independent and combined effects of soil warming and drought stress during anthesis on seed set and...

DROUGHT STRESS

Independent and Combined Effects of Soil Warming andDrought Stress During Anthesis on Seed Set andGrain Yield in Two Spring Wheat VarietiesD. F. Weldearegay1,*, F. Yan2,*, D. Jiang3 & F. Liu1

1 Department of Agriculture and Ecology, Faculty of Sciences, University of Copenhagen, Taastrup, Denmark

2 Jilin University, College of Plant Science, Changchun, China

3 Key Laboratory of Crop Physiology and Ecology in Southern China, Ministry of Agriculture/Hi-Tech Key Laboratory of Information Agriculture of

Jiangsu Province, Nanjing Agricultural University, Nanjing, China

Introduction

At the end of this century, the mean global atmospheric

temperature is predicted to rise by 2.9 �C (IPCC 2007).

One of the attributes of climate warming is irregular

increase in mean and variance of temperature at the soil

surface (Jones and Mann 2004). In addition, global

warming will not only affect air and soil temperatures but

also influence the amount and distribution of precipita-

tion leading to more frequent drought spells in the sum-

mer in many European countries including Denmark

(Christensen and Christensen 2007). Therefore, it is cru-

cial to understand how crops like spring wheat respond

to climate change and to explore whether there are varie-

ties that can better adapt to soil warming and drought

stress during their critical period of development.

To date, numerous studies have been carried out evalu-

ating the impacts of increased air temperature on wheat

Keywords

abscisic acid; anthesis; drought; grain yield;

seed set; soil warming; Triticum aestivum

Correspondence

F. Liu

Department of Agriculture and Ecology

Faculty of Sciences

University of Copenhagen

Højbakkegaard Alle 13

DK-2630 Taastrup, Denmark

Tel.: +45 353 33416

Fax: +45 353 33478

Email: [email protected]

*These authors contributed equally to the

research work.

Accepted February 24, 2012

doi:10.1111/j.1439-037X.2012.00507.x

Abstract

Increase in soil temperature together with decrease in soil moisture during

anthesis of spring wheat (Triticum aestivum L) crops is predicted to occur

more frequently in a future climate in Denmark. The objective of this study

was to investigate the responses of two Danish spring wheat varieties (Trappe

and Alora) to soil warming (H), drought (D) and both (HD) during anthesis.

The plants were grown in pots in a climate-controlled glasshouse. In H, the soil

temperature was increased by 3 �C compared with the control (C). In both D

and HD treatments, the plants were drought-stressed by withholding irrigation

until all of the transpirable soil water had been depleted in the pots. Results

showed that, particularly under D treatment, Alora depleted soil water faster

than Trappe. In both varieties, flag leaf relative water content (RWC) was sig-

nificantly lowered, while spikelet abscisic acid (ABA) concentration was signifi-

cantly increased by D and HD treatments. Compared with the C plants, D and

HD treatments significantly reduced ear number, ear to tiller ratio, shoot bio-

mass, grain yield, harvest index and seed set but hardly affected tiller number

and 1000-kernel weight, whereas H treatment alone only decreased shoot bio-

mass and reduced seed set. When analysed across the varieties and the treat-

ments, it was found that the reduction in seed set was closely correlated with

the increase in spikelet ABA concentration, indicating that D and HD treat-

ments induced greater spikelet ABA concentrations might have caused seed

abortion. It was concluded that the grain yield reduction under D and HD

treatments during anthesis in spring wheat is ascribed mainly to a lowered seed

set and wheat varieties (i.e. Alora) with more dramatic increase in spikelet

ABA concentration are more susceptible to D and HD treatment.

J. Agronomy & Crop Science (2012) ISSN 0931-2250

ª 2012 Blackwell Verlag GmbH 1

crop development and yield (e.g. Leaky et al. 2009, Wang

et al. 2011), while very little is known about how soil

warming may affect wheat in a temperate climate. Gavito

et al. (2001) reported that soil temperature had significant

effects on root development, biomass and nutrient uptake

of winter wheat during vegetative growth. However,

Hartley et al. (2007) found that soil warming remarkably

increased soil respiration but had no effects on plant

development and biomass accumulation for wheat. More

recently, Patil et al. (2010a,b) reported that soil warming

by 5 �C during the whole growing season of winter wheat

speeded up crop vegetative growth. In addition, these

authors also found that soil warming did not reduce grain

yield but reduced soil N losses and increased mildew

attack on the crop. Climate change scenarios predict that

more frequent heat waves may occur from late spring to

early summer when the wheat crop enters the reproduc-

tive stages, for example, the anthesis stage. At this stage,

the plants are very sensitive to any environmental pertur-

bations and a few days of soil warming may have signifi-

cant effects on crop growth and yield. However, until

now this aspect has not been investigated.

Drought is one of the most important factors limiting

crop productivity. Crop plants are very sensitive to

drought stress particularly during the early reproductive

stage (Saini and Westgate 2000). Severe drought during

anthesis could decrease grain yield up to 50 % by reduc-

ing the number of grains (Saini and Westgate 2000). Sev-

eral physiological mechanisms may explain the increased

seed abortion caused by anthesis drought (Liu et al.

2005). It has been suggested that phytohormones are

involved in the regulation of crop reproductive develop-

ment under drought stress. For instance, abscisic acid

(ABA) concentration in crop reproductive structures

increases significantly when the plants are drought-

stressed during flowering (Morgan 1980, Morgan and

King 1984). In wheat, seed set is negatively correlated

with the endogenous ABA concentration under drought

(Westgate et al. 1996); and application of ABA to the leaf

sheath of well-watered plants inhibits floret development,

decreases the number of fertile florets and grain set (Mor-

gan 1980, Waters et al. 1984). Similarly, Liu et al. (2003)

showed that reduction of pod set in soybean is associated

with increasing ABA concentration in flowers and young

pods. Based on these findings, it is possible that crop

varieties which have lower ABA concentration in the

reproductive organs under anthesis drought may perform

better in achieving greater grain yield than those with

higher ABA concentrations. However, to date this possi-

bility has not been explored. Nevertheless, it should be

noted that increases of ABA concentration in the xylem

sap can also act as an earlier signal inducing stomatal clo-

sure and reduction of leaf expansion growth during soil

drying, which reduces transpiration water loss thereby

preventing dehydration of the shoots and enhancing the

chance for survival under prolonged drought (Liu et al.

2005). It has been reported that exogenous ABA applica-

tion increases pod set in soybean by maintaining a

favourite leaf water status and high photosynthetic rates

under anthesis drought stress (Liu et al. 2004), indicating

a protective role of the hormone in sustaining crop yield

under stressful conditions (Liu et al. 2005).

Impact of abiotic stress factors on crop plants has been

mostly studied by applying a single stress factor such as

drought or high temperature in controlled experiments

(Guilioni et al. 2003). However, field crops often encoun-

ter multiple stresses such as drought and elevated soil

temperature simultaneously during a certain stage of

development (Zhang et al. 2008). In fact, the combination

of drought and high temperature is the major stressful

factor that restricts wheat growth and production in

many regions (Khan et al. 2009). Earlier studies have

demonstrated that combined drought and air heating sig-

nificantly decreased crop growth and yield (Savin and

Nicolas 1996, Wang and Huang 2004), whereas the effects

of combined drought stress and soil warming on crop

yield remain unknown. In this study, the plant water sta-

tus, spikelet ABA concentration and yield responses of

two Danish spring wheat varieties to soil warming,

drought and both at anthesis were investigated. We

hypothesized that the two wheat varieties perform differ-

ently in response to single or combined soil warming and

drought treatments at anthesis stage; and the phytohor-

mone ABA level in the plant plays an important role in

determining seed set and grain yield formation of spring

wheat varieties grown under multiple stress.

Materials and Methods

Experimental set-up

A pot experiment in a glasshouse was conducted at the

experimental station of the Faculty of Life Sciences, Uni-

versity of Copenhagen, Taastrup, Denmark in 2010–2011.

The pots had a volume of four litters (25 cm in height

and 15.2 cm in diameter) and were filled with 2.4-kg peat

material (Sphagnum, 32 % organic matter, pH = 5.6–6.4

and EC = 0.45 ms cm)1). In the glasshouse, air tempera-

ture and relative air humidity were 16/12 �C (day/night)

and 62 %, respectively. Two spring wheat varieties, viz.

Trappe and Alora, currently being widely cultivated by

seed companies, farmers, seed multipliers and research

organizations in Denmark were used. The seeds were sown

on 5 November 2010 directly into the pots at a density of

six seeds per pot. When the plants were 15 cm in height,

thinning was carried out leaving three plants with even

Weldearegay et al.

2 ª 2012 Blackwell Verlag GmbH

size in each pot. Each variety had 25 pots but only 20 rep-

resentative pots with uniform seedlings were selected for

treatment. Each treatment had five pots (five replications).

During the vegetative growth phase (before anthesis), all

plants were drip irrigated with nutrient solution (in total

2.5 g N, 1.3 g P and 1.8 g K were applied to each pot) to

prevent any deficiency of nutrients. During treatment,

there were no nutrients given to the plants to avoid the

potential effect of nutrient availability on the treatments.

Four treatments were imposed during anthesis (anthesis

started on 31 January 2011); soil warming (H), in which

the pots were placed on a heating carpet and the soil tem-

perature was increased by 3 �C compared with the control

(C) for 13–14 days; drought (D), in which the irrigation

was withheld from the pots for 13–14 days until the tran-

spiration rate of the stressed plants was 10 % of the C

plants; and combined soil warming and drought (HD).

Measurements

Soil water status in the pots was determined by weighing

the pots daily at 9:00 am and was expressed as the frac-

tion of transpirable soil water (FTSW) (Liu and Stutzel

2002, Jøgensen et al. 2010). The total transpirable soil

water (TTSW) in the pots was the difference between the

pot weight at 100 % holding capacity (i.e. pot weight of

4.7 kg) and pot weight when the transpiration rates of

the D and HD plants decreased to 10 % of the C plants.

Then, the daily value of FTSW was estimated by the ratio

between the amount of transpirable soil water remaining

in the pot and TTSW:

FTSW ¼ ðWTn �WTf Þ=TTSW ð1Þ

where WTn is actual pot weight on a given date and WTf

is pot weight at the time when the transpiration rate of

the D and HD plants were 10 % of the control plants.

By the end of the treatment, relative water content

(RWC) of flag leaf (one leaf per pot) was determined

according to the protocol by Jensen et al. (2000). The

RWC was calculated as follows:

RWC ¼ ðFW� DWÞ=ðTW� DWÞ � 100% ð2Þ

where FW is the leaf fresh weight, DW is the leaf dry

weight and TW is the leaf turgid weight. On the same

day, spikelet samples were collected (one spike per pot)

and were frozen immediately in liquid N for the analysis

of ABA concentration. The samples were finely ground

under liquid nitrogen, extracted in distilled water using

about 1 ml per 30 mg fresh weight. The extraction was

carried out at 4 �C on a shaker overnight. The extracts

were centrifuged at 12 000 g for 10 min at 4 �C and the

supernatants were assayed for ABA by enzyme-linked

immunosorbent assay (ELISA) using monoclonal

antibody (AFRC MAC 252) according to Asch (2000). No

cross-reactions of the antibody with other compounds in

the crude spikelet extracts were detected when tested

according to Quarrie et al. (1988).

At maturity, tiller number, ear number, shoot biomass,

grain yield, harvest index and 1000-kernel weight were

recorded. Seed set was scored by observing the presence

or absence of seed in all florets of five spikes from each

pot and calculating the ratio of fertile florets to total flo-

rets (expressed as a percentage).

Statistical analysis

The experiment was arranged in a completely randomized

design with five replications (five pots for each treat-

ment). The pots were rearranged every 3 days in the

glasshouse to avoid any placement effects on the treat-

ments. Data were subjected to three-way analysis of vari-

ance (Three-way anova) procedures (SAS 9.2, Cary, NC,

USA). Standard errors of the means (S.E.) were calcu-

lated.

Results

Soil water status

Alora depleted the plant available soil water much faster

than did Trappe under D treatment, while the two varie-

ties had similar rate of soil water depletion under HD

treatment (Fig. 1). During the treatment period, FTSW in

the C and H pots was similar and was kept at 0.77–0.93

for both varieties (data not shown).

Flag leaf RWC and spikelet ABA concentration

The RWC of flag leaf was similar for the two varieties

(Fig. 2a; Table 1); when analysed across the two varieties,

it was found that RWC was significantly affected by H

and D treatments, and there were significant interactive

effects between H and D on RWC (Fig. 2a; Table 1). Both

variety and treatment had significant effects on spikelet

ABA concentration (Fig. 2b; Table 1). Between the two

varieties, Alora had significantly higher spikelet ABA con-

centration than Trappe (Fig. 2b; Table 1). For both varie-

ties, D and HD treatments significantly increased spikelet

ABA concentration, while the increases were much evi-

dent in Alora than in Trappe (Fig. 2b; Table 1).

Tiller and ear number

At maturity, tiller number per pot was significantly

greater for Alora than for Trappe, and the H, D and HD

Soil Warming and Drought Stress Effects on Wheat


treatments did not affect tiller number for both varieties

(Fig. 3a; Table 1). Ear number per pot was affected by

both the variety and D and HD treatment (Fig. 3b;

Table 1) and was significantly greater in Alora than in

Trappe. For both varieties, D and HD treatments signifi-

cantly decreased ear number per pot as compared with

the C and H treatments (Fig. 3b; Table 1).

Ear to tiller ratio was significantly affected by both the

variety and the D and HD treatment (Fig. 3c; Table 1).

Across the treatment, Alora had significantly higher ear to

tiller ratio than Trappe. For both varieties, D and HD

treatments decreased the ear to tiller ratio but the reduc-

tion was more pronounced for Trappe than for Alora

(Fig. 3c; Table 1).

Shoot biomass, grain yield and yield components

Shoot biomass was affected significantly by both the vari-

ety and the treatment. Between the two varieties, shoot

0

500

1000

1500

Trappe AloraVariety

Spi

kele

t AB

A c

once

ntra

tion

(pm

ol g

–1 F

W)

(b)0

20

40

60

80

100

120

140

Flag

leaf

RW

C (%

)

C H D HD (a)

Fig. 2 Flag leaf relative water content (RWC) and spikelet ABA con-

centration of two spring wheat varieties grown under soil warming

(H), control (C), combined soil warming and drought (HD) and

drought (D) treatments at anthesis. Error bars indicate S.E. (n = 5).

0.0

0.2

0.4

0.6

0.8

0 2 4 6 8 10 12 14 16DAT

HD

0.0

0.2

0.4

0.6

0.8

1.0FT

SW

TrappeAlora

D

Fig. 1 Changes of the fraction of transpirable soil water (FTSW) for

two spring wheat varieties grown under drought (D) and combined

soil warming and drought (HD) treatments. DAT denotes days after

onset of treatment. Error bars indicate the standard error of the mean

(S.E.) (n = 5).

Table 1 Three-way anova for flag leaf relative water content (RWC), spikelet ABA concentration (ABA), tiller number per pot (T), ear number per

pot (E), ear to tiller ratio (E : T), shoot biomass (SB), grain yield (GY), harvest index (HI), 1000-kernel weight (TK) and seed set (SS) of the two

spring wheat varieties (V) as affected by the soil warming (H), drought (D) and combined soil warming and drought (HD) treatments (data are pre-

sented in Figs 2–5)

Factor RWC ABA T E E : T SB GY HI TK SS

V ns *** *** *** *** *** ** ns ** ***

H * ns ns ns ns ** ns ns ns *

D *** *** ns *** *** *** *** *** ns ***

V · H ns ns ss ns ns ns ns ns ns ns

V · D ns *** ns ns *** ns ns ns ns ns

H · D * ns ns ns ns ns ns ns ns ns

V · H · D ns ns ns ns ns ns * *** ns ns

*, **, and *** indicate significance level at P £ 0.05, P £ 0.01 and P £ 0.001, respectively; ns, no significance.

Weldearegay et al.


biomass was significantly higher for Alora than for Trappe

(Fig. 4a; Table 1). For both varieties, H, D and HD

treatments significantly decreased shoot biomass as

compared with the C treatment. There was no interactive

effect between the variety and the treatment on shoot

biomass.

Grain yield was affected by the variety and the D and

HD treatments. Alora had significantly higher grain yield

compared with Trappe (Fig. 4b; Table 1). For both varie-

ties, D and HD treatments significantly decreased grain

yield, while H treatment alone did not significantly affect

grain yield. There was significant interactive effect

between the variety and the treatment on grain yield

(Table 1). Harvest index was similar for the two varieties

(Fig. 4c; Table 1). There was a significant interactive effect

of the variety and the treatment on harvest index. Com-

pared with the C plants, harvest index was significantly

decreased by D and HD treatments in Alora, while it was

decreased only by HD treatment in Trappe.

Trappe had significantly higher 1000-kernel weight

than Alora; however, across the treatment, there was no

difference in 1000-kernel weight (Fig. 5a; Table 1). Seed

set was significantly affected by both the variety and the

treatment (Fig. 5b; Table 1). Across the treatments, seed

set was significantly higher for Trappe than for Alora;

when analysed across the varieties, it was found that C

plants had the highest seed set, H plants had slightly

lower seed set than C plants and D and HD plants had

the lowest seed set.

Relationships between spikelet ABA concentration, seed

set and grain yield

To analyse the sensitivity of the measured bio-physiological

parameters and yield traits to H, D, and HD treatments,

the spikelet ABA concentration, seed set and grain yield

of the H, D and HD plants were expressed relatively to

those of the C plants. It was found that, across the

varieties and the treatments, there was a clear negative

linear relationship between the relative seed set and the

0

10

20

30

40

Tille

r num

ber p

er p

ot

C H D HD (a)

0

10

20

30

Ear

num

ber p

er p

ot

(b)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Trappe AloraVariety

Ear

to ti

ller r

atio

(c)

Fig. 3 Tiller per pot (a), ear per pot (b) and ear to tiller ratio (c) of

two spring wheat varieties grown under control (C), soil warming (H),

drought (D) and combined soil warming and drought (HD) treatments

at anthesis. Error bars indicate S.E. (n = 5).

0

15

30

45

60

75

1000

-ker

nal w

eigh

t (g)

C H D HD (a)

50

60

70

80

90

100

Trappe AloraVariety

See

d se

t (%

)

(b)

Fig. 4 Shoot biomass (a), grain yield (b) and harvest index (c) of two

spring wheat varieties grown under control (C), soil warming (H),

drought (D) and combined soil warming and drought (HD) treatments

at anthesis. Error bars indicate S.E. (n = 5).



relative spikelet ABA concentration (Fig. 6); and the rela-

tive grain yield was positively correlated with the relative

seed set (Fig. 7).

Discussion

It is well known that wheat plants are very sensitive to

drought and high-temperature stresses particularly when

those occur during anthesis (Balouchi 2010, Wang et al.

2011). It is also widely recognized that interspecies and

intraspecies differences in drought or high-temperature

tolerance exist which provide the opportunity for select-

ing varieties that are suitable for a future drier and war-

mer climate. In the present study, we imposed soil

warming (H), drought (D) and the combined soil warm-

ing and drought (HD) treatments to two Danish spring

wheat varieties during anthesis. The results showed that

although soil warming treatment alone hardly affected

most of the bio-physiological and yield traits investigated,

it significantly reduced shoot biomass (Fig. 4a) and seed

set (Fig. 5b; Table 1). D and HD treatments negatively

influenced the flag leaf RWC and grain yield while signifi-

cantly increased spikelet ABA concentration in both varie-

ties; however, the magnitude of changes differed between

the two varieties. For instance, under D treatment, Alora

depleted the plant available soil water much faster than

did Trappe (Fig. 1a) and which had resulted in slightly

0

20

40

60

80S

hoot

bio

mas

s (g

pot

–1) C H D HD (a)

0

15

30

45

Gra

in y

ield

(g p

ot–1

)

(b)

0.0

0.2

0.4

0.6

Trappe AloraVariety

Har

vest

inde

x

(c)

Fig. 5 1000-kernel weight (a) and seed set (b) of two spring wheat

varieties under control (C), soil warming (H), drought (D) and com-

bined soil warming and drought (HD) treatments at anthesis. Error

bars indicate S.E. (n = 5).

r2 = 0.76 (P = 0.024)

0.7

0.8

0.9

1.0

1.1

0 1 2 3 4 5 6Rel. Spikelet ABA concentration

Rel

. See

d se

t

TrappeAlora

Fig. 6 Relationship between relative spikelet ABA concentration and

relative seed set of two spring wheat varieties exposed to soil warm-

ing (H), drought (D) and both (HD) treatments during anthesis. Error

bars indicate S.E. (n = 5).

r2 = 0.79 (P = 0.018)0.4

0.6

0.8

1.0

1.2

0.7 0.8 0.9 1 1.1Rel. Seed set

Rel

. Gra

in y

ield

Trappe

Alora

Fig. 7 Relationship between relative seed set and relative grain yield

of two spring wheat varieties exposed to soil warming (H), drought

(D) and both (HD) treatments during anthesis. Error bars indicate S.E.

(n = 5).

Weldearegay et al.


lower RWC of the flag leaf. Coincided with this, the

increase in spikelet ABA concentration was also much

pronounced for Alora than for Trappe (Fig. 2b). This

result is in good agreement with that reported by Asch

et al. (2009) in maize, who found that plants grown in

fast drying soils increased xylem ABA concentration more

dramatically than those grown in slow drying soils. In

comparison with Trappe, the more dramatic increase in

spikelet ABA concentration under D and HD treatment

for Alora might have caused the greater reduction of seed

set in this variety as exemplified by the negative linear

relationship between the relative seed set and the relative

spikelet ABA concentration (Fig. 6). Such a relationship

implies that ABA level in the reproductive organs during

anthesis plays a crucial role in determining seed number

and hence final grain yield of wheat crops under D and

HD treatments, consistent with earlier findings in

drought-stressed soybean (Liu et al. 2003, 2004) and

wheat plants (Westgate et al. 1996). However, Asch et al.

(2001) concluded that ovary ABA concentration does not

induce kernel abortion in field-grown maize under anthe-

sis drought stress. In addition, Liu et al. (2003) suggested

that the ABA accumulated in the reproductive organs of

soybean might be transported from the roots via the

xylem, and greater ABA concentrations in the reproduc-

tive organs might indicate higher ABA concentrations in

the xylem sap. If this was also the case here for wheat,

one would have expected that the ABA concentration in

the xylem sap would have been higher in Alora as com-

pared with Trappe under D and HD treatments. A strong

xylem-borne ABA signal is expected to induce early sto-

matal closure hereby curtailing the transpiration rates.

This was obviously not the case here as the Alora plants

depleted the soil water much faster than did Trappe, the

latter had much lower ABA concentration in the spikelet

(Fig. 2b). Moreover, as the sensitivity of plant physiologi-

cal processes to ABA level differs at different stages, even

a slight increase in ABA concentration may cause repro-

ductive failure at its critical stage, for example, floral initi-

ation stage (Saini and Westgate 2000). Here, the ear to

tiller ratio decreased more pronounced in Trappe than in

Alora in response to D and HD stresses, indicating there

were more sterile tillers in Trappe (Fig. 3c). This could

have been due to that the floral initiation in the late til-

lers of Trappe occurred just around the end of the D and

HD treatment when the stresses were most severe; a mod-

erate increase of ABA concentration in the plants might

have caused tiller sterile and hence a reduced ear to tiller

ratio. However, for Alora, the floral initiation in the later

tillers might have been started after the termination of

the D and HD treatments and therefore might have not

severely been affected by the treatments. However, these

speculations need to be verified in future studies.

In the present study, it was seemingly that the soil

warming treatment alone had minor effect on most of the

traits investigated. Nevertheless, soil warming significantly

decreased shoot biomass and seed set in both wheat varie-

ties (Figs 4a and 5b). These results indicate that a 3 �C

increase in soil temperature in this experiment caused a

reduction in biomass accumulation and probably a

decrease in carbohydrate availability for reproductive

development. These effects could be due to the fact that a

higher temperature around the roots has caused greater

respiration rates and larger carbohydrate losses in the

wheat plants (Hill et al. 2007). On the contrary, Gavito

et al. (2001) observed that an increase in soil temperature

from 10 to 15 �C significantly increased leaf and stem

biomass of winter wheat during vegetative growth. The

reasons for this discrepancy are unknown, most probably

due to the differences in the growing conditions (e.g. the

optimal growing temperature could be differing between

wheat varieties investigated) and the plant developmental

stages between the two experiments.

Combined H and D treatment, viz. HD treatment, is

expected to have more pronounced effects on the wheat

plants than those caused by a single H or D treatment.

This was seemingly true in the present study even though

the differences between D and HD plants were mostly not

significant. Nonetheless, the flag leaf RWC was signifi-

cantly lower in HD than in D plants (Fig. 2a) indicating

that the combination of H and D treatment had resulted

in more severe plant water deficit than D treatment alone.

This might have caused the relatively higher spikelet ABA

concentration in the HD than in D plants (P = 0.283).

Furthermore, as has been discussed previously, greater

ABA concentrations in the spike might have led to an

increased seed abortion and thus to a reduced seed set in

the two wheat varieties under HD treatment (Fig. 6).

Compared with seed set, 1000-kernel weight was less

sensitive to either D or HD treatment for both wheat

varieties (Fig. 4a). Thus, the grain yield reduction under

D and HD treatment was attributed mainly to decrease in

seed number. This conclusion is supported by the posi-

tively linear relationship between the relative grain yield

and the relative seed set (Fig. 7). However, it should be

noted that grain yield is the product of individual grain

weight and grain number; and the two yield components

often compensate each other to achieve a maximal grain

yield under suboptimal growth conditions (Liu et al.

2005). It has been observed that a lowered seed set caused

by anthesis drought stress could lead to bigger seeds at

maturity in maize (Andersen et al. 2002). However, such

effect was not evident in the present study.

Collectively, our results showed that soil warming alone

had very little effect on leaf water status, spikelet ABA

concentration and grain yield of spring wheat varieties,



but significantly affected shoot biomass and tended to

reduce seed set. Compared with the H treatment, D and

particularly the HD treatment had more pronounced

effects on seed set and grain yield of spring wheat plants.

Between the two varieties, Alora was more susceptible to

HD and D treatments in terms of reductions in seed set

and grain yield. It was concluded that the grain yield

reduction under anthesis D and HD treatments in spring

wheat is ascribed mainly to a lowered seed set, and wheat

variety (i.e. Alora) with more pronounced increase in

spikelet ABA concentration is more susceptible to D and

HD treatment.

Acknowledgement

Financial support from ViVa – Water Research Initiative

at Faculty of Life Sciences, University of Copenhagen and

the National Natural Science Foundation of China

(31028017) is gratefully acknowledged.

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independent and combined effects of soil warming and drought stress during anthesis on seed set and...

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