Levels of gibberillins (GAs) 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 GAs accompanied by a moderate rise in ABA occurred before the onset of
chlorophyll degradation. As senescence advanced, no GAs could be 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
kinetin or GA(3) was applied to detached leaves, the loss of
chlorophyll and the rise in ABA were reduced. Bound GAs were detected in senescent leaves. They were not found in the
kinetin-treated leaves, which contained a relatively high level of free GAs. The results suggest that senescence in detached romaine lettuce leaves is connected with a depletion of free GAs and
cytokinins, which is thereafter followed by a great surge in ABA.