Endogenous Gibberellin and Abscisic Acid Content as Related to Senescence of Detached Lettuce Leaves

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Abstract
Levels of gibberillins (GA) and of abscisic acid (ABA) in attached leaves of romaine lettuce (Lactuca sativa L.) declined as the leaf became older. The time course of changes in hormone levels, determined in detached lettuce leaves kept in darkness, revealed that a sharp decline in GA accompanied by a moderate rise in ABA occurred before the onset of chlorophyll degradation. As senescence advanced, no GA were detected and a considerable rise of ABA was observed. A similar sequence of hormonal modifications, but more pronounced, was observed in the course of accelerated senescence induced by either Ethephon or water stress. When kinetic or GA3 was applied to detached leaves, the loss of chlorophyll and the rise in ABA were reduced. Bound GA were detected in senescent leaves. They were not found in the kinetin-treated leaves, which contained a relatively high level of free GA. The results suggest that senescence in detached romaine lettuce leaves is connected with a depletion of free GA and cytokinins, which is thereafter followed by a great surge in ABA.