Analysis of factors related to varietal differences in the yield of rice (Oryza sativa L.) under Free-Air CO2 Enrichment (FACE) conditions

Enhancing crop traits that increase grain yield under elevated CO2 concentrations is an important option for increasing the future productivity of rice. Here, we compared the growth and yield of five varieties with different genetic background under Free-Air CO2 Enrichment (FACE) conditions to identify traits responsible for varietal differences in yield increase under elevated CO2. Three high-yielding and two standard rice varieties grown under FACE conditions commonly had (1) shorter growth periods, (2) higher dry matter production, (3) higher numbers of spikelets (sink capacity) and panicles; and (4) higher yield than those grown under ambient CO2. Yield enhancement by elevated CO2 (FACE/Ambient), however, differed significantly among varieties, ranging from 1.10 to 1.25. The greater response of the sink capacity, defined as the product of spikelet number and single grain mass, was the main factor involved in yield increase. Three high-yielding varieties (Momiroman, Takanari, and Hokuriku 193) had greater sink capacity than two standard varieties and the sink capacity of these varieties significantly increased under FACE condition. However, yield enhancement in elevated CO2 was lower in Hokuriku 193 than in Momiroman and Takanari. In Hokuriku 193, sink production was relatively low while dry matter production was similar to the others. Therefore, larger increase in sink production efficiency per unit of dry matter production under FACE was found to be a particularly important varietal trait, suggesting that efforts to develop varieties suited to the predicted elevated CO2 condition should focus on this and the related traits.