Grain growth of different rice cultivars under elevated CO2 concentrations affects yield and quality

Elevated CO2 concentrations (E-[CO2]) will likely increase grain yield but decrease grain protein content and increase grain chalkiness, thus degrading the grain quality of rice. Grain growth is the key process that determines both grain yield and quality, hence its responses to E-[CO2] were studied in a free-air CO2 enrichment (FACE) with the focus on variability among the spikelet positions within a panicle. We investigated the changes in grain growth in superior spikelets (SS) and inferior spikelets (IS) for three cultivars of rice differing in grain size and grain number: Akita 63, Koshihikari and Takanari. E-[CO2] increased the grain yield by 15% across 2 years in the three cultivars. The grain growth responses were dependent on spikelet position: the grain mass was increased only in IS, while grain N content was reduced in SS but slightly increased in IS. The increase in IS grain mass was greatest in Takanari followed by Koshihikari with Akita 63 showing a slight decrease. E-[CO2] significantly changed the occurrence of grain chalkiness, and the extent of the increased occurrence of basal white and white back kernels was greatest in Koshihikari followed by Akita 63 with Takanari being least affected. The varietal difference in the responses to E-[CO2] should be studied further with respect to the variability of grain growth and distribution of carbon and nitrogen within a panicle for taking better advantages of the increasing [CO2] for rice grain yield and quality in the future.