Rice carbon balance under elevated CO2

Season-long effects of elevated CO2 concentration ([CO2]) on the carbon balance of the rice (Oryza sativa) canopy are reported here. The experiment was conducted in six sunlit, semiclosed growth chambers for an entire growing season. Rice plants (cv. Nipponbare) were grown at 350 mu mol mol(-1) [CO2] (ambient) in three chambers, or at 650 mu mol mol(-1) (elevated) in the other three chambers. Canopy net photosynthesis and night-time respiration were determined in the chambers by mass balance. Both canopy gross photosynthesis and total respiration, through the entire growing season, were increased by the CO2 enrichment. But CO2-induced variations in canopy carbon gain were mainly caused by changes in canopy photosynthesis. The enhancement of daily canopy gross photosynthesis by elevated [CO2] was 33.4% for the first 3 week, but it declined gradually and disappeared by heading. Enhancement of daily net carbon gain also decreased as rice plants grew. These results show that the increase in rice biomass by elevated [CO2] results more from the increase in carbon gain at early rather than later stages of growth.