Effects of elevated atmospheric CO2 and drought stress on individual grain filling rates and durations of the main stem in spring wheat

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 CO2 and drought stress on the grain filling rate and duration, and the weight of individual kernels. Spring wheat (Triticum aestivum L.) was grown in a free air CO2 enrichment (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 370 μmol mol−1 of atmospheric CO2 concentrations 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 CO2 or 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.