floret anthesis and pollination in wild oats ( avena fatua )
TRANSCRIPT
Floret anthesis and pollination in wild oats (Avena fatua)'
M. V. S . RAJU, G. J . JONES, AND G. F. LEDINGHAM Department of Biology, University of Regina, Reginn, Sask., Canndn S4S OA2
Received October 25. 1984
RAJU, M. V. S., G. J. JONES, and G. F. LEDINGHAM. 1985. Floret anthesis and pollination in wild oats (Avenn fatun). Can. J. Bot. 63: 2187-2195.
Avena fatua L. (wild oats), an introduced annual, is a successful weed in the cultivated fields of the Canadian prairies. Its inflorescence is a determinate panicle consisting of many spikelets, each of which contains two or three florets. During anthesis, the lodicules in each floret swell after water uptake and cause the lemma to diverge and to establish a wide angle between it and the palea. The essential organs in the floret are exposed to the environment and subsequently the anthers dehisce releasing pollen. The pollen grains are dropped on the stigmatic branches, thus effecting self-pollination. Following pollination, the floret closes because of the collapsing of lodicules. The pollen on the stigma germinates after the floret has closed. Anthesis, both in the field and in the growth cabinet, shows a daily rhythm and occurs in the afternoon. This rhythmic floret opening seems to be temperature sensitive. The ambient temperature range for anthesis in the field is 25-28°C. The wild oat is primarily a chasmogamous species and enforced cleistogamy in the florets can be induced experimentally.
RAJU, M. V . S., G. J. JONES et G. F. LEDINGHAM. 1985. Floret anthesis and pollination in wild oats (Avena fatun). Can. J. Bot. 63: 2187-2195.
La folle avoine (Avena fatua L.), une plante annuelle introduite, est une mauvaise herbe qui rkussit trts bien dans les champs cultivks des prairies canadiennes. Son inflorescence est une panicule dkterminke, formke de plusieurs kpillets contenant chacun deux ou trois fleurs. Durant I'anthtse, les lodicules de chaque fleur gonfle suite a ]'absorption d'eau, ce qui cause 1'Ccartement du lemma et I'ktablissement d'un angle plus large entre lui et le palka. Les organes essentiels de la fleur sont alors exposks au milieu ambiant et, par la suite, il y a dkhiscence des anthkres et libkration du pollen. Les grains de pollen tombent sur le stigmate et I'autofkcondation s'effectue. Suite a la pollinisation, la fleur se referme du a I'affaissement des lodicules. Le pollen sur le stigmate germe aprks la fermeture de la fleur. L'anthtse, au champ et en cabinet de croissance, montre un rythme journalier, et elle se produit en aprts-midi. Ce rythme d'ouverture des fleurs parait sensible 21 la tempkrature. La tempkrature ambiante favorable a la tempkrature. La tempkrature ambiante favorable a I'anthkse au champ est 25-28°C. La folle avoine est avant tout une espkce chasmogame et la cltistogamie imposke peut &tre induite expkrimentalement.
[Traduit par le jounal]
Introduction Avenu fatua L. (wild oats), an introduced annual, is an
aggressive and a persistent weed in the cultivated fields of the Canadian prairies. The success of the species is often attributed to the ability of caryopses to remain dormant for several years in the field (Johnson 1935; Banting 1966). In a review on the breeding systems in grasses, Connor (1979) reported the pres- ence of chasmogamy and cleistogamy in many autogamous species emphasizing the importance of and the need for a de- tailed description of the structure and behaviour of florets to obtain a better understanding of the reproductive biology of grasses. Although the local populations of wild oats are con- sidered autogamous (Naylor 1983), the reproductive biology of this species is little known; it is uncertain whether the species is indeed cleistogamous or chasmogamous. The occurrence of facultative cleistogamy in grasses in different ecological hab- itats is not uncommon (Hackel 1906; Percival 1965). Avena scabrivalis is dimorphic producing chasmogamous florets in Chile and cleistogamous florets in Uruguay (Hackel 1906). Such a behaviour of florets of the same species in two different geographical areas prompted us to investigate the pattern of anthesis in the local wild oat populations; furthermore, the chances of chasmogamous florets producing genetic variability are better than those of cleistogamous florets. It is also well
'This paper is dedicated to the memory of the late Dr. J. M. Naylor, Professor of Biology, University of Saskatchewan, Saskatoon, who, in the course of discussion with the first author in 1982, stressed the need for a detailed study of the floral and reproductive biology for a better understanding of the genetic control of caryopsis dormancy in the wild oat (Avena fntua L.) in Canada.
known that the lodicules in chasmogamous grasses aid in the opening of florets. However, there are neither appropriate de- scriptions nor quantitative data available relating to changes in the lodicules and florets of grasses. Therefore, the floral biology of wild oat plants, selected from local populations and grown in experimental plots or in controlled-environment growth cabinets, was studied. The present paper reports the results concerned with floret anthesis and pollination of the local wild oats.
Materials and methods Seven different strains of wild oats (Avena fatun L.) maintained at
the Research Station, Agriculture Canada, Regina, were grown sepa- rately in closely placed plots in the arboretum of the Research Station. Two strains, CS 40 and AN 5 1 (Jana et a/. 1979), were grown in a growth cabinet at the University of Regina for purposes of comparison with plants grown in the field. A photoperiod of 16 h light: 8 h dark and a constant temperature of 22 2 2°C were maintained in the growth cabinet. The light intensity there was 300 p,E m-' S- ' and the relative humidity ranged from 50 to 60%. The photoperiod was adjusted from 0600 to 2200 (real time) to simulate approximately the natural July-August day length. Observations at all reproductive stages of development of the plants in these two localities were made and appropriate spikelets and florets were collected for examination.
To determine if the florets would pollinate and produce caryopses when floret opening was prevented, spikelets emerging through the flag-leaf sheath were selected. In each spikelet, the glumes at their distal ends were sealed with Scotch tape to prevent the spikelets from opening. The treated spikelets were collected at different time after 2 weeks and examined for the presence of embryos.
Most data presented in this paper are a result of observations made during at least two consecutive summers, especially on field-grown plants.
Can
. J. B
ot. D
ownl
oade
d fr
om w
ww
.nrc
rese
arch
pres
s.co
m b
y U
NIV
CH
ICA
GO
on
11/1
0/14
For
pers
onal
use
onl
y.
CAN. J . BOT. VOL. 63, I985
Can
. J. B
ot. D
ownl
oade
d fr
om w
ww
.nrc
rese
arch
pres
s.co
m b
y U
NIV
CH
ICA
GO
on
11/1
0/14
For
pers
onal
use
onl
y.
RAJU ET AL.
FIGS. 10-13. Avena fatua. Fig. 10. Stigmatic branches just before anthesis showing papillae (Pa). X370. Fig. 1 I . Stigmatic branches with pollen adhered to them 24 h after anthesis. x200. Fig. 12. Higher magnification showing pollen adhering to stigmatic branches. x450. Fig. 13. A stigmatic branch 24 h after anthesis with papillae (Pa) and germinating pollen with pollen tubes (Pt). X 1500.
Observations The inflorescence of Avena fatua L. is a determinate panicle
in which the spikelets at the time of anthesis are pendant. The first formed spikelets are found at the distal ends of the main and lateral axes (Figs. 1, 2, and 19). Each spikelet in the panicle contains two or three florets together enclosed by the first and second glumes (Raju and Ramaswamy 1983). The first (proximal) and the second (middle) florets are usually perfect, each consisting of a lemma, a bikeeled palea, two lodicules, three stamens, and a pistil. The third (distal) floret is usually
imperfect, often abnormal with rudimentary floral parts (Fig. 6). However, three perfect florets occur in spikelets in some wild oat strains, especially in the first formed panicles.
The floret contains three stamens with short filaments and long basifixed anthers (Figs. 8 and 18). In cross section, the anthers appear four lobed and the anthers on maturity dehisce longitudinally with two slits along their sides, releasing pollen (Figs. 14 and 15). At the time of pollination, the anthers are situated at approximately the level of the stigmatic branches (Figs. 7 and 8). After dehiscence of the anthers, the filaments
RGS. 1-9. Avena fatua. A, anther; F,-F,, first to third florets; L, lodicule; S, stigma. Fig. 1. Panicle 2 days after emergence through flag-leaf sheath containing some open florets. x 112. Fig. 2. Four-day-old panicle with florets in the distal spikelets open. x 112. Fig. 3. Distal-most spikelet showing the concurrently opened first and second florets. The third floret is still closed. Note the remains of dehisced anthers. x 2 . Fig. 4. A spikelet with the second floret open and the first floret, which was open on the previous day, closed. Note the dry anthers and elongated filaments. x2 . Fig. 5. Distal part of a 8-day-old panicle with closed spikelets and hanging dehisced anthers. X 1 . Figs 6-9. Scale bar = 1 cm. Fig. 6. Immature second floret with young anthers and plumose stigmatic branches. The third floret is imperfect and attached to the rachilla. Fig. 7. First floret just before anthesis showing mature lodicules and anthers. Note the curved stigmatic branches. Fig. 8. First floret immediately after anthesis. Note-the swollen lodicules, dehisced anthers, and curved stigma with pollen on them. Fig. 9. Pistil with incurved stigmatic branches with pollen (arrowheads).
Can
. J. B
ot. D
ownl
oade
d fr
om w
ww
.nrc
rese
arch
pres
s.co
m b
y U
NIV
CH
ICA
GO
on
11/1
0/14
For
pers
onal
use
onl
y.
2190 CAN. J . BOT. VOL. 63. 1985
FIGS. 14-18. Avena fatua. A, anther; Aw, awn; F, filament; FI-F3, first to third florets; GI and Gz, first and second glumes; L, lodicule; Le, and Le2, lemmas of first and second florets; P, palea; Pe, peduncle; Ra, rachilla. Scale bars = 1 mm. Fig. 14. Divarication of a spikelet during anthesis. First and second glumes are stretched apart because of the opening of the first floret. The dehisced anthers are still at the level of stigma. Fig. 15. Glumes of a spikelet are removed exposing the lemmas of two florets. First floret is open. Note the palea and the dehisced anthers. Fig. 16. A dissected first floret to show the curved stigmatic branches with pollen and dehisced anthers still attached to elongated filaments. Swollen lodicules are obvious at the base of ovary. Fig. 17. Differences in the morphology of lodicules. The first floret at anthesis has swollen lodicules and the second floret in the same spikelet has unswollen lodicules. Fig. 18. Different stages of lodicule morphology. (A) Lodicules isolated 10 min after the closure of the floret. Note collapse of basal parts. (B) Enlarged lodicules during anthesis. (C) Lodicules before anthesis. Note also undehisced anthers.
elongate and often the dried anthers appear hanging from the closed florets (Figs. 2-5). The mature pistil has two feathery or plumose stigmas which in mature florets remain outwardly curved (Fig. 7). These essential organs together with the lodi- cules are enveloped by the palea and the lemma (Fig. 15).
Pollination Anthesis in wild oats is the divergence of the lemma and
palea of the floret. Because the empty glumes cover the florets, they must also diverge each time a floret in that spikelet opens (Fig. 14). During anthesis, the mature anthers dehisce releasing pollen, some of which is trapped by the adjacent stigmatic branches (Figs. 8-1 1, 14, and 15). Following pollination, the florets and spikelets close. The pollen grains trapped by the stigma germinate after the florets have closed. About 12-24 h
after closure of the florets, the stigmatic branches shrivel (Fig. 9). An examination of such stigmas revealed the germi- nation of pollen (Figs. 11- 13). Early stages of embryo devel- opment were observed about 24-40 h after pollination in the florets. Observations on plants both in the field and in the growth cabinet indicate that the florets are chasmogamous.
Mechanism of anthesis During anthesis, the two abaxial lodicules placed alternately
to the abaxial stamen swell and push the lemma (Figs. 7 ,8 , and 16-18) away from the more centrally fixed pistil and palea in the floret. The glume that is situated immediately external to the lemma of the floret is also pushed out causing divarication of the glumes (Figs. 1-4, 14, and 15). As a result of this floret opening, the turgid anthers and the stigmatic branches are ex-
Can
. J. B
ot. D
ownl
oade
d fr
om w
ww
.nrc
rese
arch
pres
s.co
m b
y U
NIV
CH
ICA
GO
on
11/1
0/14
For
pers
onal
use
onl
y.
RAJU ET AL. 2191
posed to the environment. The opening of the floret, although a gradual process, is achieved in a few minutes in the field (mean 2 SD, 2.38 2 0.92 min; N = 20; range, 1-4 min), similar to the time observed in the growth cabinet.
The florets open with divaricate glumes, establishing a max- imum angle as measured between florets (between two awns) (Figs. 1, 2, and 4; Table 1). The angles in the florets both in the field and in the growth cabinet were very similar. The florets remain open much longer in the growth chamber (mean 2 SD 78.35 2 18.59 min; N = 20; range, 40-1 10 min) than those in the field (mean -C SD 27.75 2 10.45 min; N = 20; range, 15-50 min). The daily time range of anthesis when examined consecutively for several days appears quite variable in the field-grown plants (Fig. 21). In the growth chamber, on the other hand, it is less variable.
Observations in the field showed that the maximum number of opened florets (peak anthesis) was present when the tem- perature range was 25-28°C. When the temperature was over 30°C, most of the opened florets tended to close, but when it dropped to 30°C or less, some of them opened again while others remained closed. The florets that did not open again contained lodicules that were either burst or damaged. The time range of peak anthesis and the corresponding temperature of the environment both in the field and in the growth cabinet are given in Fig. 21.
The lodicules have been reported to be important in the opening of the grass florets (Kerner 1895; Connor 1979). In wild oats, they are uniformly thick in the radial direction before the opening of the mature floret. Tangentially, each lodicule has a basal wider part and a distal narrower portion (Figs. 7 and 18, C). Upon full imbibition with water, which occurs during anthesis, the turgid lodicules become more or less flask shaped with a radial-tangentially swollen proximal part and a distal attenuated portion (Figs. 8 and 18, B; Table 2). The radial-tangential enlargement of the lower parts of the lodi- cules as a result of water uptake causes the lemma to be pushed away, opening the floret. The anthers, which are placed at the level of stigmatic branches, dehisce, dropping pollen over the stigma and effecting pollination. While drying, the filaments elongate significantly and, at their distal ends, the dry anthers still remain attached (Figs. 8 and 16). While filaments are elongating, the dry anthers may drop pollen. After the florets have closed, the anthers with their elongated filaments remain hanging from spikelets for several days if there is no wind (Fig. 5). The closed florets with externally hanging anthers have often been considered, especially in grasses, as indi- cations of anemophily (Proctor and Yeo 1972; Stebbins 1974).
An examination of lodicules immediately after the closure of the florets showed that they, especially their proximal bulged parts, were collapsed or burst exuding small amounts of water; the lodicules were reduced in size (Fig. 18, A; Table 2).
Pattern of anthesis The sequence of events in the anthesis of wild oats described
earlier is repeated as many times as the number of perfect florets in a spikelet. Therefore, each spikelet, which contains two or three florets, opens and closes two or three times. Anthesis, which involves opening and closing, begins in the first floret and proceeds in a sequence to the second and the third. Such a sequence of anthesis corresponds to the acropetal sequence of initiation of florets (unpublished data) in which the earliest initiated floret opens first. Accordingly, the lodicules which cause floret opening of the first floret are devel-
TABLE 1. Mean (* SD; N = 20) maximum angle (degrees) of divarication of glumes during floret opening
Floret type Angle
First (primary) floret 31.90t3.65 Second (secondary) floret 31.9025.34 Third (tertiary) floret 23.4523.72 First and second florets together 55.425.31
NOTE: The minimum angle between glu~nes (or lemma awns) is considered to be zero.
TABLE 2. Mean ( 2 SD; N = 20) dimensions (millimetres) of lodi- cules at different stages of anthesis in the florets of wild oats (Avena
fatua) in the growth cabinet
Treatment
Before During After TY pe anthesis anthesis" anthesis"
Length (height) 2.01 20.11 2.01 20 . 12 1.9320.07 Tangential width
(at base) 0.57t0.02 1.17k0.24 0.5620.14 Radial width
(thickness at base) 0.38t0.02 0.6420.03 0.1920.03
"Measurements laken when the floret was fully open. "Measurements taken 10 min after the floret has closed.
opmentally more mature than those of the other florets (Fig. 17). The occurrence of exceptions, especially in the field, with concurrent opening of first and second or second and third florets in the same spikelet is not uncommon (Fig. 3). In such instances, the lemmas of the two florets move away from the center and consequently the glume divarication angle becomes double that of a single opened floret (Figs. 3, 4, 14, and 15; Table 1). Some rare exceptions (three instances in more than 400 spikelets examined in the field; two instances in 96 spike- lets in the growth chamber) were also observed where the second floret had opened earlier than the first. In such in- stances, the first florets, which did not open, were dissected and found to have damaged lodicules. No instance of simulta- neous opening of all the three florets nor the first and third florets was observed in the growth cabinet or in the field.
The sequence of floret anthesis in an entire panicle in the field-grown plants was difficult to trace, mainly because of wind and changes in temperature. Therefore, an emerging pan- icle was selected from a plant in the growth cabinet to follow the sequential opening of florets. Drawings were made of a panicle at various stages of development until its complete emergence from the flag-leaf sheath and a concomitant recording of the opening of florets, both first and second, was made and is indicated numerically in a sequence in the final composite ideograph (Fig. 19). The sequence of numbers in the numerator refers to the time of opening of the first (primary) floret and the denominator refers to that of the second (second- ary) floret. Repetition of the same number indicates concurrent opening of florets. In the ideograph, the florets in all spikelets, except two at lower nodes, were opened. The unopened florets were imperfect and contained rudimentary parts.
An analysis of floret opening in each spikelet clearly indi- cated a pattern with the proximally placed first floret opening earlier than the distal second floret. By following the sequence of floret opening in the panicle consisting of many functional and two aborted spikelets (Fig. 19), no definite pattern could be
Can
. J. B
ot. D
ownl
oade
d fr
om w
ww
.nrc
rese
arch
pres
s.co
m b
y U
NIV
CH
ICA
GO
on
11/1
0/14
For
pers
onal
use
onl
y.
CAN. 1. BOT. VOL. 63, 1985
FIG. 19. An ideograph of a panicle of Avena fatua. The associated numbers indicate the sequence of floret opening in each spikelet; see text for details.
established as some florets opened at the same time. On the other hand, an overall pattern in the entire panicle was ob- served. More florets in the distally placed spikelets of the entire panicle opened earlier than those in the proximal spikelets. Similarly, the distal spikelets of branches opened earlier than the proximal ones of that particular branch (Fig. 19).
Time of anthesis To identify the peak time of anthesis, observations were
made continuously, both night and day, on wild oat plants in
the field and in the growth cabinet. While the time range of anthesis in the field plants was more variable than that of the plants in the growth chamber, the florets in both locations opened only in the afternoon. Anthesis in the field was con- sidered maximum when six or more florets were open in a panicle in each of the selected 20 panicles. Similarly, the peak anthesis in growth cabinet was noted when four or more florets per panicle were open in each of the five panicles selected for observation.
A large number of florets of wild oats in the field opened
Can
. J. B
ot. D
ownl
oade
d fr
om w
ww
.nrc
rese
arch
pres
s.co
m b
y U
NIV
CH
ICA
GO
on
11/1
0/14
For
pers
onal
use
onl
y.
RAJU ET AL. 2193
No. of consecutive days
FIG. 20. Time required for the completion of anthesis in a panicle of Avena fatua in the field (0; N = 20) and in the growth cabinet (0; N = 5) .
between 1600 and 2000 and in the growth cabinet between 1600 and 1730 in a 24-h period. The range of peak anthesis in the growth chamber wasmore restricted-than that in the field (Fig. 2 1). Kerner (1 895) had earlier reported that floret opening in grasses was dependent on weather conditions, especially the temperature. Observations on plants in the field and in the growth chamber indicated that the temperature was indeed an important controlling factor in floret opening (Fig. 21). The smaller time range of anthesis observed in the growth cabinet seems to be related to the constant temperature in it. In the field, during the period of observation, the temperature ranged from 15°C at night to 35°C at noon or early afternoon. How- ever, the peak anthesis in the field was observed always in the afternoon at a time when the temperature range was 25-28°C (Fig. 21).
Observations on seven different strains of wild oats grown in the field under the same conditions showed that the pattern of floret opening was similar in all. The temperature of the envi- ronment was measured at the level of panicles with a thermom- eter to determine its range when anthesis occurred (Fig. 21). The florets began to open in the afternoon as the temperature dropped from high to low and peak anthesis was observed at a range of 28-25°C. In the growth cabinet, where a constant temperature of 22°C was maintained, the florets opened in the afternoon. Thus, the florets of wild oats, both in the field and in the growth cabinet, always opened in the afternoon, ex- hibiting a daily rhythm. Some differences in the time of peak anthesis were, however, noticed in both locations (Fig. 21).
Selected panicles, both in the field and in the growth cabinet, were monitored continuously for anthesis. Maximum anthesis was observed 4-6 days after the emergence of the panicle from the flag-leaf sheath. It gradually decreased and was completed in 14 days (Fig. 20).
Experimental cleistogamy In a review of cleistogamous flowers, Uphof (1938) men-
tioned many external factors, such as temperature, light, soil condition, water stress, etc., that induce cleistogamy in
TABLE 3. Number of caryopses with embryos in a panicle of Avenrc fatua produced when the florets were not allowed to open by sealing the glumes of spikelets with a narrow strip of Scotch tape. They were
examined 20 days after treatment
No. of florets % caryopses treated TY pe with embryos
48 First floret (proximal) 100 48 Second floret (distal) 96
flowers. In a more recent review, Campbell et nl. (1983) also reported the occurrence of cleistogamy in large number of grasses and indicated its importance in the abundant production of caryopses. As the florets of Avena fatun are chasmogamous, attempts were made to induce cleistogamy in them by tying the spikelets (mature) with strips of Scotch tape. This experiment was conducted in the growth cabinet with the hope of deter- mining caryopsis formation by self-pollination and self- fertilization. The results are presented in Table 3. They indicate that both first and second florets in almost all spikelets of the panicle had produced caryopses by the enforced self- pollination and self-fertilization. Examination of the caryopses showed that embryos were present in -them (Table 3).
Discussion Results of the present study on Aveno fatua deal with two
aspects of floral biology: floret opening and pollination. Quan- titative data have been provided to show that floret anthesis occurs because of the enlargement of lodicules and divarication of glumes. During anthesis, the lodicules swell owing to water uptake and push the lemma radially causing the tloret to open. Regardless of the morphology of the lodicules (Mehlenbacher 1970; Kam 1974), it is sufficient to consider them as struc- turally specialized organs adapted to the function of anthesis. While studying the water relations in rice panicles during water stress, O'Toole et al. (1984) have demonstrated that the water potential is directly related to the anthesis of florets, low water potential being associated with lack of anthesis. They further suggested that water stress of the panicle may decrease the turgor pressure in the lodicule and thus inhibit floret opening in wild oats.
Enlarged lodicules of wild oats keep the expanded lemma in tension. 'This is removed by bursting (damaged presumably by excessive turgor pressure) or shrinkage (because of low turgor) of lodicules and the floret eventually closes. In the field-grown plants, anthesis occurred in a daily cyclic fashion in the after- noon (Fig. 21). The lack of anthesis at higher temperatures (30°C or higher) in the field and its occurrence on the arrival of lower temperatures (below 30°C) indicate that temperature of the environment may be an important factor controlling floret opening in wild oats. Anthesis was also observed in the growth cabinet where a constant temperature of 22°C was maintained. The lower limit of temperature for floret opening was not determined in the present study. The low temperature of 14"C, as reported by Kerner (1895) for some grasses, could be the lower limit for wild oats also. It appears, therefore, that the wild oats show a basic pattern of rhythm in floret opening both in the field and in the growth cabinet. However, in the field, many environmental variables including temperature may alter this basic pattern of floret opening by advancing or retarding it. Appropriate experiments need to be conducted to determine the precise relationship between temperature and floret opening.
Although a rhythm was observed in the panicle of wild oats
Can
. J. B
ot. D
ownl
oade
d fr
om w
ww
.nrc
rese
arch
pres
s.co
m b
y U
NIV
CH
ICA
GO
on
11/1
0/14
For
pers
onal
use
onl
y.
CAN. J . BOT. VOL. 63. 1985
U U U U U 2 3 4 5 6
U U U U U U U U U U U U U 8 9 10 11 12 13 14 15 16 17 18 19 2 0
Number of consecutive days
Frc. 21. Mean time range of peak anthesis per panicle of Avena fatua in the growth cabinet (-; N = 5) and in the field (---; N = 20) recorded every afternoon consecutively for 20 days. Anthesis occurred only in the afternoon. The temperature at which peak anthesis occurred is also indicated (0, field; 0, growth chamber).
with its florets opening daily in the afternoon both in the field and in the growth cabinet, the individual florets lacked such a rhythm. They opened only once as a result of the collapse of the lodicules after the florets had opened.
Wild oat florets usually open and almost immediately the stigmas spread out considerably and the anthers dehisce. Although the florets are pendant, the anthers at the time of dehiscence are among the plumose branches of the stigma and some of the pollen is trapped by the receptive stigma. Although pollination takes place in open florets, selfing is favoured and no mechanisms are present in wild oats to prevent its occur- rence. Pollen dropping from the anthers is, no doubt, scattered widely by wind or air currents, but various reports indicate that outcrossing is the exception rather than the rule (Derrick 1933; Aamodt et al. 1934; Imam and Allard 1965; Naylor 1983).
Studies of the morphology and behaviour of the florets of local populations of wild oats suggest that the florets are pre- dominantly chasmogamous. However, the results of experi- mental cleistogamy indicate that the florets could resort to cleistogamy in case the lodicules fail to swell because of injury, high temperature, or any other reason. Thus, the ability to produce caryopses either by chasmogamy or cleistogamy would undoubtedly add to the success of Avena fatua in the
prairies where the environmental conditions are generally un- predictable.
The success of wild oat as a weed which has no effective means of vegetative reproduction may be related to two im- portant features. Firstly, the seeds must be produced in abun- dance and, secondly, they should be protected from effects of adverse environment and from losing their viability. As de- scribed in this paper, the weed does possess specialized floral contrivances for successful autogamy which possibly assures abundant seed production as in other grasses (campbell et al. 1983). The development of caryopses in all perfect florets in a wild oat panicle agrees with the dictum that the self-pollinating annual grasses produce a significantly greater number of seeds than the analogous perennial grasses (Stebbins 1957, 1974). The wild oat caryopses thus produced and dispersed need to be protected, since they are known to persist in the soil for several years without losing their germinability (Banting 1966). The embryo envelopes of protective value in the caryopses will be described in a separate paper. It is, however, sufficient to mention that the caryopses of wild oats do contain envelopes, as in other related species, of physical and physiological im- portance in dormancy and germination (Mann and Harlan 1915; Bewley and Black 1978; Werker 1980-1981).
Can
. J. B
ot. D
ownl
oade
d fr
om w
ww
.nrc
rese
arch
pres
s.co
m b
y U
NIV
CH
ICA
GO
on
11/1
0/14
For
pers
onal
use
onl
y.
RAJU ET AL. 2195
Genetic variation, especially in annuals, is known to occur in fully inbreeding populations (Imam and Allard 1965; Kannenberg and Allard 1967; Allard and Kannenberg 1968; Hamrick and Allard 1972; Jain 1976). Many studies on the genetic control of some selected physiological processes, such as germination, dormancy, and a-amylase production in wild oat caryopses, have been conducted and considerable vari- ability reported for local populations (see Naylor 1983).
The local wild oat populations, which are chasmogamous and self-pollinated (autogamous), retain considerable genetic variability (Naylor 1983) probably by limiting hybridization as in some species of Epilobium (Raven 1972). However, the possible occurrence of limited variability either by gei- tonogamy or xenogamy in a predominantly chasmogamous Avena fatua in the prairies is not completely excluded. Evi- dence from experimental cleistogamy also indicate that the wild oat is not an obligate chasmogamous species.
IMAM, A. G., and R. W. ALLARD. 1965. Population studics in pre- dominantly self-pollinated specics. VI. Gcnetic variability betwcen and within natural populations of wild oats from differing habitats in California. Genetics, 51: 49-62.
JAIN, S. K. 1976. The evolution of inbreeding in plants. Annu. Rev. Ecol. Syst. 7: 469-495.
JANA, S. , S. N. ACHARYA, and J. M. NAYLOR. 1979. Dormancy studies in secd of Avena fatictr. 10. On the inheritance of germi- nation behaviour. Can. J . Bot. 57: 1663-1667.
JOHNSON, L. P. V. 1935. General preliminary studies on the phys- iology of delayed germination in Avena farica. Can. J. Rcs. Sect. C, 13: 283-300.
KAM, Y. K. 1974. Developmental studies of the florct in Oryzopsis virescens and 0 . hytnenoides (Gramineae). Can. J. Bot. 52: 125-149.
KANNENBERG, L. W., and R. W. ALLARD. 1967. Population studies in predominantly self-pollinated species. VIII. Genetic variability in the Fesruca microsrachys complex. Evolution (Lawrence, Kans.), 21: 227-240.
KERNER, A. 1895. The natural history of plants. Blakie, London. Acknowledgement (Translated by F. W. Oliver.)
This investigation was supported by a research grant from MANN? A., and H. V. HARLAN. 1915. Morphology of the barley grain
Agriculture Canada. with reference to its enzyme-secreting areas. U.S. Dep. Agric. Bull. NO. 183. pp. 1-32.
MEHLENBACHER, L. E. 1970. Floret develoument, embrvolopv. and . .,. systematic position of Oryzopsis hende~soni (Gramineae). Can. J.
AAMODT, 0 . S., L. P. V. JOHNSON, and J. M. MANSON. 1934. Natural and artificial hybridization of Avena sariva with A. fatua
Bot. 48: 1741-1758.
and its relation to the fatuoids. Can. J. Res. 11: 701-727. NAYLOR, J. M. 1983. Studies on the genetic control of some phys- iological processes in seeds. Can. J. Bot. 61: 356 1-3567.
ALLARD, R. W., and L. W. KANNENBERG. 1968. Population studies O.TOOLE, J. C,, T. C. HSIAO, and 0. S. NAMUCO, 1984. Panicle in predominantly self-pollinated species. XI. Genetic divergence water relations during water stress. Plant Sci. Lett. 33: 137-143. among the members of the Fesrrtca microsrachy.~ complex. Evo- lution (Lawrence, Kans.), 22: 517-528. PERCIVAL, M. S. 1965. Floral biology. Pergamon Press, Oxford, . . .-
BANTING, J. D. 1966. Studies on the persistence ofAvenn fatun. Can. J. Plant Sci. 46: 129-140.
BEWLEY, J. D., and M. BLACK. 1978. Physiology and biochemistry of seeds in relation to germination. Vol. I . Development, germi- nation and growth. Springer-Verlag, Berlin.
CAMPBELL, C. S. , J . A. QUINN, G. P. CHEPLICK, and T. J . BELL. 1983. Cleistogamy in grasses. Annu. Rev. Ecol. Syst. 14: 1 1-41.
CONNOR, H. E. 1979. Breeding systems in the grasses: a survey. N.Z. J . Bot. 17: 547-574.
DERRICK, R. A. 1933. Natural crossing with wild oats, Avenn fnrua. Sci. Agric. 13: 459.
HACKEL, VON E. 1906. ~ b e r Kleistogamie bei den Grasern. Oesterr. Bot. Z. 56: 143- 154.
HAMRICK, J. L., and R. W. ALLARD. 1972. Microgeographical vari- ation in allozyme frequencies in Avenn barbnta. Proc. Natl. Acad. Sci. U.S.A. 69: 2100-2104.
U . K .
PROCTOR, M., and P. YEO. 1972. The pollination of flowers. Taplinger Publishing Co., New York.
RAJU, M. V. S., and S. N. RAMASWAMY. 1983. Studies on the inflorescence of wild oats (Avenn fntua). Can. J. Bot. 61: 74-78.
RAVEN, P. H. 1972. Evolution and endemism in the New Zealand species of Epilobium. In Taxonomy, phytogeography and evo- lution. Edited by D. H. Valentine. Academic Press, London. pp. 259-274.
STEBBINS, G. L. 1957. Self-fertilization and population variability in higher plants. Am. Nat. 91: 337-354.
1974. Flowering plants: evolution above the species level. Harvard University Press, Cambridge, MA.
UPHOF, J. C. T. 1938. Cleistogamic flowers. Bot. Rev. 4: 21-49. WERKER, E., 1980-1981. Seed dormancy as explained by the anat-
omy of embryo envelopes. Isr. J. Bot. 29: 22-44.
Can
. J. B
ot. D
ownl
oade
d fr
om w
ww
.nrc
rese
arch
pres
s.co
m b
y U
NIV
CH
ICA
GO
on
11/1
0/14
For
pers
onal
use
onl
y.