Mechanism of Monocarpic Senescence in Rice

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Abstract
During grain formation stage (90 to 110 days), the youngest flag leaf of rice (Oryza sativa L. cv. Jaya) remained metabolically most active (as indicated by cellular constituents and enzyme activities) and the third leaf the least active. At the grain development stage (110 to 120 days) the above pattern of age-related senescence of the flag leaf completely changed and it senesced at a faster rate than the second leaf which remained metabolically active even up to grain maturation time (120 to 130 days), when both the flag and the third leaf partially senesced. Removal of any leaf temporarily arrested senescence of the remaining attached leaves, that of flag leaf did not hasten senescence of the second leaf, while that of either the second or the third accelerated senescence of the flag. Removal of the inflorescence after emergence or foliar treatment of intact plant with kinetin equally delayed senescence and produced an age-related, sequential mode of senescence or leaves. Both translocation and retention of 32P by the flag leaf were maximum at the time of grain formation and that by the second leaf was maintained even up to grain maturation time. The induction of senescence of the flag leaf was preceded by a plentiful transport of 32P to the grains. Kinetin treatment decreased the transport of 32P, prolonged its duration, and almost equally involved all of the leaves in this process. The pattern of senescence of isolated leaf tips was similar to that of attached leaves. The level of endogenous abscisic acid-like substance(s) maintained a close linearity with the senescence behavior of the leaves of intact and defruited plants during aging, and the rise in abscisic acid in the flag leaf was also preceded by higher 32P transport to the grains.