Effect of Senescence on Levels of Free Abscisic Acid and Water Potentials in Cotyledons of Bean

Following imbibition and germination, levels of free abscisic acid (ABA) and water potentials in senescing bean cotyledons (Phaseolus vulgaris L. `Kinghorn') showed peaks on days 4 and 6 after planting and then declined as senescence intensified. Water potential displayed a final peak just prior to abscission which was not matched by a corresponding rise in abscisic acid, but this presumably reflected a generalized loss of synthetic capacity at that advanced stage of senescence. The changing water status of the tissue may be attributable to an imbalance between mobilization and transport of metabolites, with temporary water stress an apparent consequence of that asynchrony. These observations suggest that changes in the level of ABA were not directly related to advancing senescence but may have been a response to marked changes in water potential of the tissue.