984

A. aprica Fabaceae Yates & Broadhurst, 2002

A. cochlocarpa Fabaceae Yates & Broadhurst,2002

A. genistifolia Fabaceae Purdie, 1977

A. longifolia Fabaceae Weiss, 1984

A. mearnsii Fabaceae Pieterse & Boucher, 1997

A. melanoxylon Fabaceae Farrell & Ashton, 1978

A. nilotica Fabaceae Radford et al., 2001

A. pulchella Fabaceae Shea et al., 1979; Monket al., 1981

A. saligna Fabaceae Tozer, 1998

A. sieberiana Fabaceae Sabiiti & Wein, 1988

A. strigosa Fabaceae Shea et al., 1979

A. suaveolens Fabaceae Auld, 1986b; Auld & Tozer,1995; Tozer & Auld, 2006

A. sylvestris Fabaceae Floyd, 1966

Albizialophantha

Fabaceae Dell, 1980

Bonamiagrandiflora

Convolvulacae Hartnett & Richardson,1989

Bossiaeaaquifolium

Fabaceae Shea et al., 1979

Calystegiamacrostegia

Convolvulaceae Tyler, 1995

Cassianemophila

Fabaceae Griffin & Friedel, 1984

Ceanothusgregii

Rhamnaceae Arianoutsou & Margaris,1981

C. leudodermis Rhamnaceae Arianoutsou & Margaris,1981

C. megacarpus Rhamnaceae Hadley, 1961; Tyler, 1995

C. sanguineus Rhamnaceae Orme & Leege, 1976

C. velutinus Rhamnaceae Shearer, 1976

Cistus clusii Cistaceae Pugnaire & Lozano, 1997

C. ladanifer Cistaceae Juhren, 1966; Montgomery& Strid, 1976; Ferrandiset al., 1999a

C. monspeliensis Cistaceae Juhren, 1966

C. salvifolius Cistaceae Juhren, 1966; Ferrandiset al., 1999a

C. villosus Cistaceae Montgomery & Strid, 1976

Commersoniafraseri

Malvaceae Floyd, 1976

Convolvulusoccidentalis

Convolvulaceae Horton & Kraebel, 1955

Cytisusscoparius

Fabaceae Robertson et al., 1999

Daviesiamimosoides

Fabaceae Purdie & Slatyer, 1976;Purdie, 1977

Dillwyniaretorta

Fabaceae Purdie & Slatyer, 1976

Dodonaeatriquetra

Sapindaceae Floyd, 1966

(Continued )

172 Seeds

Depending on the amount of combustible material,

seeds on the soil surface may be exposed to a range of

temperatures: .600�C (Lonsdale and Miller, 1993),

600�C (Sweeney, 1956), 386�C (Floyd, 1966), 121,

302�C (Smith et al., 2004), 227�424�C (Mott, 1982),

156�C (Heyward, 1938) and 81�213�C (Beadle, 1940).

Temperatures simulating those that occur on the soil

surface during fires are lethal to seeds, even after short

periods of exposure (Wright, 1931; Stone and Juhren,

1951; Cushwa et al., 1968; Zabkiewicz and Gaskin, 1978;

Sharma et al., 1985; Auld, 1986b). Seeds of Acacia

aneura, Cassia nemophila and Dodonaea viscosa at 0, 1

or 2 cm in the soil were killed by slow-burning litter fires

when temperatures exceeded 80�C (Hodgkinson and

Oxley, 1990). Seed death probably can be attributed to

the fact that high temperatures were maintained for much

longer periods of time under slow- than under fast-

burning fires in this study. Fire in the habitat significantly

reduced the number of Cistus ladanifer, C. salvifolius and

Halimium ocymoides seeds m22 in soil samples from

depths of 0�2 and 2�5 cm, compared to those from a

nonburned control area (Ferrandis et al., 1999a). In the

burned area, the number of dead seeds was significantly

higher in the 0�2 than in the 2�5 cm layer of soil. Also,

many seeds of the three species germinated in the habitat

during the first post-fire spring. Thus, depletion of the

seed bank was due to seed death as well as germination.

However, whereas seeds of C. ladanifer and H. ocy-

moides germinated from depths of 0�2 and 2�5 cm,

most seeds of C. salvifolius that germinated did so from

the 0�2 cm soil layer.

Soil temperature profiles have been constructed as var-

ious types of vegetation were burned (see Sackett and

Haase, 1992 for methods). Soil is a good insulator. Thus,

with increases in depth, temperatures decline

(Figure 6.14), and the duration of high temperatures

increases (e.g., Portlock et al., 1990; Bradstock et al.,

1992; Bradstock and Auld, 1995). Further, with an


Species Family References

Galactiatenuiflora

Fabaceae Williams et al., 2004

Geraniumbicknellii

Geraniaceae Abrams & Dickmann, 1982

Halimiumocymoides

Cistaceae Ferrandis et al., 1999a

Helianthemumscoparium

Cistaceae Tyler, 1995

Indigoferahilaris

Fabaceae Martin, 1966

I. hirsuta Fabaceae Williams et al., 2004

I. stricta Fabaceae Martin, 1966

Iliamna corei Malvaceae Caljouw et al., 1994

I. longisepala Malvaceae Harrod & Halpern, 2009

I. remota Malvaceae Schwegman, 1990

I. rivularis Malvaceae Steele & Geier-Hayes, 1989

Jacquemontiacurtisii

Convolvulaceae Spier & Snyder, 1998

Kennediacoccinea

Fabaceae Shea et al., 1979

K. prostrata Fabaceae Shea et al., 1979

K. rubicunda Fabaceae Floyd, 1966

Lespedezacuneata

Fabaceae Wong et al., 2012

Lotushemistrata

Fabaceae Sweeney, 1956

L. salsuginosis Fabaceae Tyler, 1995

L. scoparius Fabaceae Hanes, 1971; Tyler, 1995

L. strigosus Fabaceae Tyler, 1995

Pomaderrisapetala

Rhamnaceae Cremer & Mount, 1965

Pultenaeaprocumbens

Fabaceae Purdie, 1977

Rhus copallina Anacardiaceae Cain & Shelton, 2003

R. glabra Anacardiaceae Cain & Shelton, 2003

R. javanica Anacardiaceae Kamada et al., 1987

R. typhina Anacardiaceae Marks, 1979

Stylosanthesspp.

Fabaceae Mott, 1982

Tephrosiacapensis

Fabaceae Martin, 1966

Thermopsismacrophylla

Fabaceae Borchert, 1989

Ulex europaeus Fabaceae Ivens, 1982

80(a)

(b)

1 cm

2

57 10

1 cm2

5

810

60

40

20

Tem

pera

ture

(˚C

)

0

120

80

40

00 20 40 60

Time (min.)80 100

FIGURE 6.14 Soil temperatures at various depths during and after a

cool (a) and hot (b) burn. From Auld (1986b), with permission.


increase in the amount of water in the soil, there is a

decrease in conduction of heat (Beadle, 1940; Portlock

et al., 1990). Some temperatures recorded at soil depths of

2�3 cm during fires are: 112�C (Beadle, 1940), 111�C(Shea et al., 1979), 110�C (Sweeney, 1956), 85�C (Floyd,

1966), 59�67�C (Beadle, 1940) and 40�85�C (Auld,

1986b). Thus, while seeds may be exposed to lethal tem-

peratures on the soil surface, they are exposed to much

lower temperatures a few centimeters beneath the surface.

During a fire in Mediterranean gorse shrublands in Spain,

temperatures of 215.3 to 409.9�C were recorded at the soil

surface, and temperatures varied from 55.9 to 229.6�C at a

depth of 1 cm, from 23 to 57.5�C at 3 cm and from 21 to

32�C at 5 cm. In this study, highest (69.2%) germination

occurred at a depth of 1 cm and lowest (7.8%) at 5 cm

(Baeza and Vallego, 2006).

Temperatures approximating those recorded at soil

depths of 2�3 cm during fires have been used to simulate

the effects of fire on breaking PY. Dry heat at temperatures

of 60 to 100�C causes seeds of many species to become per-

meable (Table 6.9). At low temperatures, duration of heat is

not as critical as the temperature per se in breaking dor-

mancy. However, at high temperatures (100, 120�C) seedsbecome sensitive to temperature duration (Auld and

O’Connell, 1991). By knowing the soil temperature profiles

expected under various kinds and loads of fuel (litter) and

temperatures required for dormancy break, it is possible to

predict germination of seeds in (and on) the soil following

fire (Auld and O’Connell, 1991).

Accuracy of predictions of germination following fire

also is enhanced by information about distribution of

seeds in the soil seed bank and maximum depth from

which seedlings can emerge (Laterra et al., 2006).

Further, there is an interaction between depth of heat pen-

etration and depth of emergence. For example, Williams

et al. (2004) measured temperatures at soil depths of 0, 3,

10 and 30 mm during fire in Eucalyptus savannas in

Queensland, Australia. During an early season fire, the

maximum temperature was 40�C (at 10 mm), but during a

late season fire the maximum temperature was 65�C (at

30 mm). The depth at which seeds of the legumes

Galactia tenuiflora and Indigofera hirsuta germinated fol-

lowing each fire was positively correlated with the depth

to which the soil was heated. Consequently, more seeds

germinated following the late- than the early-season fire

(Williams et al., 2004).

In some cases, fire has been used to promote germina-

tion of rare species whose seeds have PY. For example,

only three plants of Iliamna corei (Malvaceae) remained

in the wild, but following a prescribed fire on 7 May

1993, 492 seedlings appeared in a roughly 10 m3 15 m

plot (Caljouw et al., 1994). Following a fire in the habitat

in mid-May of the following year, 194 seedlings appeared

in a roughly 5 m3 8 m plot (Edwards, 1995).

TABLE 6.9 Examples of species with physical

dormancy whose seeds have been made permeable

with dry heat treatments, using temperatures

recorded in fires.

Species Optimum

Temp. (�C)for Loss of

Dormancy

References

Acacia mangium 60 Hopkins & Graham,1984b

A. suaveolens 60�80 Auld, 1986a

Acacia spp. 70 Floyd, 1976

Bossiaeaheterophylla

60 Auld & O’Connell, 1991

Ceanothusmegacarpus

100 Hadley, 1961

C. sanguineus 105 Gratkowski, 1973

Cistus albidus 100 Vuillemin & Bulard,1981

C. monspeliensis 100 Vuillemin & Bulard,1981

Commersoniafraseri

80 Floyd, 1976

Daviesia alata 80 Auld & O’Connell,1991

Dillwyniabrunioides


Dodonaeatriquetra

90 Floyd, 1976

Geraniumbicknellii

90 Abrams & Dickmann,1984

G. solanderi 62 Warcup, 1980

Glycineclandestina

80 Auld & O’Connell,1991

Gompholobiumglabratum


Hardenbergiaviolacea


Kennediarubicunda

80 Floyd, 1976

Mirbeliaplatyloboides


Platylobiumformosum


Pultenaeadaphnoides


P. linophylla 40 Auld & O’Connell, 1991

Rhus javanica 55 Washitani, 1988

R. lanceolata 82 Rasmussen & Wright,1988

Seringiaarborescens

80 Floyd, 1976

Sphaerolobiumvimineum


Trifoliumdubium

62 Warcup, 1980

Ulex europaeus 60�80 Zabkiewicz & Gaskin,1978

174 Seeds

Fire frequently is used to promote germination of

seeds with PY in the field; consequently, it is an impor-

tant tool in programs to help control invasive species

(Pieterse and Cairns, 1986; Lonsdale and Miller, 1993).

Although fire will kill seeds on the soil surface and in

dung, e.g., Acacia nilotica, it may promote germination

of seeds in the soil, resulting in appearance of seedlings

of the invasive species (Radford et al., 2001). For effec-

tive control of the species, these seedlings will need to be

destroyed by additional fires (Radford et al., 2001) or other

management strategies. Fire also may promote germination

of native species, such as the weedy, native shrubs Rhus

copallina and R. glabra, which can cause significant

problems in natural regeneration of Pinus spp. in the south-

eastern United States (Cain and Shelton, 2003).

Fire is used in native habitats to promote the breaking

of PY in seeds of desirable species, e.g., Acacia celastrifo-

lia, A. drummondii, A. extensa, A. lateriticola (Smith et al.,

2004) and A. pulchella (Portlock et al., 1990; Smith et al.,

2004). However, good management of natural vegetation

in which legumes and other species with PY are an impor-

tant component (and where propagation of such species is

desirable) may require that fire be suppressed until seed-

lings have enough time to reach fire-tolerant growth stages

(Auld, 1996). It should be noted, however, that fire in the

habitat did not promote germination of seeds of some spe-

cies with PY, including Acacia drepanolobium (Okello and

Young, 2001), A. nilotica, A. raddiana, A. seyal, A. sieberi-

ana, Faidherbia albida (Danthu et al., 2003), Mimosa

tenuiflora (Camargo-Ricalde and Grether, 1998) and

Prosopis caldenia (de Villalobos et al., 2002).

Exposure to dry heat at 100�C for 5 min caused the

first and second layers of the impermeable endocarp in

Rhus ovata fruits to develop many small cracks, especially

near the micropyle. However, when the first and second

layers of the endocarp were removed with a dental drill,

no cracks were found in the third layer in the region of the

micropyle (Stone and Juhren, 1951). Thus, seeds were still

impermeable to water. Dry heat caused the hilum fissure

to open in seeds of Ceanothus sanguineus (Orme and

Leege, 1976), and it caused the water-gap to open in fruits

of Rhus lanceolata and also the development of cracks

leading from the water-gap (Rasmussen and Wright

(1988). Although Rasmussen and Wright referred to the

water-gap on fruits of R. lanceolata as the hilum, this can-

not be correct because a hilum is located on a seed coat

and not on the endocarp.

Wet heat treatments sometimes are used to simulate

burning under wet conditions. Warcup (1980) subjected soil

samples collected in southeastern Australian forests to steam

at 60 or 71�C for 30 min and obtained increased germination

of species in the Geraniaceae, Fabaceae, Convolvulaceae

and Rhamnaceae. Dormancy was broken by soaking seeds

of Lespedeza cyrtobotrya in water at 70�C (Iwata, 1966) and

those of Cassia nicticans at 80�C (Cushwa et al., 1968).

Dipping seeds of Acacia decurrens (Beadle, 1940), A. exten-

sa, A. myrtifolia, A. pulchella, A. strigosa andMirbelia dila-

tata (Shea et al., 1979) into boiling water caused them to

become permeable (see Table 6.5).

Another aspect of fire at a habitat site is the removal of

living plant parts and litter; consequently, solar radiation

increases temperatures at the soil surface. Samples of dry

forest soil placed in direct sunlight in southeastern

Australia for 6 days reached a maximum of 62�C, andtemperatures were above 50�C for several hours each day.

These increased temperatures stimulated a two-fold

increase in germination of Geranium solanderi,

Dichondra repens and Trifolium dubium seeds (Warcup,

1980). Soil surface temperatures were 20 to 40�C higher

in burned than in nonburned plots in Idaho, USA, the

spring following a fire, and this probably is one reason

why seeds of Ceanothus sanguineus continued to germi-

nate long after the fire occurred (Orme and Leege, 1976).

Post-fire soil temperatures in New South Wales, Australia,

in summer were $40�C for several hours each day down

to depths of 45 mm and exceeded 60�C down to 4 mm;

however, soil temperatures did not reach 40�C in burned

sites in winter or in nonburned sites in summer (Auld and

Bradstock, 1996). Soil heating at 40�C broke dormancy in

some seeds of eight legumes, and 60�C broke it in those of

15 species (Auld and O’Connell, 1991).

Low Winter Temperatures

Two observations, especially on Melilotus alba and

Trifolium pratense, have caused researchers to be quite

interested in the role of temperate-zone winter environ-

mental conditions in breaking PY (Dunn, 1939). (1)

Seeds germinate in late winter and/or early spring before

they are exposed to the high temperature fluctuations of

summer. (2) Seeds sown in spring do not become perme-

able until the following late winter and/or spring, after

they have been subjected to winter conditions. Various

studies have attempted to determine what factor(s) of the

winter environment cause seeds to become permeable.

Alternately freezing at about 25 or 215�C and thaw-

ing seeds of Medicago sativa was only moderately (23%

germination) effective in breaking PY; controls germi-

nated to 8%. Germination increased in M. sativa seeds

after the first freeze, but subsequent freezing and thawing

increased germination very little (Midgley, 1926).

Storage of Melilotus alba seeds on moist and on dry sub-

strates at 5, 210 and 22�C failed to improve germination.

In fact, moist substrates at low temperatures caused the

few seeds that became permeable to imbibe, and they

subsequently died.

Moist treatments at 5 and 210�C were effective in

breaking PY in seeds of Vicia villosa (Dunn, 1939).


However, seedlings slowly decayed at 5�C, and partially

imbibed seeds were killed by freezing at 210�C (Dunn,

1939). Germination of four selections of Trifolium repens

increased at 20 and 30�C after 5 mo of exposure to low

(5 to 15�C) temperatures and high (60�100%) RH

(Burton, 1940). A higher percentage of Lespedeza cyrto-

botrya seeds was permeable in spring, if they overwin-

tered on the ground under snow in Japan, than if they

overwintered on the upright dead plants. However, the

rate of seed decay also increased under the snow (Iwata,

1966).

Seeds of Melilotus spp. stored dry (1) at room tem-

peratures and (2) at 85% RH at 7�C did not become per-

meable, whereas those stored dry in an open shed (in

Wisconsin or Iowa, USA) became permeable (Helgeson,

1932). In a series of experiments conducted in Iowa

(Martin, 1945), Melilotus spp. seeds kept wet or dry

at 23, 2, 10 and 15�20�C did not lose their PY. On the

other hand, seeds became permeable when sown on soil

in the field or when kept on a porch and in an open

garage in both cotton-stoppered jars and in closed jars at

a high RH. Few seeds on the soil or in the two types of

jars became permeable from November through early

March, but between 15 March and 10 April in a number

of years between 1929 and 1942 impermeability reached

72�97%. Martin (1945) thought that fluctuations of tem-

perature in the realm of freezing were responsible for

breaking dormancy. However, alternate freezing and

thawing did not promote loss of PY in seeds of Dalea

foliosa (Baskin and Baskin, 1998d), Lupinus texensis

(Davis et al., 1991), Senna marilandica, S. obtusifolia

(Nan, 1992), Desmanthus illinoensis (Latting, 1961),

Iliamna corei (Baskin and Baskin, 1997) or Sida spinosa

(Baskin and Baskin, 1984f).

Seeds of Aeschynomene virginica frozen at 210�C for

30 days germinated to 51% when incubated at 30/15�Cfor 2 wk. Seeds on wet sand at 5�C for 30 days germi-

nated to 8% when transferred to 30/15�C for 2 wk, and

those incubated continuously at 30/15�C germinated to

10% (Baskin et al., 2005). Seeds of Trifolium pratense

incubated on wet filter paper at 5�C for 8 wk germinated

to 76, 13 and 62% when tested at 10, 23 and 20/10�C,respectively, while control seeds incubated continuously

at these temperatures germinated to 20, 16 and 19%,

respectively (Van Assche et al., 2003).

Two Steps in Breaking Physical Dormancywith Low Temperatures

The breaking of PY in some seeds requires two steps: (1)

low winter temperatures to make seeds sensitive to alter-

nating temperatures, and (2) the alternating temperature

regimes of early spring to cause the sensitive seeds to

become water-permeable. (Van Assche et al., 2003) found

that seeds of Melilotus albus, Medicago lupulina, Lotus

corniculatus and Trifolium repens became sensitive dur-

ing incubation on wet filter paper at 5�C for 8 wk, and

sensitive seeds became water-permeable and germinated

when they were transferred to simulated spring

temperatures (15/6 or 20/10�C). Control seeds kept at 5,

15/6, 20/10 and 23�C and those moved from 5 to 10�C or

moved from 5 to 23�C germinated to significantly lower

percentages than those transferred from 5 to 15/6 or 20/

10�C (Van Assche et al., 2003). Thus, the cold wet condi-

tions of winter made seeds sensitive to the alternating

temperature regimes of early spring. However, if seeds

were moved from winter conditions to relatively high

constant temperatures, they lost their ability to respond to

spring alternating temperatures. In the field, buried seeds

of these four species, as well as those of Vicia cracca,

exhibited annual cycles in their ability to respond to

spring temperatures, i.e., the seeds had an annual sensitiv-

ity/nonsensitivity cycle (Van Assche et al., 2003).

Incubation on Moist Substrate

It seems reasonable that a continuously moist substrate

could be an important environmental clue for some seeds

(and fruits) with PY to become water permeable.

However, substrate moisture has not received much atten-

tion as a dormancy-breaking factor, and experiments on

this factor need to be carefully controlled to prevent tem-

perature from becoming an interacting factor.

Clearly, seeds of some species incubated on a continu-

ously moist substrate do not become permeable, at least

not very quickly. None of the water-impermeable fruits of

Stylobasium spathulatum increased in mass (i.e., did not

imbibe any water) when incubated on wet filter paper at

room temperatures for 3 yr (Baskin et al., 2006b). At the

end of 534 wk of incubation on wet sand at 12/12 hr alter-

nating temperature regimes of 15/6, 20/10, 25/15 and 30/

15�C, seeds of Sophora chrysophylla had germinated to

60 (91% viable, including seeds that had germinated and

those still viable at end of study), 75 (89), 52 (81), 61

(78) and 23 (28)%, respectively, and fruits of Rhus sand-

wicense to 11 (85), 16 (89), 28 (73), 53 (76) and 33

(45)%, respectively (Baskin et al., unpubl.).

In some species, seeds incubated on a continuously

moist substrate show a long delay, but they do eventually

become fully imbibed. Fifteen seeds of Alphitonia pon-

derosa (Rhamnaceae) were incubated on wet filter paper

at room temperature (23�C), and at 1- or 2-day intervals

for 24 days and at 7-day intervals (after 24 days) until all

seeds were fully imbibed, each seed was removed from

the wet paper, dried with a towel, weighed and immedi-

ately returned to the wet substrate. The number of days

required for each of the 15 seeds to imbibe was: 86, 114,

176 Seeds

163, 163, 163, 163, 163, 468, 488, 510, 517, 566, 573,

656 and 769. Further, when a seed began to imbibe,

$1 mo was required for it to become fully imbibed

(Baskin et al., unpubl.).

Acacia nilotica subsp. tomentosa which grows along

rivers in Sudan and is subjected to flooding in the wet

season has water-impermeable seeds. Soaking the seeds

in containers of water placed outdoors for 0 to 24 wk

resulted in an increase in imbibition (and germination)

with an increase in soaking time up to 18 wk; germination

decreased after 21 wk of soaking (Warrag and Eltigani,

2005). The authors note that 18 wk corresponds with the

average length of the natural flooding period in the natu-

ral riverine habitat of the taxon. In contrast, flooding

reduced the breaking of PY in seeds of Stylosanthes

hamata (Jones, 2002).

Burial in Soil in the Field

Compared to seeds on the soil surface, burial in soil may

promote the breaking of PY, e.g., Acacia tortilis (Loth

et al., 2005), Biserrula pelecinus (Loi et al., 1999),

Dichrostachys cinerea (Van Staden et al., 1994),

Ornithopus compressus cvv. Santorini and Charano and

accessions GEH723-1A and GRC5045-2-2 (Revell et al.,

1999; Taylor and Revell, 2002), Trifolium clypeatum

(Zeng et al., 2005b) and wild accession (3788) of Vigna

unguiculata (Lush et al., 1980), or it may not, e.g.,

Abutilon theophrasti (Webster et al., 1998), Aeschynomene

virginica (Baskin et al., 2005), Ornithopus compressus cv.

Aquila (Revell et al., 1999), Trifolium lappaceum and T.

subterraneum (Zeng et al., 2005b). The promoting effect

of burial in soil on the breaking of PY may be related to

increased soil moisture, but this has not been investigated.

Seeds of Parkinsonia aculeata with PY were buried at

depths of 0, 3 and 20 cm in the open and under artificial

cover (shade cloth), ground cover and canopy cover in the

wet-dry tropics of northern Australia in January (during the

wet season) (van Klinken et al., 2006). Dormancy break

was greater in the open than under any kind of cover, and

it was higher at 0 than at 3 or 20 cm and lowest at 20 cm.

Thus, seeds had a gap- as well as a depth-detecting mecha-

nism. In another study of dormancy break in P. aculeata,

seeds were buried at a depth of 2 cm in various kinds of

habitats throughout the range of the species in northern

Australia; controls were stored dry in the laboratory at 25

6 5�C (van Klinken et al., 2008). Seeds from the field and

laboratory were tested for germination (dormancy break)

periodically from 35 to 1281 days after start of the study.

When all data were pooled, there was a significantly

greater loss of PY when seeds were buried at wet and

warm than at relatively dry and warm sites.

On the other hand, the failure of PY to be broken

when seeds are buried may be related to temperature. In

fact, Taylor and Ewing (1996) suggest that buried seeds

with PY do not become permeable because temperatures

are not appropriate for the two stages of dormancy break

to occur. Seeds of Sophora alopecuroides were placed

at depths of 0, 2 and 7 cm in native habitat sandy soil at

two sites in Inner Mongolia, China, in January 2006. By

December 2006, a high percentage of seeds at one site

had become permeable with only about 22, 30 and 38%

impermeable seeds remaining at 0, 2 and 7 cm, respec-

tively, but at the other site about 90, 25 and 42%,

respectively, of the seeds were still impermeable (Hu

et al., 2009b).

Evidence that temperature plays an important role in

the breaking of PY of buried seeds is seen in results of

studies where seeds have been buried at different depths.

Breaking of PY decreased with depth of burial below

2 cm for seeds of Biserrula pelecinus (Loi et al., 1999),

Medicago polymorpha (Zeng et al., 2005b), Ornithopus

compressus (Loi et al., 1999), Trifolium glanduliferum,

T. lappaceum, T. spumosum (Zeng et al., 2005b) and

T. subterraneum (Taylor and Ewing, 1996; Loi et al.,

1999; Zeng et al., 2005). Significant dormancy break

occurred in seeds of Trifolium tomentosum and T. stella-

tum placed on the soil surface in Syria during summer,

but about 90% or more of the seeds buried at depths of 5

and 10 cm remained water-impermeable (Russi et al.,

1992c). Perhaps the decrease in dormancy break with an

increase in soil depth is related to a decrease in maximum

as well in the amplitude of daily temperature fluctuations.

We do not know. Also, if heat from fires is required to

break PY, seeds buried too deeply to be heated enough

for dormancy break would remain impermeable after a

fire (Brown et al., 2003a).

The breaking of PY in seeds of Abutilon theophrasti

increased with an increase in depth of burial from 1 to

10 cm in the field during winter (Cardina and Sparrow,

1997). Further, there was a steady decline in PY in seeds

of this species stored dry at 4�C (Cardina and Sparrow,

1997). Thus, the lower temperatures associated with

increased depth of burial of A. theophrasti seeds may

have promoted the breaking of dormancy.

Microbial Action

One reason frequently given for the loss of impermeabil-

ity of seeds is the action of microbes (e.g., Krefting and

Roe, 1949; Mayer and Poljakoff-Mayber, 1989; Fenner,

1985; Lonsdale et al., 1988; see Baskin and Baskin,

2000), but data to support this idea are very limited

(Baskin and Baskin, 2000). In fact, little is known about

the effects of microbes on seeds with PY. Gogue and

Emino (1979) found that nonscarified seeds of Albizia

julibrissin incubated in nonsterilized and sterilized natural

soil germinated to 30 and 11%, respectively, after 7 days.


After 30 days, seeds of A. julibrissin germinated to 1, 3,

3, and 2% in the presence of the soil fungi Fusarium sp.,

Rhizoctonia sp., Pythium sp. and in the control, respec-

tively; after 60 days seeds germinated to 10, 22, 5 and

3%, and after 90 days to 21, 38, 7 and 9%. Rhizoctonia

sp. was the most effective in breaking PY.

Photomicrographs revealed that hyphae had altered the

appearance of the seed coat, presumably due to secretion

of enzymes. Seeds of Senna angustifolia nontreated, trea-

ted with a native isolate of Azotobacter sp. and treated

with a native isolate of Azospirillum sp. germinated to 58,

85 and 77%, respectively (Lakshmanan et al., 2005).

Seeds of Vigna minima imbibed within 186 hr when

placed on blotters soaked with soil suspensions but failed

to imbibe after 30 days on blotters moistened with distilled

water (Gopinathan and Babu, 1985). These authors

hypothesized that microbes softened the seed coat of

V. minima seeds at the lens. No significant differences

were found in germination percentages of Trifolium sub-

terraneum seeds incubated for 4 and 12 mo in the presence

of fungi, root nodule bacteria or seed coat saprophytes at

various intensities and in sterile conditions (Aitken, 1939).

Seeds of Abutilon theophrasti have a characteristic

group of fungi associated with the seed coat that includes

Cladosporium cladosporioides, Alternaria alternata,

Epicoccum purpurascens and Fusarium sp. (Kremer

et al., 1984). These fungi inhibit the establishment of soil

microbes on the surface of seeds of this species placed on

soil (Kremer, 1986a). Another factor preventing establish-

ment of soil microbes is the production of antimicrobial

compounds, including phenolics, by the seed coat

(Kremer, 1986b). The fact that Kremer (1987) found bac-

teria within the subpalisade cell layer in the seed coat that

exhibit antifungal activity makes this story even more

intriguing. Thus, fungi associated with the seed coat in

some species may prevent the growth of bacteria and pro-

long the life of seeds (see Chapter 7, “Seed Death due to

Microorganisms” on p. 253).

Effects of Animals on Seeds withPhysical Dormancy

Observations of intact seeds in feces or in regurgitated

material of animals (e.g., Timmons, 1942; Jackson and

Gartlan, 1965; Lieberman et al., 1979; Gill, 1985) have

caused many biologists to ask: Does being eaten have any

subsequent effects on seed germination? There are at least

four possibilities. (1) Seeds are destroyed, i.e., masticated

and/or digested (Gardener et al. 1993a; Swank, 1944;

Smit and Rethman, 1996; Michael et al., 2006b). Only 40

and 25% of nondamaged seeds of Acacia dudgeoni that

passed through cattle and sheep, respectively, were recov-

ered, and with increased time in the gut, especially cattle,

germination decreased (Razanamandranto et al., 2004).

(2) Seeds germinate while they are in the animal’s diges-

tive tract, but the resulting seedlings die (e.g., Janzen,

1981a,b; Janzen et al., 1985; Gardener et al., 1993a).

Germination during passage through a digestive tract may

occur if seeds are infected by insect larvae. For example,

infestation of Acacia albida seeds by bruchid beetles pro-

motes germination. Thus, if infected seeds are eaten by

cattle, they germinate during passage through the animal,

and subsequently the seedlings die (Hauser, 1994).

(3) Dormancy is broken at least in a portion of the

seeds, and defecated or regurgitated seeds germinate to

higher percentages than those that have not been

ingested, e.g., Acacia constricta (Cox et al., 1993),

A. cyclops (Glyphis et al., 1981; Gill, 1985), A. erioloba

(Hoffman, et al., 1989), A. ligulata (Letnic et al., 2000),

A. nilotica (Miller, 1995), A. tortilis (Lamprey, 1967;

Miller, 1995), Adenocarpus decorticans (Robles et al.,

2005), Albizia saman (Jolaosho et al., 2006), Biserrula

pelecinus (Malo and Suarez, 1995), Cassia fistula

(Todaria and Negi, 1992), Cistus ladanifer (Malo and

Suarez, 1996; Ramos et al., 2006a), C. monospeliensis

(Ramos et al., 2006a), Dichrostachys cinerea (Van

Staden et al., 1994), Fumana ericoides, F. thymifolia

(Ramos et al., 2006a), Halimium umbellatum (Manzano

et al., 2005), Helianthemum apenninum, H. violaceum

(Ramos et al., 2006a), Medicago sativa (Swank, 1944),

Ornithopus compressus (Peco et al., 2006), Prosopis chi-

lensis (Winer, 1983; Campos et al., 2008), P. juliflora

(Shiferaw et al., 2004), P. pallida (Lynes and Campbell,

2000), Retama sphaerocarpos (Robles et al., 2005), Rhus

glabra (Krefting and Roe, 1949), R. hirta (Swank, 1944),

R. trilobata (Auger et al., 2002), Trifolium campestre

(Russi et al., 1992a), T. pratense, T. sativa, (Swank,

1944), T. stellatum, T. tomentosum (Russi et al., 1992a)

and Robinia pseudoacacia (Swank, 1944). Seeds of

Acacia ligulata collected from feces of birds (probably

spiny-cheeked honeyeaters) in Queensland, Australia,

germinated to higher percentages than fresh seeds col-

lected from the trees. Further, a dry high temperature

treatment (100�C for 30 min) was very effective in

breaking PY of seeds from the trees but had little effect

on breaking PY of seeds from bird scats. However, scari-

fication (rubbing with sandpaper and removal of aril if

present) was highly effective in breaking PY of seeds

from bird scats but not those from the trees (Letnic

et al., 2000). Copraphagy by chimpanzees means they

extract and eat seeds from their own feces. This habit

results in seeds, e.g., Dalium spp., being made permeable

the first time and then being destroyed the second time

they pass through the gut (Krief et al., 2004).

(4) Dormancy is not broken, and defecated or regurgi-

tated seeds germinate to the same percentages as control

seeds, e.g., Acacia erioloba (Barnes, 2001), Burkea

178 Seeds

africana (Razanamandranto et al., 2004), Caesalpinia

paraguariensis (Ortega-Baes et al., 2001), Gleditsia tri-

acanthos (Speroni and de Viana, 2000), Prosopis africanus

(Razanamandranto et al., 2004), P. ferox (Ortega-Baes

et al., 2002), P. laevigata (Sanchez de la Vega and

Godinez-Alvarez, 2010), P. torquata (Campos et al., 2008)

and Sphaeralcea coccinea (Gokbulak, 2002). In some

cases, defecated seeds germinate to the same or to lower

percentages than those in the nondefecated controls

(Ramos et al., 2006b). In some species, a large number of

seeds passes through the gut of an animal virtually

unharmed. For example, in one pile of elephant dung in

Zimbabwe, Dudley (1999) found 5,690 seeds of Acacia

erioloba. Various birds and small mammals were observed

eating the bonanza of seeds in the elephant dung (Dudley,

1999). However, the effect of the secondary consumers on

the seeds is not known. In other species, only a small per-

centage of the seeds with PY pass through the gut

unharmed (alive) and have the potential to germinate

(Michael et al., 2006b).

Usually seeds with PY are fed to only one kind of

animal, and the assumption is made that the effect of

having passed through the digestive system on germina-

tion would be the same regardless of the species of ani-

mal. However, in the case of Prosopis flexuosa the

effect of a digestive system on seed germination varied

with the animal: cattle and rodents increased germina-

tion; horses showed some increase in germination, but

not as much as cattle and rodents; foxes had no effect on

germination; and European wild boar destroyed many of

the seeds and thus decreased germination (Campos and

Ojeda, 1997).

The way in which a digestive system breaks PY is

unknown (Cavanagh, 1980), but it is assumed to be via

acid (Lamprey et al., 1974) and/or mechanical (Cavanagh,

1980) scarification. Germination percentages increased

from 20 to 31% when time of retention of Trifolium cam-

pestre seeds in the digestive system of sheep increased

from 24 to 48 hr, but they decreased from 50 to 46% for

T. stellatum and from 25 to 14% for T. tomentosum (Russi

et al., 1992a). Even prolonged exposure to the acidic con-

tents of a digestive system may not result in loss of imper-

meability of 100% of the seeds. For example, Leucaena

leucocephala, Stylosanthes scabra and Macroptilium

atropurpureum seeds germinated to about 90, 45 and 15%,

respectively, after 240 hr in the rumen of cattle (Gardener

et al., 1993b). Malo and Suarez (1995) examined seeds of

Biserrula pelecinus under a microscope and found no

scratches on the seed coats after seeds had passed through

the digestive system of cattle, indicating that mechanical

scarification had not occurred. However, the number of

seeds of Trifolium campestre recovered from sheep dung

was higher than that of T. stellatum (Russi et al., 1992a).

Since seeds of T. campestre are smaller than those of

T. stellatum, Russi et al. (1992a) reasoned that there was a

higher probability of large seeds being destroyed by masti-

cation and rumination than small ones. Water-permeable

seeds of Dichrostachys cinerea recovered from scats of

nyala antelope had a raised lens, indicating that the water-

gap was open, as well as small cracks on the seed coat

(Kelly et al., 1992).

In evaluating germination data of seeds with PY sepa-

rated from fecal or regurgitated material, attention should

be paid to the amount of time between deposition of the

material and retrieval of the seeds. Fermentation of fecal

material could increase the temperature; consequently,

seeds would receive a wet-heat treatment after they are

deposited. Also, waste materials may be dropped on the

soil surface in open areas where seeds would be exposed

to the daily temperature fluctuations that occur in the hab-

itat. These daily temperature changes may be high enough

to break PY. Gill (1985) suggested that the large fluctua-

tion and high temperatures experienced by seeds of

Acacia cyclops deposited in open sunny sites contribute

to the breaking of PY.

There is an interaction between the seeds of some

Acacia spp., bruchid beetles that lay their eggs on the

seeds and mammals and birds that eat the seeds. Gazelles

feed on the indehiscent pods of Acacia gerrardii,

A. raddiana and A. tortilis in the Negev Desert of Israel

and serve as a dispersal agent for the seeds. Further, seeds

germinate better after they have passed through the ani-

mal’s digestive system than they do if sown on the soil,

and seeds infested with larvae of a bruchid beetle germi-

nate to slightly higher percentages than noninfected ones.

Thus, both the bruchid beetle larvae and the gazelles

enhance germination, but a high percentage of the seeds

is lost to each predator (Halevy, 1974). In the Namib

Desert of southern Africa, a similar relationship exists

between seeds of A. erioloba, a bruchid beetle and mam-

malian herbivores. Freshly matured seeds eaten by an ani-

mal have a greater chance of germinating before the

bruchid beetle larva eats enough of the embryo to kill it,

than seeds not eaten for several weeks or months after

maturation (Hoffman et al., 1989). Seeds of A. tortilis in

central Africa (Tanzania) are eaten by elephants, impalas,

dikdiks and gazelles, and passage through the digestive

system improves germination. Therefore, seeds containing

a bruchid beetle larva are more likely to germinate before

the parasite destroys the embryo if seeds are eaten by a

mammal than if they are not eaten (Lamprey et al., 1974;

Pellew and Southgate, 1984). Seeds of the swollen-thorn

species of Acacia in Central America are eaten and dis-

persed by birds. Seeds containing a developing larva of a

bruchid beetle are broken apart by the bird’s digestive

system, while noninfected ones are unlikely to be

destroyed during passage through the digestive system.

However, seeds removed by birds immediately after


maturation may be dispersed before they are predated by

the beetles (Janzen, 1969b).

Although animals may serve as effective dispersal

agents of seeds and passage through their digestive sys-

tems may overcome PY, we should not lose sight of the

fact that PY can be (and probably usually is) broken with-

out the aid of animals. That is, if seeds are not eaten by

animals environmental factors will result in the loss of

PY. Thus, just because seeds are eaten by an animal does

not mean that they are dependent on the animal for the

breaking of PY. For example, consider the case of

Enterolobium cylocarpum (Mimosoideae). This Central

American tree has received much attention because its

fruits apparently were eaten (and its seeds dispersed) by

large herbivores in the early part of the Pleistocene.

However, these animals became extinct about 10,000

years ago (Janzen and Martin, 1982), and today intro-

duced cattle and horses eat the fruits and disperse the

seeds (Janzen, 1981c). The species is not dependent on

the herbivores for germination because seeds will germi-

nate in response to dry heat treatments of 45�50�C(Hunter, 1989), temperatures that frequently occur at the

soil surface in the tropics.

Seeds of Acacia tortilis placed on the soil surface in

full sun in a savanna in Tanzania failed to germinate.

However, both seeds covered by elephant dung and

those buried to a depth of 1 cm in soil at the same site

germinated to about 38%. Under the grass canopy,

seeds on the surface, covered by dung and buried 1 cm

deep germinated to 0, 23 and 43%, respectively, and

under a tree canopy to 4, 10 and 23%, respectively (Loth

et al., 2005).

Feeding activities of animals, especially insects, may

result in seeds with PY being destroyed, but in some

cases the seed only has a hole cut in it, i.e., it effectively

has been mechanically scarified. Seed bugs and weevils

scarified some Gossypium thurberi seeds, and they germi-

nated to higher percentages than those that were not

attacked (Karban and Lowenberg, 1992). About 60% of

Astragalus australis var. olympicus seeds were predated

by the weevil Tychius sp., and about 11% of the damaged

seeds were viable and permeable (Kaye, 1999a). Seeds of

Lotus corniculatus also were scarified by a weevil seed

predator; however, even when as much as one-third of the

cotyledon tissue was eaten about 50% of the seeds germi-

nated and 50�70% of the resulting seedlings survived

(Ollerton and Lack, 1996). Seeds of some Acacia species

that have been infected with larvae of bruchid beetles can

germinate if they are imbibed before the larvae destroy

the embryo (Or and Ward, 2003; Walters and Milton,

2003). Some seeds of Ulex europaeus predated by the

seed weevil Exapion ulicis (Sixtus et al., 2003b) and

seeds of Vicia sativa predated by larvae of the tortricid

moths Cydia lunulana and C. nigricana (Koptur, 1998)

were viable and permeable. Seeds of Acacia sieberiana

with bruchid exit holes in them sown in soil germinated

to 17%, whereas none of the control seeds germinated

(Mucunguzi, 1995).

Larvae of the tortricid moths Cydia lunulana and

C. nigricana scarified many seeds of Vicia sativa, and

seeds with ,10 to 10�25% damage (5volume of seed

missing) germinated faster than nondamaged seeds that

were nicked with a razor blade; nonscarified seeds did

not germinate (Koptur, 1998). Nymphs and adults of the

bug Hyalymenus tarsatus broke PY in seeds of Sesbania

drummondii, resulting in imbibition and germination of

the seeds (Ceballos et al., 2002). In laboratory experi-

ments, seeds of Gleditsia japonica were made water-

permeable by the bean weevil (Bruchidius dorsalis)

(Takakura, 2002). Seeds of A. tortilis and A. nilotica

chewed by the rodent Mastomys natalensis germinated to

higher percentages than those in the controls (Miller,

1995). In feeding experiments, the wild mouse

(Apodemus speciosus) refused to eat (or chew) seeds of

Gleditsia japonica (Takakura, 2002).

LONG-TERM STORAGE OFPERMEABLE SEEDS

After seeds with PY have been made permeable, how

long will they remain viable in dry storage? From a habi-

tat or species restoration perspective being able to make

seeds permeable and then store them dry for a period of

time before they are sown is an advantage. Dry seeds

flow easily through planting equipment, and they will not

imbibe and germinate until after it rains, thus helping to

ensure survival of the seedlings.

The absolute period of time that scarified seeds will

remain viable in dry storage has not been determined;

however, evidence from various species suggests that we

are talking about many months, and perhaps many years.

Different seed lots of Lathyrus maritimus made perme-

able by acid scarification showed no decrease in germi-

nation following 37, 180 or 225 days of dry storage at

room temperatures (Lemmon et al., 1943). Seeds of

Acacia farnesiana and Prosopis cineraria made perme-

able with a seed gun or by burning with a “glow burner”

showed no loss of viability during 2 mo of storage at

30�C and 80% RH or at 0�4�C in a closed container

(Lauridsen and Stubsgaard, 1987). Seeds of Leucaena

leucocephala made permeable either by mechanical or

acid scarification germinated to 49 and 78%, respec-

tively, immediately after treatment, and after 15 mo dry

storage at room temperature they germinated to 82 and

70%, respectively (Omokanye et al., 1995). Seeds of

Lupinus havardii and L. texensis made permeable by

acid scarification were stored at 75, 52, 23 and 11% RH

180 Seeds

at both 4 and 22�C. Viability of L. havardii seeds stored

at 75% RH at 22�C had declined significantly after

4 mo, and had that of L. texensis seeds stored at 52%

RH at 22�C declined significantly after 6 mo. At the

other combinations of storage conditions, viability had

not declined significantly after 12 mo (Mackay, 2005).

Seeds of Sophora moorcroftiana made permeable by

alternately dipping seeds in hot (90�C) and cold (0�C)water, germinated to 79% after 69 wk of dry storage at

room temperatures (Baskin et al., 2007a).

SEEDS WITH PHYSICAL ANDPHYSIOLOGICAL DORMANCY

A number of species occurring in various plant families

have seeds with a combination of physical and physiolog-

ical dormancy (Table 6.10). Breaking of PY under natural

conditions already has been discussed; thus, emphasis

now will be placed on how physiological dormancy of

the embryo is broken. Species with combinational

dormancy can be divided into two categories, depending

on the conditions required to break PD: (1) afterripening

at high temperature, and (2) dormancy break by cold

stratification.

Winter annuals, including Geranium carolinianum,

G. cicutarium, G. columbinum, G. dissectum, G. lucid-

dum, G. molle, G. pusillum, Lathyrus aphaca, Medicago

arabica, Ornithopus compressus, Stylosanthes spp. and

Trifolium subterraneum, as well as some perennials and

shrubs, such as Dichrostachys cinerea, Diplopeltis

heugelii, Geranium pratense, Malva neglecta and Parkia

pendula, have seeds in which PD is broken by afterripen-

ing at high temperatures. Further, PD generally is broken

before seeds become water permeable. Thus, the way to

monitor loss of PD is to scarify seeds at intervals during

the afterripening period and test them for germination.

PD usually is broken within 1�4 mo following seed mat-

uration (Barrett-Lennard and Gladstones, 1964; Baskin

and Baskin, 1974b; Gardener, 1975; Chauhan et al.,

2006a; Van Assche and Vandelook, 2006). In the field,

PD is broken during summer, and seeds germinate in

autumn, when soil moisture becomes nonlimiting, assum-

ing seeds become water-permeable during summer and/or

autumn.

PD was broken in seeds of Ornithopus compressus at

60/16 but not at 20�C (Barrett-Lennard and Gladstones,

1964) and in those of Trifolium subterraneum at 40,

40/15, 60/15 but not at 15�C (Quinlivan and Nicol, 1971).

Afterripening occurred in water-impermeable seeds of

Diplopeltis huegelii during 13 mo of dry storage at room

temperatures; however, it occurred in 6 wk for seeds

dipped in hot (88�92�C) water for 5 min and then stored

at 13, 23 or 50% RH at 23�C (Turner et al., 2006a).

TABLE 6.10 Species whose seeds have impermeable

seed coats (physical dormancy) and physiologically

dormant embryos and the temperature treatment

required to break physiological dormancy. C5 cold

stratification, W5dry at summer temperatures,

PD5physiological dormancy.

Species Family Treatment

Required

to Break

PD

References

Alysicarpusvaginalis

Fabaceae W Singer &Pitman, 1988

Ceanothussanguineus

Rhamnaceae C Gratkowski,1973

Ceanothusspp.

Rhamnaceae C Quick & Quick,1961

Cerciscanadensis

Fabaceae C Afanasiev, 1944;Geneve, 1991

C. siliquastrum Fabaceae C Profumo et al.,1978; Gebre& Karam,2004; Pipiniset al., 2011

Cotinuscoggygria

Anacardiaceae C Olmez et al.,2008; Guner& Tilki,2009; Denget al., 2010

Cuscutaepithymum

Convolvulaceae C Meulebroucket al., 2008

Cyclocarpapaliurus

Fabaceae C Fang et al.,2006


Fabaceae W Bell & VanStaden, 1993

Diplopeltishuegelii

Rhamnaceae W Turner et al.,2006a

Discariatoumatou

Rhamnaceae C Keogh &Bannister,1994

Erodiumcicutarium

Geraniaceae W Van Assche &Vandelook,2006


Geraniaceae W Baskin &Baskin, 1974b

G. dissectum Geraniaceae W Van Assche &Vandelook,2006

G. lucidum Geraniaceae W Van Assche &Vandelook,2006

G. molle Geraniaceae W Van Assche &Vandelook,2006

G. pratense Geraniaceae W Van Assche &Vandelook,2006

G. pusillum Geraniaceae W Van Assche &Vandelook,2006

(Continued )


Like seeds of winter annuals that have permeable

seed coats, those with PY germinate to higher percen-

tages at low (5, 10, 15, 15/6) than at high (20/25, 20/10,

25/15�C) temperatures (Nakamura, 1962; Baskin and

Baskin, 1974b), if they are made permeable in the early

stages of the loss of PD. With a decrease in PD, scari-

fied seeds exhibit an increase in germination rate

(Ballard, 1958; Van Assche and Vandelook, 2010) and

maximum temperature (Baskin and Baskin, 1974b) of

germination.

Seeds of the winter annual Trifolium subterraneum

made permeable shortly after they matured germinated

to 8.5% and at a slow rate (Ballard, 1958). However,

germination percentage increased greatly when imbibed

seeds with dormant embryos were exposed to increased

CO2 concentration (0.3 to about 5%) (Ballard, 1958).

The implication of these results is that embryo dormancy

could be lost more quickly in permeable seeds that

become buried than in those remaining on the soil sur-

face. This is assuming that CO2 levels are a little higher

in the soil than they are in the air above the soil. Low

concentrations of O2 (0.1�1.0%) also break embryo dor-

mancy in seeds of T. subterraneum (Ballard and Grant

Lipp, 1969), but it is doubtful that O2 would decline to

these levels unless the soil is flooded. Under laboratory

conditions, PD in permeable seeds of Stylosanthes humi-

lis has been overcome by cadmium, copper and

zinc ions (Delatorre and Barros, 1996), ethylene

(Ribeiro and Barros, 2006; Barros and Ribeiro, 2006),

ethrel, benzyladenine, thiourea (Burin et al., 1987;

Vieira and Barros, 1994) and selenium compounds

(Pinheiro et al., 2008a,b).



Required

to Break

PD

References

G. robertianum Geraniaceae W Van Assche &Vandelook,2006

G. sylvaticum Geraniaceae W? Kallio &Piiroinen,1959

Koelreuteriapaniculata

Sapindaceae C Garner, 1979;Rehman &Park, 2000a,b

Lathyrusaphaca

Fabaceae W Van Assche &Vandelook,2010

L. nissolia Fabaceae W Van Assche &Vandelook,2010

Malvaneglecta

Malvaceae W Van Assche &Vandelook,2006

M. parviflora Malvaceae W Sumner &Cobb, 1967;Chauhanet al., 2006b;Michaelet al., 2006a

Medicagoarabica


M. truncatula Fabaceae W Bolingue et al.,2010


Fabaceae W Barrett-Lennard& Gladstones,1964

Parkia pendula Fabaceae W? Rizzini, 1977

Rhusaromatica

Anacardiaceae C Heit, 1967b; Liet al., 1999b

R. trilobata Anacardiaceae C Heit, 1967b

Sicyosangulatus

Curcurbitaceae W Qu et al., 2012

S. deppei Curcurbitaceae W Brechu-Francoet al., 1992

S. lanceoloidea Curcubitaceae W Baskin et al.,unpubl.

Stylosanthesspp.

Fabaceae W McKeon &Mott, 1984

Thermopsislupinoides

Fabaceae C Kholina et al.,1999

Tiliaamericana

Malvaceae C Barton, 1934

T. cordata Malvaceae C Heit, 1967b

T. europea Malvaceae C Heit, 1967b

T. japonica Malvaceae C Heit, 1967b

T. platyphyllos Malvaceae C Nagy & Szalai,1973

(Continued )



Required

to Break

PD

References

T. tomentosa Malvaceae C Heit, 1967b

Trifoliumdubium


T. subterraneum Fabaceae W Quinlivan &Nicol, 1971

Vicia hirsuta Fabaceae W Van Assche &Vandelook,2010

V. sativa Fabaceae W Van Assche &Vandelook,2010

182 Seeds

The presence of embryo dormancy possibly could pre-

vent germination of any seeds that became permeable

during early to midsummer. Seedlings from seeds that

germinate after a summer rain are likely to die due to

drought stress when the soil dries out again (Baskin and

Baskin, 1971g). However, McKeon and Mott (1984)

point out that embryo dormancy in Stylosanthes spp.

never has been shown to inhibit germination in the field,

because most seeds are impermeable while the embryo is

dormant. Embryo dormancy is lost by the time Trifolium

subterraneum seeds become permeable (Quinlivan and

Nicol, 1971). Gardener (1975) thought embryo dormancy

would prevent germination of Stylosanthes spp. seeds in

late spring before they dry to about 8�10% moisture

content, at which point the seed coats become imperme-

able to water.

Wetting (by amounts of precipitation too low to allow

germination) followed by soil drying can influence germi-

nation after PY is broken. For example, alternate wetting

(imbibed) and drying of permeable seeds of Stylosanthes

humilis increased final germination from 46 to 70%

(McKeon, 1984). Also, it should be noted that for seeds

of three species each of Medicago and Trifolium, stored

in the field during summer in Syria, Russi et al. (1992b)

concluded that thickness of the seed coat was correlated

to loss of PY. That is, seeds with relatively thin seed

coats were more likely to become permeable during sum-

mer than those with thick coats.

Seeds of Geranium carolinianum have PY1 PD, and

PD is broken during summer, when the seeds are water

impermeable. Seeds become water permeable in two

temperature-dependent steps (Figure 6.15). Freshly

matured dormant seeds of this species incubated at

$20�C under either wet or dry conditions became highly

sensitive after 1 to 4 mo, depending on the storage tem-

perature. After seeds became sensitive, exposure to

#20�C caused the water-gap to open, and seeds became

water-permeable. In contrast to seeds of G. carolinianum,

those of G. dissectum became permeable at $20�C if

kept continuously dry (i.e., only one step for dormancy

break). However, seeds stored wet became permeable

only after they had received a period of drying at $20�C,thus exhibiting two-step dormancy break (Gama-

Arachchige et al., 2012).

In a quantitative analysis of the thermal requirements

for stepwise breaking of PY in seeds of G. carolinianum,

FIGURE 6.15 Conceptual model for breaking of seed dormancy of the winter annual Geranium carolinianum in nature. From Gama-Arachchige

et al. (2012), with permission


Gama-Arachchige et al. (2013a) found that step I was a

chemical process as indicated by Q10 values of 2.0 to 3.5,

whereas step II was a physical process as indicated by

Q10 values of 0.02 to 0.1. The base temperature (Tb) for

sensitivity induction was 17.2�C and was constant for all

fractions of the seed population. When seeds that have

been made sensitive (step I) are exposed to a temperature

lower than the sensitivity-induction temperature they

become water permeable (Figure 6.16). Exposure of

sensitive seeds to a low temperature causes a temperature

difference to occur across the seed coat, and the

palisade and subpalisade layers shrink differentially

(Figure 6.16A), thereby causing a small gap to form

between them (Figure 6.16B). Then, (1) palisade cells

imbibe water, which causes them to swell, resulting in

formation of a blister, (2) the hinged valve opens and

(3) the valve cover is dislodged, thereby fully opening the

water-gap (step II is completed) (Figure 6.16C�F).

In some species with both PY and PD, seeds must be

imbibed and given a cold stratification treatment to break

PD. Length of the cold stratification treatment required to

break PD varies with the species: Cotinus coggygria, 2 mo

(Olmez et al., 2008; Deng et al., 2010) and 4 mo (Guner

and Tilki, 2009); Ceanothus spp., 3 mo (Quick, 1935;

Gratkowski, 1973); Cercis siliquastrum, 4 mo (Gebre and

Karam, 2004), 3 mo (Pipinis et al., 2011); Cuscuta epithy-

mum, 2 mo (Meulebrouck et al., 2008); Koelreuteria

paniculata, 3 mo (Garner, 1979); Thermopsis lupinoides,

8�10 days (Kholina et al., 1999); Tilia spp., 3 mo (Barton,

1934; Heit, 1967b; Nagy and Szalai, 1973); and Rhus

aromatica, 21 days (Li et al., 1999b). Freshly matured,

scarified seeds of Cotinus coggygria var. cinerea germi-

nated to 13, 34 and 41% in 0, 0.1 and 1.0 mM GA3, respec-

tively, but after 16 mo of dry storage scarified seeds

germinated to 37, 79 and 69%, respectively (Deng et al.,

2010). Cold stratification of fresh scarified seeds for 15, 30,

FIGURE 6.16 Model of step II in the breaking of PY in seeds of Geranium carolinianum. Diagrams show median longitudinal sections of seeds in

the region of the water-gap (hinged valve). (a) Sensitive seed is exposed to low temperature, which causes differential shrinking of the palisade (PaL

and Wpa) and subpalisade (SpaL) layers. (b) A gap forms between the palisade and subpalisade layers. (c) Cells of the seed coat imbibe water causing

them to swell. (d) A blister forms in the water-gap region. (e) The hinged valve breaks loose at the end away from the micropyle. (f) The hinged valve

is dislodged, thereby fully opening the water-gap. Cr, cracks on the palisade layer; Hv, hinged-valve (composed of PaL and Wpa); Mi, micropyle; Pa,

palisade cells; PaL, elongated palisade cells of the water-gap; SpaL, elongated subpalisade cells of the water gap; Wpa, water-gap palisade cells; Wg,

water-gap opening; *, subpalisade cells with a smooth outer periclinal cell wall; and **, subpalisade cells with a corrugated outer periclinal cell wall.

From Gama-Arachchige et al. (2013a), with permission.

184 Seeds

45, 60 and 75 days followed by incubation at 5, 10, 15, 20,

25, 30 and 35�C resulted in a maximum of 57% germina-

tion, after 45 days cold stratification and incubation at 10�C(Deng et al., 2010). In Cercis canadensis, growth potential

of the embryo increased, and penetration resistance of the

testa decreased during cold stratification (Geneve, 1991).

Since seeds must be imbibed before embryo dor-

mancy can be broken by low (0�10�C) temperatures

during winter, seeds sown or dispersed in autumn may

not germinate until at least the second spring after mat-

uration (Heit, 1967b). One reason for such a long delay

in germination of some species is that appropriate envi-

ronmental conditions to overcome PY do not occur in

the habitat until the following summer. If seed coats

become permeable in response to fluctuating tempera-

tures during summer, seeds can be cold-stratified dur-

ing winter and germinate the following spring.

Obviously, one could shorten the time required for

germination by acid-scarifying the seeds and then cold

stratifying them (Barton, 1934; Afanasiev, 1944; Heit,

1967b). Hot water treatments or mechanical scarifica-

tion prior to cold stratification also are effective

(Quick, 1935; Afanasiev, 1944; Garner, 1979).

Scarified seeds of Cercis siliquastrum germinated to

48% when treated with 1.4 mM GA3 (Gebre and

Karam, 2004). Ertekin (2010) obtained 91% germina-

tion of permeable C. siliquastrum seeds by soaking

them in 500 mg/l polystimulin growth regulators (PS-

A6-PS-K) for 24 hr and then cold stratifying them for

100 days. While all species of some genera such as

Tilia and Cercis appear to have PY1 PD, only some of

the species in other genera have PY1 PD. Some Rhus

spp. have only PY, but others have PY1 PD (Heit,

1967b; Li et al., 1999b). Ceanothus spp. from the high

montane zone in California have PY1 PD, while most

maritime species have only PY (Quick, 1935).


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