Effects of elevated atmospheric CO
2
and drought stress on
individual grain filling rates and durations of the
main stem in spring wheat
Aiguo Li
a,∗, Yuesheng Hou
b, Anthony Trent
c,1aNEI/NIH, Building 6 Rm 313, 6 Center Dr., MSC 2740, Behesda, MD 20892-2740, USA bWeed Science Laboratory, USDA-ARS, Washington State University, Pullman, WA 99164, USA cDepartment of Plant, Soil and Entomological Sciences, University of Idaho, Moscow, ID 83844-2339, USA
Received 1 May 2000; received in revised form 21 August 2000; accepted 14 September 2000
Abstract
Rate and duration of individual grain growth determine final kernel weight and are influenced by environmental factors. The objectives of this research were to assess the effects of elevated CO2and drought stress on the grain filling rate and duration,
and the weight of individual kernels. Spring wheat (Triticum aestivumL.) was grown in a free air CO2enrichment (FACE)
system on the demonstration farm at the University of Arizona Maricopa Agricultural Center with a split-block design of four replications. Mainplots were 550 or 370mmol mol−1of atmospheric CO2concentrations and subplots were two irrigation
treatments. The weights of individual kernels from upper, middle, and lower spikelets of the main stem spike were fitted into nonlinear cumulative logistic curves as a function of accumulated thermal units using SAS proc NLIN. Rate and duration of individual grain filling varied greatly depending on floret positions and environmental factors. The combination of these changes determined the final weight of individual kernels. The rank order of kernel weights among kernel positions within a middle and lower spikelet was not affected by either elevated CO2or water stress treatments in this study. Elevated CO2
often stimulated the rate of individual grain filling, whereas the well-watered condition extended duration of individual grain filling. Furthermore, kernels further from the rachis or nearest to the rachis were affected proportionately more than those towards the center of a spikelet. The information from this research will be used to model wheat grain growth as a function of climate. © 2001 Elsevier Science B.V. All rights reserved.
Keywords:Elevated CO2; Drought stress; Wheat; Grain filling rate; Grain filling duration; Yield
Abbreviations: A, ambient CO2 concentration; ATU,
accumu-lated thermal unit; D, drought stress condition; E, elevated CO2
concentration; FACE, free air CO2 enrichment; MS, main stem;
W, well-watered condition
∗Corresponding author. Tel.:+1-301-402-0964; fax:+1-301-402-1883.
E-mail address:[email protected] (A. Li).
1Idaho Agricultural Experiment Station Research Paper No. 99722.
1. Introduction
Individual kernel weight, one of the three yield components in wheat, is determined by both rate and duration of grain filling (Wiegand and Cuellar, 1981). Individual kernel weight of mature grains varies among various positions within a spike, and even 0168-1923/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved.
within a spikelet (Bremner, 1972). At 9 days after anthesis, the largest kernels are located in the central spikelet, whereas at maturity, the largest kernels are more towards the base of a spike. Within a spikelet, when assimilate supply is limited, the first kernel (proximal) is the largest; when assimilate supply is sufficient, the second and third kernels exceed the first kernel in weight (Bremner and Rawson, 1978; Simmons and Moss, 1978). Generally, the distal ker-nels (fourth and fifth) within a spikelet are smaller than the proximal kernels (first and second) in size (Simmons and Crookston, 1979).
Grain growth in wheat consists of three phases: a lag, a linear, and a plateau phase (Wheeler et al., 1996). During the initial lag phase, which follows anthesis and only lasts for a few days in wheat, the number of cells per kernel is determined (Brocklehurst, 1977) and there is little increase in grain dry weight. Grain dry weight then increases linearly until a maximum dry weight is achieved. Differences in grain filling rate or duration during this phase are important in ex-plaining variation in individual final kernel dry weight (Pinthus and Sar-Shalom, 1978; Simmons and Crook-ston, 1979; Gebeyehou et al., 1982). After the linear phase, grain dry weight remains stable while the grain dries, or declines slightly, depending on the cultivar. Proximal florets usually reach anthesis 2–4 days ear-lier than distal ones (Evans et al., 1972; Simmons and Crookston, 1979). This behavior can lead to a longer grain filling duration of proximal kernels because the cessation of grain filling occurs at approximately the same time for all kernels in a spikelet (Simmons and Crookston, 1979). Further, some kernels grow faster than others due to a greater grain filling rate which could be caused by ample assimilate supply and dif-ferences in kernel growth potential (Bremner, 1972; Bremner and Rawson, 1978). The contribution of grain filling rate and duration to final kernel weight remains unclear. Darroch and Baker (1990) reported a positive association between the rate of linear grain filling phase and final grain weight, while Gebeye-hou et al. (1982) found that both rate and duration of grain filling were positively associated with the final grain weight.
Grain growth has been described by various math-ematical models (Simmons and Crookston, 1979; Wiegand and Cuellar, 1981; Bauer et al., 1985). Bauer et al. (1985) described individual kernel growth with
a cubic polynomial curve and determined that grain filling rates for lag phase, constant phase, and post linear phase were, respectively, 0.022, 0.049, and 0.019 mg GDD−1kernel−1, whereas others predict grain filling rates in the linear phase of 0.04 and 1.98 mg spike−1day−1 over various floret locations and in different varieties (Simmons and Crookston, 1979; Wiegand and Cuellar, 1981). Simmons and Crookston (1979) reported that distal kernels exhib-ited a lower growth rate than proximal kernels during the linear phase of grain filling.
The rate and duration of grain filling are affected by environmental factors. The relationship between temperature and grain filling rate and duration has been well documented. A higher temperature accel-erates rate and shortens duration of individual kernel grain filling (Sofield et al., 1977), while a lower temperature prolongs duration of individual kernel grain filling (Wiegand and Cuellar, 1981). The op-timum temperature for individual kernel growth is 12–18◦C (Chowdhury and Wardlaw, 1978; Wardlaw and Moncur, 1995). On average, grain filling duration is shortened about 3.3 days◦C−1and reduction in ker-nel weight of 3–5% for each 1◦C increase above the optimum temperature (Wiegand and Cuellar, 1981). The rate of individual kernel growth did not respond to irradiance in cultivars where kernel number per spike was affected by radiation, while with those in which kernel number was less affected by radiation, the rate of individual kernel growth was highly re-sponsive to radiation, especially in the distal kernels (Sofield et al., 1977). The duration of individual ker-nel growth was not influenced by radiation (Sofield et al., 1977). Nitrogen has little effect on the rate of grain growth. If available nitrogen exceeded the amount needed for the greatest kernel yield, however, the duration decreased (Bauer et al., 1985). Informa-tion on the effect of water on individual kernel growth is limited. No information on the effect of elevated CO2on rate and duration of individual grain filling is available.
2. Materials and methods
2.1. Experimental facilities and design
Spring wheat (cv. Yecora Roja) was planted on the demonstration farm at the University of Arizona Maricopa Agricultural Center using a split-block de-sign of four replications (Fig. 1). Mainplots were atmospheric CO2 concentrations of 550mmol mol−1 (elevated) or 370mmol mol−1 (ambient). The ele-vated CO2 concentration was maintained using the free air CO2 enrichment (FACE) system. Subplots were two levels of irrigation treatments: well-watered which allowed only 30% of the available water in the rooted zone to be depleted (as determined from estimates of potential evapotranspiration obtained from an on-farm meteorological station), and drought stressed (or limited-water treatment) which supplied only half as much as the well-watered treatment at each irrigation. Water was supplied by a sub-surface drip irrigation system (0.5 m tube spacing, 0.3 m emit-ter spacing, 0.2 m depth). The cumulative irrigation amounts from emergence to harvest were 600 and 275 mm for well-watered and drought stress
treat-Fig. 1. FACE facilities in the field.
ments, respectively (Lewin et al., 1992). The wheat was planted on 15 December 1992 and emerged on 1 January 1993. Final harvest was on 24 May 1993. Enrichment of CO2in the elevated CO2 plots started on the day of emergence. Plants were grown in rows spaced 0.25 m apart with 130 plants m−2and received 277 kg N ha−1 and 44 kg P ha−1 over the growing season. Air temperature was measured 2 m above the soil surface. Accumulated thermal units (ATUs) were calculated as:
ATU=X
T
max+Tmin
2 −Tb
(1) whereTmax and Tmin represent daily maximum and minimum air temperatures based on hourly readings, respectively.Tb=0 is the base temperature for wheat (Bauer et al., 1985).
2.2. Sample collection and processing
Fig. 2. Wheat plants harvested from each treatment.
Fig. 3. The illustration of spikelets removed from the main stem spike (a) and numbering specific floret positions (b). was identified and spikelet numbers on a spike were counted. The spike was separated into three sections: the upper section containing about one quarter of the spikelets, the middle section containing about one half of the spikelets, and the lower section containing about one quarter of the spikelets (Fig. 3a). Three spikelets: the middle spikelet of the middle section, the second spikelet of the upper section from the middle, and the second spikelet of the lower section from the middle, were removed from the spike of the main stem for each plant (Fig. 3a). Samples were dried for 14 days at 70◦C in an oven, put into a desiccator and allowed to cool. Then each kernel was weighed to nearest of 0.1 mg. Starting from proximal kernels to distal ones, the kernels were named the first, second, third, and fourth kernels within a spikelet (Fig. 3b).
2.3. Mathematical analysis
of the grain filling duration correspond to 0.5M. Here M represents final grain weight (g). Although total grain yield never reaches its asymptotic maximumM, L measures the duration to 0.95M in this paper. The equation of the modified logistic curve is shown below
Y =M
P
P +e−B(X−L)
(2) whereYis the estimated grain weight (g),Xthe ATU’s from emergence, M the estimated final grain weight (g), B the slope of logistic curve and related to the grain filling rate, and La measure of the completion of the grain filling process in ATUs. The grain filling duration (d) was defined as the period between an in-dividual kernel weight of 0.003 g to 95% of the final kernel weight. A significant difference in duration in this paper was defined by a difference in two kernels’ durations greater than standard errors of the comple-tion of individual kernel growth. After each model was estimated, model adequacy was assessed by residual analysis. Following this, models were compared us-ing a full model dummy variable procedure (Bates and Watts, 1988). Duguid and Brûle-Babel (1994) have defined the maximum rate of grain filling (R) based on logistic model parameters as:R= 1
4MB. Although an exact test forRis not possible, simultaneous con-trasts of parametersMandBprovide an approximate test. All computations were carried out using SAS 6.12 (SAS Institute, 1989).
3. Results
3.1. Grain growth on various positions of a spike over four treatments
3.1.1. Kernel growth on the upper spikelet
Only the first and second kernels in the upper spikelet had enough data to fit the nonlinear cumu-lative logistic curves. Under ambient CO2 and water stress conditions, the grain filling process of the first kernel was different from that of the second kernel (Table 1). This was caused by a lower grain filling rate (Table 2) and a longer grain filling duration within the second kernel (Fig. 4A). We did not find a difference in final kernel weight between the first and second kernels (Table 1). We also detected a difference in the grain filling process between the first and second
kernels under elevated CO2 and well-watered condi-tions (Table 1), in which the first kernel was 2.0 mg heavier than the second one (Table 2) due to a longer grain filling duration (Fig. 4A).
3.1.2. Kernel growth on the middle spikelet
The ranking of kernel weights within a middle spikelet was the second > the first > the third >
Fig. 4. Grain filling durations in thermal units for individual kernels on the upper (A), middle (B), and lower spikelets (C) for different treatments. AD — ambient CO2 and drought stress condition, AW — ambient CO2 and well-watered condition, ED — elevated CO2and
Table 2
Parameter estimates (±asymptotic standard errors) for the upper spikelet of the main stem over ambient (A) and elevated (E) CO2concentrations, and drought stress (D) and well-watered (W)
treatments: maximum kernel weights (mg) per kernel (M), slopes of logistic curves (B), grain filling rates (Ra), and proportion of variances explained by regressions (var.%)
Treatment Parameter Estimates
First kernel Second kernel
AD M 41.7±0.7 43.4±0.9
B 0.0104±0.0008 0.0083±0.0014
R 0.11 0.09
Var.% 99 99
AW M 45.9±0.7 44.5±0.8
B 0.0085±0.0005 0.0091±0.0006
R 0.10 0.10
Var.% 98 99
ED M 43.8±0.6 45.7±1.0
B 0.0109±0.0007 0.0105±0.0011
R 0.12 0.12
Var.% 99 98
EW M 48.5±0.8 46.5±0.8
B 0.0077±0.0005 0.0090±0.0006
R 0.09 0.10
Var.% 99 99
aR= 1
4MB, the test ofR is for difference in parametersM
andB.
3.1.3. Kernel growth on the lower spikelet
Significant differences in grain filling processes were found between the first and third, the first and fourth, the second and third, the second and fourth, and the third and fourth kernels over all four growth conditions (Table 1). As with the middle spikelet, however, grain filling processes of the first and second kernels were not different except under ambient CO2 and well-watered conditions (Table 1). The ranking of the kernel weights in the lower spikelet was the same as that of middle spikelet, i.e. the second>the first>
the third>the fourth kernels. The second kernel had a faster grain filling rate (Table 6), and a longer grain filling duration than the fourth kernel in all treatments (Fig. 4C). The first kernel weights under all four treatments were heavier than those of the third kernel due to a faster grain filling rate under well-watered condition (Tables 1 and 6) and a longer grain filling duration under drought stress condition (Fig. 4C). The weight of the first kernel was higher than that of the fourth kernel due to both a longer grain filling
duration and a faster grain filling rate (Tables 1 and 6). The third kernel weighed more than the fourth kernel for various reasons. Under ambient CO2, the third kernel had a greater grain filling rate than the fourth kernel, and difference in grain filling duration was not detected. Under elevated CO2 and drought stress conditions, however, both a longer grain filling duration and a faster grain filling rate contributed to the increase in weight of the third kernel (Tables 1 and 6).
3.2. Effects of elevated CO2on the grain growth 3.2.1. Kernels on the upper spikelet
Elevated CO2 affected grain filling processes of both the first and second kernels at the upper spikelet under either drought stress or well-watered condition (Table 3). Under drought stress condition, elevated CO2 increased the grain filling rate of the first and second kernels and led to final kernel weight increases of 2.1 and 2.3 mg, respectively, compared to those un-der ambient CO2(Table 2). Under well-watered con-dition, elevated CO2resulted in an increase in the first kernel weight of 2.6 mg due to a longer grain filling
Table 3
Individual kernel grain filling processes of the main stem upper spikelet; maximum grain weights (mg) per kernel (M), and grain filling rates (Ra) contrast over ambient (A) and elevated (E) CO
2
concentrations, and drought stress (D) and well-watered (W) treat-ments
Contrasts First Kernel Second Kernel
d.f. F d.f. F
4MB, the test ofRis for difference in parametersM
andB.
duration, but had no effect on the second kernel weight (Tables 2 and 3).
3.2.2. Kernels on the middle spikelet
Elevated CO2influenced the grain filling processes of all the kernels on the middle spikelet over two water regimes (Table 5). Under drought stress condi-tions the weight of both the first and fourth kernels increased 4.0 mg by elevated CO2, due to an enhanced grain filling rate. However, this did not occur under well-watered conditions (Table 4). Under drought stress condition, elevated CO2 shortened the grain filling duration (Fig. 4B) and increased the grain filling rate of the second and third kernels (Tables 4 and 5). However, a faster grain filling rate may have compensated for the shorter grain filling duration and difference in final kernel weight was not detectable (Table 4).
3.2.3. Kernels on the lower spikelet
The grain filling processes of the first, second, third and fourth kernels at the lower spikelet over two water treatments were influenced by elevated CO2
Table 4
Parameter estimates (±asymptotic standard errors) for the middle spikelet of the main stem over ambient (A) and elevated (E) CO2
concentrations, and drought stress (D) and well-watered (W) treatments: maximum kernel weights (mg) per kernel (M), slopes of logistic curves (B) and grain filling rates (Ra), and proportion of variances explained by regressions (var.%)
Treatments Parameters Estimates
First kernel Second kernel Third kernel Fourth kernel
AD M 47.3±0.9 52.1±1.4 45.6±1.8 31.5±1.2
B 0.0102±0.0008 0.0082±0.0009 0.0081±0.0013 0.0099±0.0017
R 0.12 0.11 0.09 0.08
Var.% 98 97 95 97
AW M 52.1±1.1 55.9±1.0 48.0±1.0 33.2±1.6
B 0.0073±0.0005 0.0079±0.0005 0.0087±0.0007 0.0092±0.0017
R 0.10 0.11 0.10 0.08
Var.% 98 99 98 97
ED M 51.3±0.7 52.2±0.6 46.6±1.0 35.5±1.3
B 0.0101±0.0006 0.0106±0.0006 0.0103±0.0012 0.0101±0.0015
R 0.13 0.14 0.12 0.09
Var.% 99 99 98 97
EW M 53.9±0.9 56.3±1.0 48.2±0.9 36.1±1.5
B 0.0082±0.0005 0.0085±0.0006 0.0089±0.0006 0.0081±0.0013
R 0.11 0.12 0.11 0.07
Var.% 99 99 99 98
aR= 1
4MB, the test ofRis for difference in parametersMandB.
(Table 7). Elevated CO2 increased the grain filling rate of the first and fourth kernels, and led to increases in the final kernel weight of 2.8 and 6.3 mg under drought stress conditions, respectively, and 5.0 and 7.5 mg under well-watered conditions, respectively (Tables 6 and 7). Elevated CO2 increased the second kernel weight by 2.8 mg under drought stress condi-tion which was due to an increased grain filling rate. This did not occur under the well-watered condition though (Tables 6 and 7). The fourth kernel was more responsive to elevated CO2than the other kernels.
3.3. Effects of drought stress on the grain growth
3.3.1. Kernels on the upper spikelet
Table 5
Individual kernel grain filling processes of the main stem middle section; maximum grain weights (mg) per kernel (M), grain filling rates (Ra) and the completion of grain filling processes (L) contrast over ambient (A) and elevated (E) CO2 concentrations, and drought stress
(D) and well-watered (W) treatments
Contrasts Kernel 1 Kernel 2 Kernel 3 Kernel 4
d.f. F d.f. F d.f. F d.f. F
AD–AW 3 11.88∗∗∗ 3 14.41∗∗∗ 3 9.00∗∗∗ 3 4.21∗∗
AD–ED 3 9.29∗∗∗ 3 5.07∗∗ 3 3.46∗ 3 4.11∗∗
AW–EW 3 18.79∗∗∗ 3 19.42∗∗∗ 3 8.41∗∗∗ 3 4.49∗∗
ED–EW 3 4.66∗∗∗ 3 3.97∗∗ 3 2.64∗ 3 2.06
MAD–MED 1 11.98∗∗∗ 1 0.02 1 0.37 1 5.66∗
RAD–RED 2 9.21∗∗∗ 2 6.48∗∗ 2 5.00∗ 2 4.04∗
MAW–MEW 1 1.67 1 0.05 1 0.01 1 1.58
RAW–REW 2 9.31∗∗∗ 2 5.57∗∗ 2 1.59 2 1.57
MAD–MAW 1 12.34∗∗∗ 1 3.17 1 1.55 1 0.66
RAD–RAW 2 12.17∗∗∗ 2 3.37∗ 2 0.82 2 1.02
MED–MEW 1 4.64∗ 1 8.11∗∗ 1 0.93 1 0.09
RED–REW 2 5.00∗∗ 2 5.13∗∗ 2 2.40∗ 2 1.05
aR= 1
4MB, the test ofRis for difference in parametersMandB.
∗Significant at 0.05 level. ∗∗Significant at 0.01 level. ∗∗∗Significant at 0.001 level.
Table 6
Parameter estimates (±asymptotic standard errors) for the lower spikelet of the main stem over ambient (A) and elevated (E) CO2
concentrations, and drought stress (D) and well-watered (W) treatments: maximum kernel weights (mg) per kernel (M), slopes of logistic curves (B), grain filling rates (Ra), and proportion of variances explained by regressions (var.%)
Treatments Parameters Estimates
First kernel Second kernel Third kernel Fourth kernel
AD M 49.5±1.0 50.9±1.2 45.5±1.1 34.2±1.3
B 0.0087±0.000 0.0096±0.0009 0.0099±0.00096 0.0112±0.0019
R 0.11 0.12 0.11 0.10
Var.% 99 98 98 97
AW M 51.3±1.2 55.6±1.2 48.2±1.2 37.5±1.6
B 0.0077±0.000 0.0076±0.0005 0.0078±0.0006 0.0084±0.0013
R 0.10 0.11 0.09 0.08
Var.% 98 99 98 98
ED M 52.3±0.9 53.7±0.9 46.9±1.1 40.5±1.2
B 0.0093±0.000 0.0094±0.0007 0.0106±0.0012 0.0108±0.0022
R 0.12 0.13 0.12 0.11
Var.% 99 99 98 98
EW M 56.3±1.0 56.5±1.0 50.6±1.2 45.0±2.3
B 0.0078±0.000 0.0079±0.0005 0.0083±0.0007 0.0090±0.0016
R 0.11 0.11 0.10 0.10
Var.% 99 99 98 96
aR= 1
Table 7
Individual kernel grain filling processes of the main stem lower spikelets; maximum grain weights (mg) per kernel (M), and grain filling rates (Ra) contrast over ambient (A) and elevated (E) CO
2 concentrations, and drought stress (D) and well-watered (W) treatments
Contrasts Kernel 1 Kernel 2 Kernel 3 Kernel 4
d.f. F d.f. F d.f. F d.f. F
∗Significant at 0.05 level. ∗∗Significant at 0.01 level. ∗∗∗Significant at 0.001 level.
3.3.2. Kernels on the middle spikelet
Drought stress influenced the grain filling pro-cesses of all the kernels on the middle spikelet under both elevated and ambient CO2levels with the excep-tion of the fourth kernel at elevated CO2 condition (Table 5). The first kernel weight increased 4.8 and 2.6 mg under ambient CO2 and elevated CO2 levels, respectively, due to a prolonged grain filling dura-tion (Fig. 4B). The final weight of the second kernel increased 4.1 mg in the well-watered treatment at elevated CO2 again due to a prolonged grain fill-ing duration (Fig. 4B). The third and fourth kernels were less responsive to drought stress conditions than the first and second kernels in this experiment (Table 5).
3.3.3. Kernels on the lower spikelet
All the grain filling processes of the individual ker-nels at the lower spikelet were influenced by drought stress conditions over both CO2 levels (Table 7). Well-watered conditions increased the final kernel weights of the first, third, and fourth kernels under elevated CO2 due to a longer grain filling duration (Fig. 4C), but not under ambient CO2(Tables 6 and 7). The final weight of the second kernel increased by well-watered condition due to a longer grain fill-ing duration under the ambient CO2 condition only
(Tables 6 and 7). Water treatment had the most effect on the fourth kernel weight.
4. Discussion
be associated with the increased individual leaf or canopy assimilation (Pinter et al., 1996; Garcia et al., 1998). Also kernels that are more proximal or distal to the rachis were affected proportionately more than those towards the center of a spikelet.
The ranking of individual kernel weights within a spikelet was the second > the first > the third >
the fourth kernels with the exception in the upper spikelet. It was not affected by elevated CO2 or wa-ter stress treatments in this experiment even though the differences among the kernels might change over various treatment and floret positions. The study of the vascular anatomy of a spikelet (Zee and O’Brien, 1971; Hanif and Langer, 1972) has indicated that the first, second, and third kernels are connected to the transfer cells in the rachis by independent vascular strands and the fourth kernel is connected to the main assimilate source by strands originating near the base of the third kernel, and so on. By this anatomical construction, the first kernel has priority to the assim-ilate supply and also it has an advantage because it is fertilized earlier than the rest of kernels on the same spikelet. However, the morphogenesis was modified in the later stage of grain growth and the second kernel exceeded the first one in weight at maturity (Bremner and Rawson, 1978). This may be because the second kernel has a larger growth potential than the other kernels. Within a spikelet the order of the individual kernel weight is determined structurally and genetically. In this experiment, water stress and elevated CO2 treatments did not change the ranking of kernel weights, even though the absolute weights of kernels were changed.
The grain filling process is determined by assim-ilate supply, intrinsic resistance to assimassim-ilate trans-portation, and kernel growth potential (Bremner and Rawson, 1978). For a specific kernel within a spikelet, the combination of these three factors, as well as en-vironmental factors, determines the grain filling rate and duration. The combination of rate and duration changes determines the final kernel weight. Elevated CO2and well-watered conditions may directly affect the grain filling process by providing more assimilate (Pinter et al., 1996), and may also potentially influ-ence the development of the plant at cellular or molec-ular levels, further impacting the growth potential and intrinsic resistance. We reported in a previous paper that the final grain weight of the main stem spike is
increased 4% by elevated CO2of 550mmol mol−1and 7% by well-watered conditions (Li et al., 2000). It is probable that the increase from distal and proximal kernels contributes more to the main stem total grain weight.
5. Conclusion
The rate and duration of individual grain filling varies greatly depending upon floret positions and en-vironmental factors. The combination of these factors determines the final individual kernel weight. The ranking order based on individual kernel weight was the second > the first > the third > the fourth ker-nels with the exception in the upper spikelet. Water stress and elevated CO2treatments in this study were not enough to change this ranking. However, kernels farther from the rachis or nearest to the rachis were influenced proportionately more than those towards the center of a spikelet.
Acknowledgements
We acknowledge the helpful suggestions on the data collection provided by Dr. Gerard W. Wall and the statistical analysis provided by Mr. William J. Price. We also acknowledge the cooperation of the USDA-ARS, Water Conservation Laboratory and the University of Arizona Maricopa Agricultural Center. Research support was provided by the United States Department of Agriculture, Agriculture Research Service, and the FACE apparatus was furnished by Brookhaven National Laboratory. This work con-tributes to the Global Change Terrestrial Ecosystem (GCTE) Core Research Program, which is part of the International Geosphere–Biosphere Program.
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