Photosynthetic rate analysis in rice (Oryza sativa L.) with reference to leaf nitrogen and nitrogen accumulation

A high yielding Japonica rice cultivar, Takanari, shows a higher rate of leaf photosynthesis after heading through ripening than cultivar Koshihikari under field condition. This contributes to the higher dry matter production in Takanari. The authors measured the rate of leaf photosynthesis in Takanari and Koshihikari from tillering stage through the ripening stages and investigated the causal factors underlying the differences in the rate of leaf photosynthesis with emphasis on leaf nitrogen content and nitrogen accumulation. The superiority of Takanari in photosynthetic rate could not be observed at tillering stage. But it appeared at panicle formation stage and was kept higher through the ripening stage. Stomatal conductance was also higher in Takanari than Koshihikari after panicle formation stage. However, the significantly higher leaf nitrogen content in Takanari was not observed until heading. From heading to ripening stages, the total nitrogen accumulation was significantly higher in Takanari but the nitrogen partitioning to the leaves was significantly lower. No significant cultivar difference was observed in photosynthetic rate at tillering stage at the same leaf nitrogen content. At heading however, the photosynthetic rate in Takanari was significantly higher than Koshihikari even at the same leaf nitrogen content. The difference in the relationship between the leaf nitrogen content and the rate of photosynthesis at an intercellular CO2 concentration of 280 +- uL/L was negligible. The superiority of Takanari in photosynthetic rate after the panicle formation stage resulted from the larger stomatal conductance even at the same level of leaf nitrogen. The increased level of leaf nitrogen due to the larger capacity of nitrogen accumulation was also attributed to the higher rate of leaf photosynthesis in Takanari after heading. These suggest that the higher stomatal conductance and leaf nitrogen content might be responsible for the higher photosynthetic rate in Takanari.