A High-Yielding Rice Cultivar "Takanari" Shows No N Constraints on CO₂ Fertilization

Enhancing crop yield response to elevated CO₂ concentrations (E-[CO₂]) is an important adaptation measure to climate change. A high-yielding indica rice cultivar "Takanari" has recently been identified as a potential candidate for high productivity in E-[CO₂] resulting from its large sink and source capacities. To fully utilize these traits, nitrogen should play a major role, but it is unknown how N levels influence the yield response of Takanari to E-[CO₂]. We therefore compared grain yield and quality of Takanari with those of Koshihikari, a standard japonica cultivar, in response to Free-Air CO₂ enrichment (FACE, +200 μmol mol^-1) under three N levels (0, 8, and 12 g m^-2) over three seasons. The biomass of both cultivars increased under E-[CO₂] at all N levels; however, the harvest index decreased under E-[CO₂] in the N-limited treatment for Koshihikari but not for Takanari. The decreased harvest index of Koshihikari resulted from limited enhancement of spikelet number under N-limitation. In contrast, spikelet number increased in E-[CO₂] in Takanari even without N application, resulting in significant yield enhancement, averaging 18% over 3 years, whereas Koshihikari exhibited virtually no increase in yield in E-[CO₂] under the N-limited condition. Grain appearance quality of Koshihikari was severely reduced by E-[CO₂], most notably in N-limited and hot conditions, by a substantial increase in chalky grain, but chalky grain % did not increase in E-[CO₂] even without N fertilizer. These results indicated that Takanari could retain its high yield advantage over Koshihikari with limited increase in chalkiness even under limited N conditions and that it could be a useful genetic resource for improving N use efficiency under E-[CO₂].