Seasonal changes in temperature dependence of photosynthetic rate in rice under a free-air CO2 enrichment

center dot Background and Aims Influences of rising global CO2 concentration and temperature on plant growth and ecosystem function have become major concerns, but how photosynthesis changes with CO2 and temperature in the field is poorly understood. Therefore, studies were made of the effect of elevated CO2 on temperature dependence of photosynthetic rates in rice (Oryza sativa) grown in a paddy field, in relation to seasons in two years. center dot Methods Photosynthetic rates were determined monthly for rice grown under free-air CO2 enrichment (FACE) compared to the normal atmosphere (570 vs 370 mu mol mol(-1)). Temperature dependence of the maximum rate of RuBP (ribulose-1,5-bisphosphate) carboxylation (V-cmax) and the maximum rate of electron transport (J(max)) were analysed with the Arrhenius equation. The photosynthesis-temperature response was reconstructed to determine the optimal temperature (T-opt) that maximizes the photosynthetic rate. center dot Key Results and Conclusions There was both an increase in the absolute value of the light-saturated photosynthetic rate at growth CO2 (P-growth) and an increase in T-opt for P-growth caused by elevated CO2 in FACE conditions. Seasonal decrease in P-growth was associated with a decrease in nitrogen content per unit leaf area (N-area) and thus in the maximum rate of electron transport (J(max)) and the maximum rate of RuBP carboxylation (V-cmax). At ambient CO2, T-opt increased with increasing growth temperature due mainly to increasing activation energy of V-cmax. At elevated CO2, T-opt did not show a clear seasonal trend. Temperature dependence of photosynthesis was changed by seasonal climate and plant nitrogen status, which differed between ambient and elevated CO2.