Soil and Water Warming Accelerates Phenology and Down-Regulation of Leaf Photosynthesis of Rice Plants Grown Under Free-Air CO(2) Enrichment (FACE)

To enable prediction of future rice production in a changing climate, we need to understand the interactive effects of temperature and elevated [CO(2)] (E[CO(2)]). We therefore examined if the effect of E[CO(2)] on the light-saturated leaf photosynthetic rate (A(sat)) was affected by soil and water temperature (NT, normal; ET, elevated) under open-field conditions at the rice free-air CO(2) enrichment (FACE) facility in Shizukuishi, Japan, in 2007 and 2008. Season-long E[CO(2)] (+200 µmol mol(−1)) increased A(sat) by 26%, when averaged over two years, temperature regimes and growth stages. The effect of ET (+2°C) on A(sat) was not significant at active tillering and heading, but became negative and significant at mid-grain filling; A(sat) in E[CO(2)]–ET was higher than in ambient [CO(2)] (A[CO(2)])–NT by only 4%. Photosynthetic down-regulation at E[CO(2)] also became apparent at mid-grain filling; A(sat) compared at the same [CO(2)] in the leaf cuvette was significantly lower in plants grown in E[CO(2)] than in those grown in A[CO(2)]. The additive effects of E[CO(2)] and ET decreased A(sat) by 23% compared with that of A[CO(2)]–NT plants. Although total crop nitrogen (N) uptake was increased by ET, N allocation to the leaves and to Rubisco was reduced under ET and E[CO(2)] at mid-grain filling, which resulted in a significant decrease (32%) in the maximum rate of ribulose-1,5-bisphosphate carboxylation on a leaf area basis. Because the change in N allocation was associated with the accelerated phenology in E[CO(2)]–ET plants, we conclude that soil and water warming accelerates photosynthetic down-regulation at E[CO(2)].