Effect of nitrogen nutrition on the carbohydrate repression of photosynthesis in leaves of Phaseolus vulgaris L.

When carbohydrates accumulate in leaves, photosynthesis is repressed. Limited nitrogen nutrition is thought to enhance this repressing effect. However, the interaction between carbohydrate and nitrogen limitation in leaf photosynthesis has not been examined intensively. In this study, we grew Phaseolus vulgaris L. plants at three different nitrogen levels, and examined the effects of sucrose feeding to the roots on the nitrogen content, carbohydrate content and photosynthetic properties of the primary leaves. Nitrogen content and photosynthetic rate were lower and the carbohydrate content was greater in plants grown with limited nitrogen than in well-fertilized plants. Sucrose feeding to the plants increased carbohydrate content and decreased photosynthetic rate and nitrogen content. The increase in carbohydrate content and the decreases in nitrogen content and photosynthetic rate occurred at the same time, and the negative relationship between the carbohydrate content and photosynthetic rate did not differ among nitrogen nutrition levels. These results show that carbohydrate accumulation in the leaves leads to a decrease in photosynthetic rate. At low nitrogen nutrition levels, carbohydrates accumulated markedly, which accelerated this effect. It appears that the nitrogen nutrition level influences leaf photosynthesis through changing the carbohydrate level rather than through modifying sensitivity of the leaf to the carbohydrate level.