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Initial stages in the onset of senescence in tobacco leaves.

A marked loss of leucine (14)C incorporation occurred in chloroplasts isolated from Nicotiana rustica L. leaves exposed to 24 hours of darkness. This loss is not due to an initial decline in RNA-synthesis potential of the chloroplasts, as was inferred from the extent of UTP incorporation by the isolated chloroplasts. Upon reillumination of the leaves, leucine incorporation by the isolated chloroplasts reverted to its original level within 3 to 4 hours, hence it is doubtful whether the period of 24 hours after detachment should be regarded as the initial phase of leaf senescence.After 48 and 72 hours of darkness, however, complete recovery of the incorporation activity was not achieved by re-illumination of the leaves, representing the apparent onset of an irreversible process. Treatment with kinetin, which markedly delayed the symptoms of senescence in these tobacco leaves, did not prevent the dark-induced decline in chloroplast protein synthesis activity. Nor, up to 24 hours of darkness, did it have any effect on the light-induced complete recovery of this synthesis. Nevertheless, after reilluminating kinetin-treated leaves that had been exposed to darkness for 48 and 72 hours, leucine incorporation in the isolated chloroplasts was resumed at a faster rate and reached a higher level than did the untreated controls.