Intraspecific variation in the response of rice (Oryza sativa) to increased CO₂– photosynthetic, biomass and reproductive characteristics

Two rice ( Oryza sativa L.) cultivars of contrasting morphologies, IR‐36 and Fujiyama‐5, were exposed to ambient (360 μl l −1 ) and ambient plus 300 μl l −1 CO 2 from time of emergence until ca 50% grain fill at the Duke University Phytotron, Durham, North Carolina. Exposure to increased CO 2 resulted in about a 50% increase in the photosynthetic rate for both cultivars and photosynthetic enhancement was still evident after 3 months of exposure to a high CO 2 environment. The photosynthetic response at 5% CO 2 and the response of CO 2 assimilation (A) to internal CO 2 (C i ) suggest a reallocation of biochemical resources from RuBP carboxylation to RuBP regeneration. Increases in total plant biomass at elevated CO 2 were approximately the same in both cultivars, although differences in allocation patterns were noted in root/shoot ratio. Differences in reproductive characteristics were also observed between cultivars at an elevated CO 2 environment with a significant increase in harvest index for IR‐36 but not for Fujiyama‐5. Changes in carbon allocation in reproduction between these two cultivars suggest that lines of rice could be identified that would maximize reproductive output in a future high CO 2 environment.