Enhancement of rice canopy carbon gain by elevated CO2 is sensitive to growth stage and leaf nitrogen concentration

Abstract

• Increasing our understanding of the factors regulating seasonal changes in rice canopy carbon gain (Cgain: daily net photosynthesis - night respiration) under elevated CO2 concentrations ([CO 2]) will reduce our uncertainty in predicting future rice yields and assist in the development of adaptation strategies. • In this study we measured CO2 exchange from rice (Oryza sativa) canopies grown at c. 360 and 690 μmol mol-1[CO2] in growth chambers continuously over three growing seasons. • Stimulation of Cgain by elevated [CO2] was 22-79% during vegetative growth, but decreased to between -12 and 5% after the grain-filling stage, resulting in a 7-22% net enhancement for the whole season. • The decreased stimulation of C gain resulted mainly from decreased canopy net photosynthesis and partially from increased respiration. A decrease in canopy photosynthetic capacity was noted where leaf nitrogen (N) decreased. The effect of elevated [CO2] on leaf area was generally small, but most dramatic under ample N conditions; this increased the stimulation of whole-season Cgain. These results suggest that a decrease in Cgain enhancement following elevated CO2 levels is difficult to avoid, but that careful management of nitrogen levels can alter the whole-season Cgain enhancement. © New Phytologist (2006).