Mobilization of Metabolites from Leaves to Grains as the Cause of Monocarpic Senescence in Rice

The pattern of senescence was studied by following the changes in chlorophyll and protein in the leaves and by measuring (32)P retention and export from source to sink during development of the rice plant (Oryza sativa L. cv. Jaya) subjected to different manipulative treatments. With the advance of reproductive development, the chronological sequence of leaf senescence was changed, so that the flag and the third leaf senesced earlier than did the second leaf. In presence of the daughter shoot of defruited plants, senescence was delayed in all three leaves of the mother plant, as compared to the same leaves of intact plants. Senescence of all three leaves was further delayed when both panicle and daughter shoots were removed from the plant. The above manipulative treatments caused the initial sequential pattern of senescence of leaves to persist. Removal of both panicle and daughter shoots caused little export of (32)P between leaves. In the presence of daughter shoots of defruited plants, export of (32)P was maximum from leaves of the mother plant to the nearest daughter shoots. This led to earlier senescence of such mother plant leaves than that of plants from which both panicle and daughter shoots were removed. The pattern of senescence and export of (32)P in the flag and the second leaf of the daughter shoot was essentially the same as that of the intact plant. Based on these findings, it was concluded that mobilization of metabolites from source to sink is the primary cause of monocarpic senescence in rice.