Relations between the leaf senescence and the reduction of roof functions in crop plants under water stress

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Reduction in leaf photosynthetic rate due to senescence decreases dry matter production and yield. However, the mechanisms of the leaf senescence are not still known. The leaf senescence is enhanced in some environmental conditions. And other organs such as roots are expected to affect leaf senescence. In this research, effects of soil moisture deficiency and root functions on leaf senescence were investigated. The obtained results are as follows.(1) Clear differences in leaf senescence during ripening were observed in soybean plants between cultivars and between plants grown under different soil moisture conditions. The plants, showing a slow leaf senescence, had a well-developed root system and showed a high rate of root exudation.(2) Reductions in soil moisture induced leaf senescence in rice and maize plants. Leaf senescence of rice plants is promoted with a small reduction in soil moisture. However, leaf senescence of a wilting mutant, growing under submerged soil conditions, was … More slower than the wild type, growing under moderately deficient soil moisture conditions, even though the mutant had a greater leaf xylem resistance to water transport and, therefore, a lower midday leaf water potential. The root exudation rate was significantly higher in the former plants than in the latter plants.(3) Reduction in leaf photosynthetic rate due to senescence in ripening stage was large in the rice, cv. Nipponbare compared with cv. Akenohoshi. Total amount of nitrogen and cytokinins transported from root to shoot was estimated to be significantly large in Akenohoshi.(4) Under deficient soil moisture conditions, exudation rate decreased significantly in rice and maize plants. And total amount of nitrogen and cytokinins also decreased in maize plants.(5) From these results, root may affect leaf senescence through the functions of nitrogen uptake and cytokinin synthesis. The reduction in transport of nitrogen and cytokinins may be responsible for the promotion of the senescence under the deficient soil moisture conditions. Less