Among the 4 uppermost leaves of greenhouse-grown plants of X. strumarium L. the content of abscisic acid (ABA) per unit fresh or dry weight was highest in the youngest leaf and decreased gradually with increasing age of the leaves. Expressed per leaf, the 2nd youngest leaf was richest in ABA; the amount of ABA per leaf declined only slightly as the leaves expanded. Transpiration and stomatal conductance were negatively correlated with the ABA concentration in the leaves; the youngest leaf lost the least amount of water. This correlation was always very good if the youngest leaf was compared with the older leaves but not always good among the older leaves. Since stomatal sensitivity to exogenous (.+-.)-ABA was the same in leaves of all 4 age groups ABA may be in at least 2 compartments in the leaf, 1 of which is isolated from the guard cells. The ability to synthesize ABA in response to wilting or chilling was strongly expressed in young leaves and declined with leaf age. There was no difference between leaves in their content of the metabolites of ABA, phaseic, and dihydrophaseic acid, expressed per unit weight.
