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.

Near isolines of ‘Nugaines’ winterTriticum aestivumthat differed in height were planted with and withoutAegilops cylindricato determine the effect of plant height on competition againstA. cylindrica.The isolines had either reduced height geneRht1,Rht2,Rht1plusRht2, or neitherRhtgenes and averaged 79, 77, 51, and 101 cm tall, respectively, when grown with or without competition fromA. cylindrica.Plants with fewer reduced height genes had the faster rates of height and weight gain, which are important traits for enhanced competitiveness. When growing in competition withA. cylindrica, the shortest isoline allowed the greatest amount ofA. cylindricaseed production but did not have the lowestT. aestivumyield. However, when compared to theA. cylindrica-free control, the shortest isoline had the greatest percent yield loss. The tallest isoline reducedA. cylindricaseed production the most, andT. aestivumyield reduction due toA. cylindricaon a percent basis was the least when averaged over 2 yr. When competing againstA. cylindrica, the tallest isoline did not always have the largest yield and yield parameters, and the shortest isoline did not always have the smallest yield and yield parameters. There is a cost to theT. aestivumplant to produce extra stem biomass that may reduce yield potential of taller plants and reduce the advantage gained by being taller than the surrounding weeds.

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The effect of elevated tropospheric ozone concentration [O3] on root processes in wheat systems of different O3 sensitivity is not well understood. Two wheat cultivars (cv. Y15 and YN19) with contrasting O3 tolerance were grown in a fully open‐air O3 enrichment platform for one season. We found that elevated O3 (EO3) (50% above the ambient O3) significantly decreased the total biomass at all key growth stages and the yield of the O3‐sensitive cultivar YN19 but did not affect those of the O3‐tolerant cultivar Y15. EO3 significantly decreased the root biomass of two wheat cultivars at the jointing and grain‐filling stages. EO3 significantly decreased the root length, length density, surface area and volume of the two cultivars at the jointing stage but increased those of YN19 at the grain‐filling stage. EO3 significantly increased the root activities (specific root respiration rates) of Y15 and YN19 at the jointing, heading and grain‐filling stages. EO3 significantly decreased the contribution of fresh root respiration to soil respiration (CRS) of YN19 at the jointing stage but increased it at the heading stage; however, it did not change the CRS of Y15 at any growth stages. This study indicates that the effects of EO3 on root morphology and activity varied among wheat cultivars, and suggest that we can breed O3‐tolerant cultivars to maintain crop yield under higher [O3] scenarios.

Rates and durations of individual phases of wheat (Triticum aestivum L.) apical development are among the most important factors that determine yield components. Because atmospheric CO2 has been increasing steadily, it is important to evaluate the effects of elevated CO2 on wheat development. This study was conducted to determine rates and durations of leaf, spikelet, and floret primordium initiation in a Free‐Air Carbon Dioxide Enrichment (FACE) system. Spring wheat (cv. Yecora Roja) was planted at the University of Arizona Maricopa Agricultural Center. The two CO2 concentrations were 550 (elevated) and 370 (ambient) μmol mol−1 CO2. Individual plant samples were collected every 3 to 4 d. We dissected the main stem (MS), coleoptile tiller (T0), primary tillers (T1, T2, and T3) and secondary tillers (T00, T01, T02, T10, Tll, and T12) and counted primordia. Apex primordium data were fitted to a four‐piece linear‐spline segmented regression model with the SAS proc NLIN. No influence of elevated CO2 (550 μmol−1 on leaf primordium initiation of MS was detected. Nevertheless, CO2 enrichment significantly increased rates of spikelet primordium initiation of MS, T1, T2, T10, and Tll, and diminished the durations of spikelet development phase of MS, TI, T2, T3, T10, and T11. Within the floret phase, CO2 enrichment significantly increased rates of floret primordium initiation of MS, TO, T1, T2, and T3, and diminished the time to the completion of floret primordium initiation of MS, T0, T1, T3, and T11. The information from this study will be utilized to predict wheat apical development and grain production in the elevated atmospheric CO2 environments of the future.