Analysis on the network of nitrogen partitioning and nitrogen metabolism affecting to the photosynthetic activities of leaves in crop plants

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To clarify the physiological mechanisms of the maintenance of photosynthetic activity, the network of nitrogen metabolism associated with the photosynthetic activity was investigated, focusing on the nitrogen absorption, assimilation and partitioning.1. The leaves of the soybean cultivar Tachinagaha senesce more slowly after flowering and maintain a higher rate of photosynthesis throughout the ripening period than the leaves of the Enrei cultivar. Tachinagaha maintained higher levels of rubisco and nitrogen of leaves than those of Enrei. The low leaf nitrogen content was caused by the smaller accumulation of nitrogen in plants, and in addition, by the greater partitioning of nitrogen to pods.2. The levels of abscisic acid in Enrei leaves were increased rapidly at the late stage of ripening. In other hand, the levels of abscisic acid in Tachinagaha leaves were stable during ripening. There was a negative correlation with the increase of abscisic acid and the decrease of rubisco.3. The leaves of a high yielding rice cultivar, Akenohoshi maintain a higher rate of photosynthesis throughout the ripening stage than the leaves of Nipponbare. The photosynthetic activity was closely correlated with leaf levels of rubisco and nitrogen. The high leaf nitrogen content was achieved by the greater accumulation of nitrogen in plants, and in addition, by the greater partitioning of nitrogen to leaves.4. There was a close correlation between the Rubisco content and the levels of rbcL and rbcS mRNAs. The maintenance of high levels of rubisco was caused by an increase in nitrogen partitioning to leaves and not by an increase in nitrogen absorption by plants treated with cytokinin.5. The depression of the Rubisco and nitrogen in rice leaves caused by water stress was closely correlated with the decrease of the level of cytokinin in root exudate. The rubisco degradation in leaves was accelerated by water stress.