double ridge to anthesis (GS2) and anthesis to grain maturation (GS3), as well as on yield and yield compo- nents. The duration of all developmental stages was ...
Euphytica 35 (1986) 483-492
DIFFERENTIAL REACTION OF WHEAT CULTIVARS TO HOT ENVIRONMENTS L. SHPILERandA.
BLUM
Department of Field Crops, The Volcani Center, ARO, POB 6, Bet Dagan, Israel Received 23 April 1985
INDEX WORDS
Triticum aestivum, grain yield components, stability, development, phenology, differentiation, tance.
heat resis-
SUMMARY
Ten to 20 spring wheat (Triticum uestivum L.) cultivars of Israeli origin were grown in three winter (normal) and two summer (abnormal) growing seasons. During the period of emergence to anthesis mean daily temperature was on the average 12°C higher and photoperiod was about 3 h longer in the summer than in the winter. Data was collected on the durations of the periods from emergence to double-ridge (GSI), double ridge to anthesis (GS2) and anthesis to grain maturation (GS3), as well as on yield and yield components. The duration of all developmental stages was reduced by high temperature. While the duration of GS2 was the most thermo-sensitive, it may also have been reduced by the longer summer photoperiod. The effect of photoperiod on GS2 could not be isolated, but the results were interpreted to show that the effect of photoperiod on the duration of GS2 was relatively small. The most heat-affected yield component was number of grains per spikelet and the least affected component was the number of spikes per plant. High temperature reduced grain weight via reduced grain growth duration and not grain growth rate. A general linear regression model of yield on its components revealed that while variation for number of spikes per plant had the greatest effect on yield variation among cultivars in the winter, variation for number of grains per spikelet and spikelets per spike were by far the most important in the summer. Grain weight was the least important component, in this respect, in all seasons. Varieties which sustained the highest yield in hot environments were able to maintain the longest duration of GS2 and the highest number of grain per spike. INTRODUCTION
Wheat (Triticum aestivum L.) is best adapted to cool growing conditions. The interest in wheat response to supra-optimal temperature is increasing as wheat is being planted outside the optimal temperature. Even in established spring,wheat growing regions, such as the Mediterranean climate, mild winter temperatures may have an important role in reducing spring wheat yields. Wheat yields are clearly reduced in hot environments (e.a. MIDMORE et al., 1984), where the term ‘hot’ is used hereon to define any supra-optimal temperatures for wheat. FISCHER & MAURER (1976) showed that a 1 “C rise in temperature above ambient during the period between the end of tillering to the beginning of grain tilling reduced yield by 4% under their test conditions. Yield reduction was associated with reduced numbers of spikes per plant and grains per spike. Yield in hot environments 483
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was reduced (MIDMORE et al., 1984) due to the acceleration of all plant developmental phases. While growth at high temperature is affected by the reduced photosynthesis at supra-optimal temperatures and the increase in respiratory loss of photosynthate (WARDLAW et al., 1980) the dominant effect of high temperature is undoubtedly on the phasic development of the plant. Of all three major developmental phases (GSl: emergence to double-ridge, GS2: double-ridge to anthesis and GS3: anthesis to maturity), the most thermo-sensitive stage was found to be GS2 (WARRINGTON et al., 1977). Reduction of the duration of GS2 under the influence of high temperature resulted in a reduction in the numbers of spikes per plant (FISCHER& MAURER, 1976) and of spikelets and/or grains per spike (HALSE&WEIR, 1970; WARRINGTON etal., 1977; JOHNSON&KANEMASU, 1983). High temperature reduces spikelet number through its effect on both the duration and rate of spikelet initiation (HALSE & WEIR, 1974). Temperature during GSl does not affect spike size but higher temperature at this phase decreases tillering and the number of spikes per plant (WARRINGTON et al., 1977). Grain weight is reduced by high temperatures (PINTHUS& SAR-SHALOM,1978; WARDLAW et al., 1980; JOHNSON& KANEMASU, 1983), as mediated by a reduction in both the duration and rate (SOFIELDet al., 1977; WARDLAW et al., 1980) of grain filling. However, as grain weight interacts with grain number per spike, high temperatures during grain filling do not always reduce grain weight, especially when grain number is small (WARRINGTONet al., 1977). The dominant effect of the thermal regime on wheat growth and yield raises the important question of the existence of genetic variation in wheat adaptation to hot environments. Several investigators found that when the vernalization and photoperiod requirements are fully satisfied, wheat varieties differ in the extent of heat effect on spikelet number per spike (HALSE& WEIR, 1974; HALLORAN, 1977). A most remarkable example is given in the work of BAGGA & RAW~ON (1977), where Kalyansona, compared with Condor showed excellent stability in grain number per spike at increasing temperatures. This and their own data led FLOOD & HALLORAN (1984) to propose that wheat genotypes have a basic development rate, irrespective of their vernalization and photoperiod requirements, and that this basic rate may be used in breeding for a wider environmental adaptation. This work was done to evaluate some of the factors responsible for yield stability of various wheat genotypes in hot environments. MATERIALSANDMETHODS
Experiments were carried out at Bet Dagan Experiment Farm on the Coastal plain of Israel, situated at a latitude of 32”04’ N, and an altitude of ca. 50-100 m. Soil type at the site is a deep fertile vertisol of relatively good structure. Various spring wheat (T. aestivum L.) cultivars were planted in the normal winter growing season as well as in the offseason summer. Seventeen, 20 and 10 cultivars were planted in the winter of 1982, 1983 and 1984, respectively. Eleven cultivars were planted in the summer of 1982 and 1984. Seven cultivars were common to all experiments. These were ‘Lachish’, ‘Barkaee’, ‘Miriam’, ‘Bethlehem’, ‘V747’, ‘V748’ and ‘V75 1’. All cultivars were highly adapted, locally bred materials. All winter plantings 484
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ENVIRONMENTS
emerged during the first ten days of December. All the summer plantings emerged during the last 8 days of July. Day length during the winter season ranged from 10 hr 04 min in December 1st to 12 hr 33 min in April 1st and during the summer season from 13 hr 48 min in July 20th to 11 hr 41 min in September 20th. The pertaining temperature data are included in Table 1. All experiments were planted in a randomized block design with two replications, at normal planting rates (120 kg/ha) in drill spacings. Plot size was never smaller than 4 m2. Experiments were managed for potential production, including nitrogen and phosphorus fertilization, and complete chemical weed and disease control. Supplemental irrigation (by sprinklers) was applied whenever required during the winter (rainy) season, so that available soil moisture was never below 60%. In the summer (dry) season, irrigation was applied on weekly intervals, and available soil moisture was never below 70%. In all experiments and replications the date of emergence and the number of seedlings per 1 m of row in 4 rows per plot, were determined. In 10 plants per replicate the date of double-ridge appearance, the date of anthesis and the date of grain physiological maturity, as judged by the total loss of green color in the glumes @INCH et al., 1984), were determined. Upon full maturity, 10 plants were sampled from each plot. In these plants, the following measurements were made: total number of spikes; number of spikelets per spike; number of grains per spike; and mean weight per grain. These measurements were used to calculate (on a per plot basis) the number of spikes per plant; number of grains per spikelet; estimated mean grain growth rate (= grain weight/grain growth duration); number of grains per plant; grain yield per plant and per m2. The number of ‘Thermal units’ for each plant developmental phase x cultivar x experiment combination was determined as the sum of daily mean air temperatures above 1 “C, based on data from a meteorological station situated 4 km from the site, at the same altitude. RESULTS
The duration of all three developmental phases (Table 1) were markedly reduced from the winter to the summer season. The mean rate of reduction across seasons was 46x, Table 1. Mean duration and range (in days) of wheat development phases (GS) in the winter and summer growing seasons, as averaged over seven varieties. T = mean daily temperature (“C) during phase, and TU = thermal units (sum of mean daily temperatures above 1“C). Developmental phase
GS 1: Emergence to double-ridge GS2: Double-ridge to anthesis GS3: Anthesis to maturity GSl + GS2 + GS3
Winter
Summer
mean range (4 (4
T (co)
TU
mean range T (4 (co) (4
TU
37.0* 2653 66.3 58-77 54.8 45-63 158.1 148-170
13.0 11.8 16.2 -
484 785 851 2140
19.9* 19.8 35.1 74.8
497 481 832 1798
15-25 14-26 24-41 59-8.5
25.1 24.4 23.4 -
* For all means, differences, between seasons are significant at p < 0.05. Euphytica
35 (1986)
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Table
2. Mean
(over seven varieties)
Spikes/ plant
Winter Summer
Spikelets/ spike
grain
AND
A. BLUM
yield components
Grains/ spikelet
in the winter
Grains/ plant
and summer
Grain weight
seasons.
Rate***
Yield/ plant
Yield/m2
nomber
SD*
number
SD
number
SD
number
SD
mg
SD
w/ day
SD
g
SD
g
SD
3.5* 3.0
1.4 0.8
18.0 13.6
1.9 1.8
2.1 1.0
0.4 0.2
129 41
55 16
40.3 35.2
3.8 4.9
0.74 1.00
0.08 0.09
5.3 1.5
2.3 0.7
649 150
213 72
* Standard deviation calculated from the analysis of variance. ** For all means differences between seasons are significant at p d 0.05. *** Grain growth rate.
70% and 36x, for GSl, GS2 and GS3, respectively. Total growth duration was reduced by 53%. These reductions could have resulted from the effects of higher temperatures and longer photoperiod in the summer. The duration of each of the three developmental phases was correlated very well with the number of thermal units across all varieties in each of the live experiments, with correlation coefficients that were better than r = 0.931 (p < = 0.01). Thermal units required for GSl and GS3 were practically the same in both winter and summer (Table l), indicating a complete dependance of the duration of these phases on the number of thermal units. However, less thermal units were required in the summer than in the winter for GS2, indicating that additional environmental factors contributed to the reduction in GS2 from winter to summer. The rate of reduction in GS2 thermal units from winter to summer differed significantly (p < 0.05) among the seven cultivars (within a range of reduction of 233 to 358 TU). It is concluded that the longer photoperiod in the summer contributed to the reduction in GS2, depending on cultivar-specific photoperiod sensitivity. Although the effects of temperature and photoperiod on the ducation of GS2 could not be separated in this experiment, their relative contributions will be further discussed below, to indicate that temperature was the dominant factor in these experiments. Grain yield and its components were reduced significantly in all varieties in the summer as compared with the winter (Table 2). For the seven cultivars common to all experiments a significant season x cultivar interaction for yield was revealed. Of all grain yield components, the largest relative reduction in the summer occurred in the number of grains per spikelet, while the smallest reduction occurred is spikes per plant. Compensation among components did not occur. For example, grain weight did not increase in the summer in response to the reduction in grain number per spike. The reduction in grain weight under high temperature in the summer could be fully accounted for by a reduction in grain growth duration (Table l), since grain growth rate was not decreased but even significantly increased in the summer (Table 2). Grain yield was regressed across cultivars on all yield components (as main effects), by the general linear model (GLM), in order to resolve in each year and season the relative contribution of individual yield components to the variation in yield among cultivars (Table 3). For the seven common varieties the analysis gave different results in the summer and winter, variation for the number of spikes per plant had the largest effect on variation for yield per m2, while in the summer the number of spikelets per spike and the number of grains per spikelet had the largest effect on yield variation 486
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ENVIRONMENTS
Table 3. Linear regression of grain yield (per m*) on grain yield components, across means of seven varieties in two seasonsand across all varieties in each growing season. Contributions from the different yield components to the explanation of yield per m2 are expressed as the percentage of the total sum of squares. The effect of grain weight is further divided into the separate effects of grain growth duration and grain growth rate. Yield component
Seven varieties
All varieties
winter mean
summer mean
1982 winter
1983 winter
1984 winter
1982
1984 summer
Spikes per plant Spikelets per spike Grain per spikelet Grain weight (Grain growth duration) (Grain growth rate)
62.2*
66.3
61.9
49.9
(0.7)
(6.8)
22.1 1.3 8.6 (3.9) (4.7)
19.5 8.1 22.6 (11.9) (10.7)
8.1 40.6 48.0 3.3
(4.1)
18.8 48.6 24.5 8.1 (1.3)
21.6 31.8 40.8 5.8 (0.5) (5.3)
Total sum of squares RZ
97.87 0.972
6.44 0.974
16.65 0.956
34.71
19.8 13.2 4.8
9.4 18.3 6.0 (3.3) (2.7) 77.51
0.978
0.995
(0.1) (3.2) 7.30 0.989
1.50 0.996
* All respective mean squares are significant at p < 0.05.
among cultivars. This basic difference between summer and winter conditions was repeated for all varieties tested in each year. Except for the winter of 1984, variation for grain weight had a relatively small effect on yield variations among cultivars. The variation for rate of grain growth was far more important than variation for grain growth duration in the summer, while both had a similar effect on the variation in yield among cultivars in the winter. Simple correlations across all cultivars in each of the live experiments, between the duration of GSl, GS2 or GS3 and the various yield components did not give very consistent results. None of the yield components was correlated with the duration of GSI The duration of GS2 was significantly correlated with the number of spikelets per spike in two out of live experiments (r = 0.61, p = 0.01; r = 0.62, p = 0.01; where p designates probability). The duration of GS2 was also significantly correlated with the number of spikes per plant in two experiments (r = 0.67, p < 0.01; r = 0.74, p < 0.01). The duration of GS3 was significantly correlated with grain weight in two winter seasons (r = 0.77, p < 0.01; r = 0.71, p < 0.01). While cultivar effects were not consistent in this respect, the environmental effect was more pronounced in relating yield to GS. For the mean over the seven common varieties, significant correlations were obtained across the 5 experiments between yield per m2 and the duration of GSl (r = 0.94, p = 0.02) and GS2 (r = 0.93, p = 0.02), but not GS3. Yield was also negatively correlated across experiments with the mean temperature during GSl (r = -0.96, p = O.Ol), GS2 (r = -0.96, p = 0.01) or GS3 (r = -0.94, p = 0.02). It is therefore evident that a genetic reduction in the duration of GS2 and more so of GSl is not necessarily and simply associated with a reduction in yield and its components in all environments. In contrast, the environmental effect was clear. Yield reduction was associated with shorter developmental phases, under the influence of high temperatures. This effect was not limited only to the difference between summer Euphytica
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Table 4. The regression coefficients for the FINLAY & WILKINSON (1963) stability analysis sion of variety mean on means for each of live growing seasons, over seven varieties) Variety
Intercept
Lachish Miriam Barkaee Bethlehem v747 V748 v751
Slope
a
SD
b
SD
18.8 -59.5 -27.1 25.8 29.0 -15.6 28.5
46.1 31.2 105.4 93.5 102.9 157.2 33.9
0.746 0.915 0.937 0.842 1.216 1.316 1.027
0.089 0.061 0.204 0.181 0.201 0.305 0.065
R”
Mean (k/W
0.958 0.987 0.874 0.877 0.924 0.861 0.987
3530 3500 3930 4030 5740 5740 4880
for yield (regres-
yield
and winter but it occurred across all 5 seasons. Therefore, while the reduction of GS2 in the summer may have been partly affected by temperature and partly by the extended photoperiod, the general association shows that yield variation across all experiments was related to temperature in each of the developmental phases. Therefore, a stability analysis for yield by the linear regression method (FINLAY & WILKINSON, 1963) across the five seasons was performed with the interpretation that the environmental index (mean over all cultivars in each experiment) represented largely the temperature environment. The results of the stability analysis, in terms of the linear regression for each cultivar, are presented in Table 4 and Fig. 1. The significant cultivar x season interaction, as revealed by the analysis of variance is exemplified by the different slopes and intercepts of the various cultivars. The magnitude of the intercept (a) represents the relative performance under low yielding (hot) 8000
0 80 10 MEAN
SEASON
YIELD
(KG/HA)
Fig. 1. Stability analysis according to FINLAY & WILKINSON (1963): regression of cultivar (over seven cultivars) yields of experiments, for 5 representative wheat cultivars (V747, Bethlehem and Miriam); regression parameters are presented in Table 4.
488
yield V748,
on mean Lachish,
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ENVIRONMENTS
conditions. Its magnitude is affected by both the slope(b) and the mean yield, whereby a large intercept may result from either a low slope or a high mean yield (over environments). A low slope represents stability (FINLAY 8z WILKINSON, 1963) across the different thermal environments. The analysis therefore considers three parameters for each cultivar: mean yield, stability (slope) and an index of performance in low yielding (hot) environments (intercept). Bethlehem (Fig. 1) is a medium-yielding stable cultivar with a relatively large intercept due to a low slope. Lachish was similiar in that respect, only having a lower mean yield. V747 had a relatively large intercept due to a high mean yield. V748 is a high yielding cultivar that due to poor stability had a small intercept. Miriam is a low yielding cultivar that has only moderate stability and subsequently a low intercept. It is therefore apparent that performance in low yielding (hot) environments, as expressed by the intercept (a) of the linear regression, was determined by both stability (b) and mean yield of the cultivar. Further analysis was therefore required in order to understand the underlying reasons for a better performance in hot environments. A stability analysis by the linear regression was performed also for each yield component and the duration of each GS. The yield intercept (Table 4), as an index of yield performance in hot environments, was correlated across the seven cultivars with all yield components, as well as with their regression parameters (intercept and slope). The most significant result was the positive association between the regression intercept for yield (as per Table 4) and the the regression intercept for the duration of GS2 (Fig. 2). Thus cultivars that performed better than others in hot environments (large intercept for yield) were characterized by tolerance of the duration of GS2 to high temperature (large intercept for the duration of GS2). Such tolerant cultivars
INTERCEPT
FOR
GS2
Fig. 2. Regression of yield intercept (a) on intercept (a) for the duration of GS2, former was taken from from the stability analysis as per Table 4; the latter was taken analysis for the duration of GS2. R2 = 0.973. Euphytica
35 (1986)
across 7 cultivars; the from a similar stability
489
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(V747, V751, Bethlehem or Lachish in Fig. 2) had either a high (e.g. V747) or a low (e.g. Lachish) mean yield (Table 4). Still, as the duration of GS2 may have been shortened also by the longer summer photoperiod, it must be allowed that such tolerant varieties under summer conditions may have been less sensitive to the longer summer photoperiod. This is unlikely as Barkaee and Miriam are positively known to be photoperiod insensitive, while having the lowest GS2 phase intercept in this analysis (Fig. 2). DISCUSSION
The winter and summer environments differed largely and mainly in temperature and photoperiod. Mean temperatures during GSl and GS2 were 12.1 and 12.6 degrees higher, respectively, in the summer than in the winter. Photoperiod during GSl and GS2 was about 10 and 13 hr in the winter and summer, respectively. These summer conditions markedly reduced the duration of all three developmental stages. While GS 1 and GS3 responded only to temperature, as evidenced by their constant requirements in thermal units, GS2 may have been reduced also under the effect of the longer photoperiod in the summer. The photoperiod-sensitive phase in GS2 is the period from double-ridge to terminal spikelet initiation (PINTHUS & NERSON, 1984), which consists of only a part of what has been defined as GS2 in this study. It is unreasonable to accept such a large effect of photoperiod on the number of thermal units of GS2, when photoperiod is effective only during part of GS2. Furthermore, GSl should be at least as sensitive as GS2 to photoperiod, while the results obtained in this study with these cultivars show that the duration of GSl was fully accounted for by temperature. The smaller number of thermal units in GS2 in the summer could also be explained by a non-linear effect of temperature on the duration of GS2, meaning that the net effect of 1 degree on the duration of GS2 is greater at higher (summer) than at lower (winter) temperatures. It is impossible to separate the exact relative effects of temperature and possibly photoperiod on the duration of GS2 in these experiments. However, in view of the above arguments the relative role of photoperiod on the duration of GS2 is considered to be small. The important consideration is the effect of the reduced duration of GS2 on the number of spikelets per spike. A reduction in the duration of GS2 under long photoperiods is not necessarily associated with a reduced number of spikelets per spike (e.g. RAWSON, 1971). Thus, the reduced number of spikelets per spike observed in the summer in this study can be largely ascribed to the effect of high temperature. This is also supported by the finding that mean (over seven cultivars) yield was highly and negatively correlated across the five experiments with mean temperatures at GS2 (r = -0.96, p = 0.01). Finally, as the longer photoperiod in the summer has resulted in the same direction of effect as the increased temperature (i.e. decreased duration of GS2), a discussion of the effect of decreased duration of developmental stages on yield and its components as a phenomenon of heat stress is legitimate. Asfoundbyothers (FISCHERL~MAURER, 1976; JOHNSON&KANEMASU, 1983; MIDMOREet al., 1984), hot environments reduce the duration of all developmental stages, and the period of double-ridge to anthesis (GS2) is indeed (WARRINGTON et al., 1977) the most sensitive in this respect. Averaged over seven varieties, the largest associated 490
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effect of high temperature was on the number of grains per spikelet and the number of spikelets per spike, while the smallest effect was on the number of spikes per plant. Subsequently, number of grains per spike was found to be the most important yield component affecting yield variation among all cultivars in the summer, as compared with number of spikes per plant in the winter. Stability analysis showed that cultivars differ in their relative adaptation to hot environments, irrespective of their yield performance in cool environments. The relative high stability of Bethlehem in this respect is interesting as it was also found to be a drought resistant cultivar in terms of both its production and physiology (BLUM et al., 1983). In view of the relative thermo-sensitivity of GS2, it was quite reasonable to find yield at high temperatures (expressed by a high intercept of the regression for yield) to be associated with a long duration of GS2 at high temperature (expressed by the high intercept of the regression for the duration of GS2 (Fig. 2). When conditions of high temperature occur together with a longer photoperiod, photoperiodinsensitive genotypes may have also a relative advantage in this respect. Photoperiod will be ineffective in reducing the duration of GS2 in such genotypes. FLUID & HALLORAN (1984) proposed that wheat genotypes have a basic development rate, irrespective of their vernalization and photoperiod requirements, and that this basic rate may be used in breeding for a wider environmental adaptation. On the basis of the study reported here, as well as on other studies (HALSE &WEIR, 1974; BAGGA& RAWSON, 1977) it may be concluded that the basic development rate is highly affected by temperature and that breeding for wider adaptation also involves reduced temperature sensitivity of the basic developmental stages, most notably of GS2. As a result of reduced thermo-sensitivity of GS2 there are wheat genotypes better able to sustain grain number per spike at a high temperature, as exemplified by Kalyansona (BAGGA & RAWSON, 1977) and some of the cultivars tested in this experiment. The reduction in the number of grains per spike in hot environments is mediated by the reduced numbers of grains per spikelet and spikelets per spike. Both components can be very simply screened for by visual observations. The number of grains per spikelet was found to be significantly correlated across all varieties in each of all seasons with spike length (correlation coefficients ranging from r = 0.85; p = 0.01 to r = 0.98; p < 0.01). Therefore, selection for long spikes with a large number of spikelets during the summer is expected to result in the improvement of wheat productivity and stability in hot environments. REFERENCES BAGGA, A. K. & H. M. RAWSON, 1977. Contrasting responses of morphologically similiar wheat cultivars to temperatures apropriate to warm temperate climates with hot summers: a study in controlled environment Amt. J. Plant Physiol. 4: 877-887. BLUM, A., J. MAYER & G. GOZLAN, 1983. Associations between plant production and some physiological components of drought resistance in wheat. Plant Cell and Environment 6: 219-225. FINLAY, K. C. & G. N. WILKINSON, 1963. The analysis of adaptation in plant breeding programme. Aust. J. Agric. Res. 14: 742-749. FISCHER, R. A. & R. 0. MAURER, 1976. Crop temperature modification and yield potential in a dwarf spring wheat. Crop Sci. 16: 855-859. FLOOD, R. G. &G. M. HALLORAN, 1984. Basic development rate in spring wheat. Agron. J. 76: 257-260. HALLORAN, G. M., 1977. Developmental basis of maturity differences in spring wheat. Agron. J. 69: Euphytica
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899-902. N. J. & R. N. WEIR, 1970. Effects of vernalization, photoperiod and temperature on phenological development and spikelet number of Australian wheat. Aust. J. Agric. Res. 21: 383-393. HALSE, N. J. & R. N. WEIR, 1974. Effect of temperature on spikelet number of wheat. Aust. J. Agric. Res. 25: 687-695. JOHNSON, R. C. & E. T. KANEMASU, 1983. Yield and development of winter wheat at elevated temperatures. Agron. J. 75: 561-565. MIDMORE, D. J., P. M. CARTWRIGHT & R. A. FISCHER, 1984. Wheat in tropical environments. II. Crop growth and grain yield. Field Crops Res. 8: 207-227. PINTHUS, M. J. &Y. SAR-SHALOM, 1978. Dry matter accumulation in the grains of wheat (Triricum uestivum L.) cultivars differing in grain weight. Ann. Bot. 42: 469-471. PINTHUS, M. J. & H. NERSON, 1984. Effect of photoperiod at different growth stages on the initiation of spikelet primordia in wheat. Aust. J. Plant Physiol. 11: 17-22. RAWSON, H. M., 1971. An upper limit for spikelet number per ear in wheat as controlled by photoperiod. Aust. J. Agric. Res. 22: 537-546. SINGH, V. P., M. SINGH & M. S. KAIRON, 1984. Physiological maturity in aestivum wheat: visual determination. J. Agric. Sci. Camb. 102: 285-287. SOFIELD, I., L. T. EVANS, M. G. COOK & I. F. WARDLAW, 1977 Factors influencing the rate and duration ofgrain filling in wheat. Aust. J. Plant Physiol. 4: 785-797. HALSE,
WARDLAW,
I. F., I. SOFIELD & P. M. CARTWRIGHT,
1980. Factors
limiting
the rate of dry matter
accumula-
tion in the grain of wheat grown at high temperature. Aust. J. Plant Physiol. 7: 387-400. WARRINGTON,
I. J., R. L. DUNSTONE
& L. M.
GREEN,
1977.
Temperature
effects
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stages on the yield of the wheat ear. Aust. J. Agric. Res. 28: 11-27.
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