Abscisic Acid Metabolism in Water-stressed Bean Leaves.

Phaseic acid (PA) and dihydrophaseic acid (DPA) are the major metabolites observed when (S)-2-(14)C-abscisic acid (ABA) is fed to 14-day excised primary bean leaves (Phaseolus vulgaris L. cv. Red Kidney). The distribution of (14)C in leaves which were wilted after feeding ABA appears to be the same as that observed in unwilted leaves. A reduction in the relative specific radioactivities of the two metabolites after wilting, compared with the specific radioactivities measured in unwilted plants, indicated that these metabolites continue to be formed endogenously after wilting. Estimates of the endogenous ABA levels showed that they rose from 0.04 mug to approximately 0.5 mug/g fresh weight within 4 hours after the beginning of a 10% wilt and remained at that level during a subsequent 20 hours of wilt. In unwilted leaves, the levels of PA and DPA were 5 times and 20 times higher than that of ABA, respectively. Both PA and DPA levels rose throughout the wilt period. PA rose from 0.20 mug to 1.0 mug and DPA from 0.8 mug to over 3 mug/g fresh weight. From these data, we calculated the rate of ABA synthesis to be at least 0.15 mug/hr.g fresh weight during this period. We have interpreted these results to mean that in wilted leaves an elevated level of ABA is maintained because the rate of synthesis and metabolism are both elevated and approximately equal.