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CROP PHYSIOLOGY & METABOLISM

Free-Air CO₂ Enrichment and Drought Stress Effects on Grain Filling Rate and Duration in Spring Wheat

^a NEI/NIH, Bld. 6 Rm 304, 6 Center Dr., Bethesda, MD 20892-2740 USA
^b Weed Science Lab., USDA-ARS, Washington State Univ., Pullman, WA 99164 USA
^c Jr., Water Conservation Lab., USDA-ARS, 4331 E. Broadway Rd., Phoenix, AZ 85040 USA
^d Associate Professor of Plant Sciences, Univ. of Idaho, Idaho Agric. Exp. Stn. Res. Paper No. 97735. Dep. of Plant, Soil and Entomological Sciences, Univ. of Idaho, Moscow, ID 83844-2339 USA

lia{at}intra.nei.nih.gov

	ABSTRACT

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Wheat grain weight is a function of rate and duration of grain growth and is affected by photosynthate supply. Drought stress reduces photosynthate production because of stomatal closure. However, this might be partially overcome by an increase in air CO₂ concentration. This study was conducted to evaluate elevated CO₂ and drought stress effects on grain-filling rate and duration for spring wheat (Triticum aestivum L.). Spring wheat (cv. Yecora Roja) was grown at two CO₂ concentrations, 550 (elevated) or 370 (ambient) µmol mol^-1 and two water treatments in a Free-Air CO₂ Enrichment (FACE) system at the University of Arizona Maricopa Agricultural Center. Plant samples were collected every 3 to 4 d from 6 d after anthesis until plant maturity. Main stem spikes were separated into upper, middle, and lower sections. Grain weight data for the intact main stem spike, each of its sections, and intact tiller spikes were fitted to a cumulative logistic model. Both elevated CO₂ and water treatments significantly influenced the grain-filling processes. Under drought stress conditions, elevated CO₂ increased grain weight in the upper and lower sections of the main stem spike by 10 and 24%, respectively. In well-watered plants, final grain weight in the midsection of the main stem spike was 8% higher than that measured under drought stress conditions. Grain weight increase under elevated CO₂ was due to a faster rate of grain filling. Effects of elevated CO₂ on the statistically derived duration of grain filling were inconclusive because of the confounding effect of blower-induced temperature changes on the process. An increase in grain weight of well-watered plants was due to a longer grain-filling period. Later-formed tiller spikes were more responsive to elevated CO₂ and drought stress than main stem spikes. Information from this study will help us understand the grain growth of wheat and provide information to establish grain growth mechanism.

Abbreviations: ATU, accumulated thermal units • FACE, free-air CO₂ enrichment • T1, T2, and T3, primary tillers

	INTRODUCTION

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THE CONCENTRATION of atmospheric CO₂ is predicted to double above pre-industrial levels by the middle of the next century. With the rise in air temperatures, more frequent drought stresses are predicted to occur. Therefore, elevated CO₂ and water stress should be evaluated for their effects on grain yield which is a function of the number of grains and their grain-filling rate and duration. Both genetic (Wiegand and Cuellar, 1981; Mashiringwani et al., 1994; Mou and Kronstad, 1994) and environmental factors (Sofield et al., 1977; Wiegand and Cuellar, 1981; Bauer et al., 1985; Wheeler et al., 1996) affect grain-filling rate and duration. Important environmental factors include temperature, irradiance, CO₂ concentration, and soil water. Higher temperatures accelerate assimilation rate and enhance movement of photosynthate from flag leaf to spike, but shorten the grain-filling duration (Sofield et al., 1977; Wiegand and Cuellar, 1981; Bruckner and Frohberg, 1987). Greater irradiance increases grain-filling rates by enhancing photosynthesis, but has little effect on duration of linear grain filling (Sofield et al., 1977). Although CO₂ enrichment to 680 µmol mol^-1 has been shown to increase grain-filling rates by 21 to 24% per spike over the temperature range of 14 to 20°C, respectively, linear grain-filling duration was unaffected by CO₂ enrichment (Wheeler et al., 1996). Information regarding the effect of water deficit on grain-filling rate and duration is inconclusive, and conflicting results exist on the importance of rate and duration of grain filling in contributing to the final yield. Some researchers have suggested that differences in final grain weight were primarily determined by the difference in grain-filling rate (Nass and Reiser, 1975). Others believe that grain-filling duration contributes more to higher grain yield than the grain-filling rate (Gebeyehou et al., 1982; Wong and Baker, 1986). Information on tiller versus main stem spike grain-filling rate and duration in the literature is limited.

The grain-filling process has been described by various mathematical functions (Nass and Reiser, 1975; Simmons and Crookston, 1979; Bauer et al., 1985; Bruckner and Frohberg, 1987; Darroch and Baker, 1990; Wheeler et al., 1996). The logistic function, which includes a lag phase, linear phase and a plateau phase, is an effective and flexible method for describing the grain-filling process (Darroch and Baker, 1990; Duguid and Brûlé-Babel, 1994). During initial lag phase, which lasted a few days, the number of grain per spike is established (Bremner and Rawson, 1978). During linear phase grain dry weight and size increase significantly. By including the lag phase in the logistic model (i.e., initial phase of grain filling), the entire time course of grain filling is quantified and grain-filling rate and maximum grain weight are estimated so that statistical comparisons can be made among models for the entire grain-filling process, or portions thereof. Univariate analysis of variance and stepwise multivariate analysis of variance have been used to determine importance of those variables in characterizing the grain-filling curves (Darroch and Baker, 1990; Duguid and Brûlé-Babel, 1994). However, direct contrast of model parameters may be more appropriate.

Our objectives in this research were (i) to estimate grain weights per main stem spike, or per spike section, and grain weights per spike for tiller spikes; (ii) to estimate the grain-filling rates and durations of the main stem spike, of its sections, and of primary tiller spikes; and (iii) to compare these parameters to assess the influence of atmospheric CO₂ concentration and drought stress on grain growth in spring wheat.

	Materials and methods

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Experimental Facilities and Design
Spring wheat (cv. Yecora Roja) was grown on the demonstration farm at the University of Arizona Maricopa Agricultural Center. The experiment was designed as a split-block with four replications. Main plots were atmospheric CO₂ concentration of 550 (elevated) or 370 (ambient) µmol mol^-1. The elevated CO₂ concentration was maintained with the Free Air CO₂ Enrichment (FACE) system, which was designed by Brookhaven National Laboratory to generate an elevated CO₂ environment in four areas, each 25 m in diameter, in an open field. The areas were circled by an array of 32 vertical vent pipes, which were metered to pump air enriched with CO₂ to the field (Lewin et al., 1992). The ambient CO₂ circles were 90 m apart to avoid contamination of elevated CO₂. In retrospect, there was a design flaw in the experiment because there were blowers in the FACE plots but none in the control plots. These blowers increased air turbulence at night in the enriched plots with a consequent slight warming of canopy temperature (Pinter et al., 1997). Subplots consisted of two levels of irrigation treatment: 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 emitter spacing, 0.2-m depth). The cumulative irrigation amounts from emergence to harvest were 600 and 275 mm for well-watered and drought stress treatments, respectively. The wheat was planted on 15 Dec. 1992 and emerged on 1 Jan. 1993. Final harvest was on 24 May 1993. Enrichment of CO₂ in the elevated CO₂ rings started on the day of emergence. Plants were grown in rows spaced 0.25 m apart with 130 plants m^-2 and 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 (ATU) were calculated as follows:

(1)

where T_max and T_min represent daily maximum and minimum air temperatures based on hourly readings. T_b = 0 is the base temperature for wheat (Bauer et al., 1985). Only positive average daily temperatures were included in the calculation of accumulated thermal units (Baker and Gallagher, 1983).

Sample Collection and Processing
Flowering began on 23 April and 50% of spikes had flowered by 26 April (a spike was defined as flowering when stamens began to be visible on the middle spikelets of the spike). Sampling for grain-filling assessment began on 2 May, with nine plants per subplot sampled every 3 to 4 d until the plants matured. From this sample, three plants were randomly subsampled. The culms on the subsampled plants were identified by locating the main stem first, which is not always the tallest, but always has fewer aborted spikelets, and a triangle-shaped first internode that is attached to seed roots when the plant is dissected. Alternatively, if main stem spike identification was difficult, the culm was identified by locating the tiller group first. The number of spikelets on the main stem spike were counted and 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. (A spike of 20 spikelets was separated into 5, 10, 5 spikelets for upper, middle and lower sections; a spike of 21 spikelets, was 5, 11, 5; of 22 spikelets, was 5, 11, and 6; of 23 spikelets, was 5, 12, 6; of 24 spikelets, was 6, 12, and 6; of 25 spikelets, was 6, 13, and 6.) Spikes on the three primary tillers (T1, T2, and T3) were kept in three separate bags. Samples were dried for 14 d at 70°C in an oven and placed in a desiccator at room temperature for 5 h. Specimens were threshed by hand and the grains weighed.

Mathematical Analysis
The grain weights from the intact main stem spike, its sections and tiller spikes were fitted to a non-linear cumulative logistic curve as a function of ATUs. A common form of this equation is (Torres and Frutos, 1990):

(2)

where Y is estimated grain weight (g), X represents the ATUs from emergence, M is estimated final grain weight (g). B is related to the grain-filling rate and L is a measure of grain-filling duration. In this form, L estimates the grain-filling duration correspond to 0.5 M. Although total grain yield never reaches its asymptotic maximum M, grain filling is thought to be complete when Y = 0.95 M (Darroch and Baker, 1990). Thus, it is desired that L measures the duration to 0.95 M. By introducing a constant P into this equation, we can estimate the duration of grain filling to any fractional point (William J. Price, 1996, personal communication). Equation [2] then becomes

(3)

where Y, X, M, B, and L are as defined before. These changes do not affect the definition or estimation of parameters M and B. When P = 1, the standard logistic Eq. [2] results. At Y = 0.95 M and X = L, Eq. [3] reduces to 0.95 M = (PM)/(P + 1), yielding P = 19. This value was used in the estimation of Eq. [3] parameters. The grain-filling rate is related to Parameter B in Eq. [3]. As grain-filling rate increases, the slope of the line also increases. However, overall rate is also a function of M. Duguid and Brûlé-Babel (1994) have defined the maximum rate of grain filling (R) on the basis of logistic model parameters as

(4)

After each model was estimated, model adequacy was assessed by residual analysis. Following this, the models were compared by a full model dummy variable procedure (Bates and Watts, 1988). Although an exact test for R is not possible, simultaneous contrasts of Parameters M and B provide an approximate test. All computations were carried out with SAS 6.12 (SAS Institute, Ver. 6).

	Results

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Convergence criteria for regression were met for all logistic models. The asymptotic correlation matrices for the parameter estimates of all models were less than 0.99, indicating the models were not over parameterized. The regression equations explained more than 91% of the variance for all models (Tables 1, 2, and 3) . Residual analyses showed no patterns or trends and all residuals were of acceptable magnitude.

View this table: [in this window] [in a new window]   Table 1 Parameter estimates (± asymptotic standard error); maximum grain weights (g) per main stem spike (M), slope of logistic curve (B), and grain-filling durations in ATU (L) for main stem spike, asymptotic correlation matrices for the parameter estimates, grain-filling rates in g ATU-1 spike-1 (R), and proportion of variance explained by regression (var. %)

View this table: [in this window] [in a new window]   Table 2 Parameter estimates (± asymptotic standard errors); maximum grain weights (g) per section of the main stem spike (M), slope of logistic curve (B), and grain-filling durations in ATU (L) for upper, middle, and lower sections of the main stem spike, grain-filling rates in g ATU-1 section-1 (R), and proportion of variance explained by regressions (var. %)

View this table: [in this window] [in a new window]   Table 3 Parameter estimates (± asymptotic standard errors), maximum grain weights (g) per tiller spike (M), slope of logistic curve (B), and grain-filling durations in ATU (L) for T1, T2, and T3 spikes, grain-filling rates in g ATU-1 spike-1 (R), proportion of variance explained by regressions (var. %)

View this table: [in this window] [in a new window]   Table 4 Main stem spike grain-filling processes, maximum grain weights (g) per main stem spike (M), grain-filling rates in g ATU-1 spike-1 (R) and durations in ATU (L) contrasted over ambient (A) and elevated (E) CO2 concentrations, and drought stress (D) and well-watered (W) treatments

View larger version (25K): [in this window] [in a new window]   Fig. 1 The main stem spike total grain weight as a logistic function of accumulated thermal units over different CO2 and water regimes. Vertical error bars represent observed means ± standard errors. AD = ambient CO2 and dry treatment; AW = ambient CO2 and well-watered treatment; ED = elevated CO2 and dry treatment; EW = elevated CO2 and well-watered treatments

View this table: [in this window] [in a new window]   Table 5 Grain-filling processes for upper, middle, and lower sections of the main stem spike, maximum grain weights (g) per section (M), grain-filling rates in g ATU-1 section-1 (R), and grain-filling durations in ATU (L) contrasted over ambient (A) and elevated (E) CO2 concentrations, and drought stress (D) and well-watered (W) treatments

View larger version (30K): [in this window] [in a new window]   Fig. 2 Logistic functions representing grain weights of upper, middle and lower sections of the main spike as a function of accumulated thermal units over dry treatment (left) and well-watered treatment (right). Vertical error bars represent predicted value ± standard errors. AD = ambient CO2 and dry treatment; AW = ambient CO2 and well-watered treatment; ED = elevated CO2 and dry treatment; EW = elevated CO2 and well-watered treatments

Grain-Filling Processes of Tiller Spikes
Grain-filling processes of T1 and T2 under both well-watered and limited-water conditions were changed by elevated CO₂ treatments (Table 6) . Under drought stress conditions, elevated CO₂ concentration increased final grain weights up to 21% for T1 spike, and 37% for T2 spike (Table 3). Under well-watered conditions, elevated CO₂ enhanced the grain weight of T1, but not T2 (Table 6). As with the main stem spike, the increases in final grain weight of tiller spikes under elevated CO₂ were attributed to faster grain-filling rates compared with ambient CO₂ (Table 6, Fig. 3) . Water treatments affected grain-filling processes of T1 and T2 under ambient CO₂ atmosphere, but not under elevated CO₂ (Table 6). Under ambient CO₂, the T2 final grain weight, when under well-watered conditions, was 0.19 g spike^-1 greater than T2 final grain weight under limited-water conditions (Table 3). However, final grain weight for T1 was unaffected by elevated CO₂ in this experiment (Table 6). Similar to the main stem spike, decreases in final grain weight of tiller spikes caused by drought stress were due to shorter grain-filling periods.

View this table: [in this window] [in a new window]   Table 6 Grain-filling processes of T1, T2, and T3 spikes, maximum grain weights (g) per spike (M), grain-filling rates in g ATU-1 spike-1 (R), and durations in ATU (L) contrasted over ambient (A) and elevated (E) CO2 concentrations, and drought stress (D) and well-watered (W) treatments

View larger version (28K): [in this window] [in a new window]   Fig. 3 Logistic functions representing grain weights of the main stem and tillers as a function of accumulated thermal units. Vertical error bars represent predicted value ± standard errors. AD = ambient CO2 and dry treatment; AW = ambient CO2 and well-watered treatment; ED = elevated CO2 and dry treatment; EW = elevated CO2 and well-watered treatments

Under elevated CO₂, the grain-filling curves for the main stem and primary tiller spikes appear less divergent than those under ambient CO₂ for both water treatments (Fig. 3), suggesting that elevated CO₂ stimulated the grain-filling process of later-formed tillers and produced higher final grain weight than in plants in the ambient CO₂ exposure.

	Discussion

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Even though both the elevated CO₂ and the well-watered treatments produced a higher final grain yield per spike on the main stem spike, their effects on the components of grain-filling process were different. The grain weight increase of a spike caused by elevated CO₂ was due to an enhanced grain-filling rate and possibly also increased number of grains per spike, whereas increasing amount of water applied prolonged grain-filling duration of the main stem spike. Our data support the results of a parallel study from Pinter et al. (1996) who found CO₂ enrichment caused a 21% yield increase in limited-water and 8% increase in well-watered treatments on the basis of large area sampling. Grain weights are determined by both assimilate supply and storage capacity of the spike (Willey and Dent, 1969; Bremner and Rawson, 1978). Elevated CO₂ was reported to enhance individual leaf assimilation rate by 32, 25, and 23% and the CO₂ effect was 12, 2, and 35% greater under water stressed condition compared with well watered at mid-morning (0930–1015 h MST), midday (1145–1230 h MST) and mid-afternoon (1300–1430 h MST), respectively. Full irrigation increased net assimilation by 13% at midmorning, 29% at midday, and 28% at mid-afternoon (Garcia et al., 1998; Wall et al., 1999). This increase in individual leaf net assimilation rate increased whole canopy photosynthesis by 20% in the well-watered conditions of this study (Kimball et al., 1995; Pinter et al., 1996) thereby increasing structural and non-structural carbohydrate accumulation (Havelka et al., 1984; Cloux et al., 1987; Estiarte et al., 1999). Greater assimilate supply is the most likely explanation for the higher grain weight and faster grain-filling rate with elevated CO₂ (Sofield et al., 1977; Wheeler et al., 1996). In this study, canopy temperature was increased by 0.6°C under elevated CO₂ over the entire growth season the blowers which may have contributed to the insignificant shorter grain-filling duration implied by smaller values of Ls in Tables 1, 2, and 3. Nevertheless, another factor contributing to smaller values of Ls was that both pre- and post-anthesis assimilate supply was greater because of elevated CO₂ (Garcia et al., 1998; Wall et al., 1996, 1999). Furthermore, ample carbohydrate supply for grain filling was also evidenced by an increase in the carbohydrate pool in leaves because of elevated CO₂, and a greater relative change in total non-structural carbohydrates occurred in water-stressed compared with well-watered plants (Nie et al., 1995; Estiarte et al., 1999). The effects of soil water conditions on grain filling in wheat are inconsistent (Bauer et al., 1985, Kobata et al., 1992). Bauer et al. (1985) reported that greater soil water supply during heading (>85 mm) resulted in higher assimilation rate and reduction in grain-filling duration, whereas Kobata et al. (1992) found that grain weight per plant increased 33%, and duration of grain filling was increased only slightly by irrigation. Under ambient CO₂ conditions, we found that final grain weight of main stem spike was 7% higher and grain-filling duration was 8% (155 ATU) longer for well-watered plants than for those grown under limited-water conditions. Greater water supply prolongs the lag phase of grain filling, and results in larger kernels (Kobata et al., 1992; Pinter et al., 1996). Pinter et al. (1996) also found that leaf senescence occurred 2 to 3 wk later under well-watered conditions than under drought stress conditions. The increased grain weight was presumably caused by the larger grain capacity for photosynthate and the longer effective period of leaf photosynthesis.

Even though elevated CO₂ resulted in an increase in the final grain weight of both the upper and lower sections on the main stem spike, the lower section was more responsive to CO₂ enrichment. Water had only a marginal effect on final grain weight in the main stem spike middle section. Elevated CO₂ increased assimilate production because of enhanced flag leaf and spike photosynthesis (Garcia et al., 1998; Wall et al., 1996, 1999). At early stages of spike growth, the sequence of morphogenesis partially determines the growth potential of spikelets resulting in the heaviest spikelets being located in the middle of the spike (Bremner and Rawson, 1978), whereas during later stages of grain growth, lower spikelets have priority for assimilate translocated from plant parts or photosynthesis products. Bremner and Rawson (1978) reported that the basal half of the spike yields about 50% more grain weight than upper half spike, which agrees with our results. These observations indicate that a spike intrinsic regulation mechanism probably exists, which allows assimilate distribution to the lower and middle sections first, then to the upper section during later grain-filling stage. This preferential flow of assimilate implies that the upper section has a smaller storage capacity than lower section. Differences in growth capacity could be attributed to the differences in rate and duration of floret primordia initiation or to the differences in lag phase of early grain growth between these sections. We observed a longer lag phase under well-watered conditions, which was associated with 6% larger kernels, than under limited-water and ambient CO₂ conditions (Pinter et al., 1996).

The grain-filling processes of later-formed tiller spikes were more responsive to elevated CO₂ or water treatments than those of earlier-formed tillers. Two carbohydrate sources for grain filling include preanthesis assimilates stored in plant parts and postanthesis photosynthates from leaf and spike photosynthesis, especially from flag leaf. Main stem and tiller spikes formed earlier have priority over later-formed tillers for assimilate formed during preanthesis or postanthesis (Kobata et al., 1992). Plants under elevated CO₂ or well-watered conditions have more carbohydrate available from photosynthesis than plants under ambient CO₂ or drought stress (Garcia et al., 1998; Wall et al., 1996, 1999). Therefore, the additional photosynthate produced due to irrigation or elevated CO₂ concentration would be of greater benefit to T2 spikes than T1 or main stem spikes since carbohydrate supply is normally less for T2 spikes. Nevertheless, more assimilate was available for the grain growth of later-formed tillers under elevated CO₂. We propose that a sufficient source of assimilate under elevated CO₂ was responsible for the increased grain-filling rates and greater final grain weights of later-formed tiller spikes under elevated CO₂ concentration.

In conclusion, elevated CO₂ increased grain yield (Pinter et al., 1996) by increasing the grain-filling rate. Later-formed tillers are more responsive to elevated CO₂. Well-watered treatment prolonged grain-filling duration to increase grain yield. These results reflect the distinct mechanisms of elevated CO₂ and water on grain growth, when reported on a per spike basis.SAS Institute 1989

	ACKNOWLEDGMENTS

 
We acknowledge the helpful suggestion on the statistical analysis provided by Mr. William J. Price and the cooperation of the USDA-ARS, Water Conservation Laboratory and the University of Arizona Maricopa Agricultural Center. We also acknowledge the following individuals for their involvement in generating unique plants for this project: Dr. Douglas J. Hunsaker, Robert L. LaMorte, and Richard L. Garcia. The primary authors finished the data procession and manuscript in their spare time. The data collection funding was provided by the University of Idaho. The operational support was provided by Grant IBN-9652614 from the NSF-DOE/NASA/USDA joint program on Terrestrial Ecology and Global Change, by grant DE-FG03-95ER-62072 from the Department of Energy, and by the Water Conservation Lab of USDA-ARS. The FACE apparatus was furnished by Brookhaven National Laboratory. This work contributes to the Global Change Terrestrial Ecosystem (GCTE) Core Research Program.

Received for publication September 4, 1997.

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