Genotypic variation in rice yield enhancement by elevated CO2 relates to growth before heading, and not to maturity group.

Maturity group (based on the number of days to maturity) is an important growth trait for determining crop productivity, but there has been no attempt to examine the effects of elevated [CO(2)] on yield enhancement of rice cultivars with different maturity groups. Since early-maturing cultivars generally show higher plant N concentration than late-maturing cultivars, it is hypothesized that [CO(2)]-induced yield enhancement might be larger for early-maturing cultivars than late-maturing cultivars. To test this hypothesis, the effects of elevated [CO(2)] on yield components, biomass, N uptake, and leaf photosynthesis of cultivars with different maturity groups were examined for 2 years using a free-air CO(2) enrichment (FACE). Elevated [CO(2)] significantly increased grain yield and the magnitude significantly differed among the cultivars as detected by a significant [CO(2)] x cultivar interaction. Two cultivars (one with early and one with late maturity) responded more strongly to elevated [CO(2)] than those with intermediate maturity, resulting mainly from increases in spikelet density. Biomass and N uptake at the heading stage were closely correlated with grain yield and spikelet density over [CO(2)] and cultivars. Our 2 year field trial rejected the hypothesis that earlier cultivars would respond more to elevated [CO(2)] than later cultivars, but it is revealed that the magnitude of the growth enhancement before heading is a useful criterion for selecting rice cultivars capable of adapting to elevated [CO(2)].