Daniel C. Walton

The effects of water stress on abscisic-acid levels and metabolism in roots of Phaseolus vulgaris L. and other plants.

SummaryAbscisic-acid (ABA) levels in roots of bean plants exposed to a—4 bar stress in the root medium increased ca. 10fold within 1 h and 16fold by the end of the 2nd h. Several types of experiments indicated that there is no transport requirement from the shoot for the increase to occur. ABA levels in roots from pea (Pisum sativum L.) and sunflower (Helianthus annuus L.) also increased in response to a—4 bar stress, although not as dramatically as in bean. When (S)-[2-14C]-ABA was fed to excised bean roots dihydrophaseic acid (DPA) was the major metabolite formed. The levels of endogenous DPA and phaseic acid increased markedly during a 27-h stress period. These results are consistent with a possible role for ABA in roots of water-stressed plants.

Asymmetry, Its Importance to the Action and Metabolism of Abscisic Acid

Unlabeled and 14C-labeled enantiomorphs of abscisic acid (ABA) were obtained through acetylcellulose chromatography and tested as inducers of abscission, as inhibitors of seed germination, and as antagonists of gibberellic acid-induced synthesis and release of α-amylase. The activity of the R isomer was either equal to or less than that of the naturally occurring S form. Greatest differences were in the inhibition of root-related growth. In excised beam axes, although uptake of S-[14C]ABA is faster, the internal concentration of R-ABA is higher because of faster conversion of S-ABA to inactive metabolic products. In axes a reversal in chirality is less important to the physiological action of ABA than to its metabolism.

The relationship between stomatal resistance and abscisic-acid levels in leaves of water-stressed bean plants

Leaf water potentials of Phaseolus vulgaris L. plants exposed to a -3.0 bar root medium were reduced to between -7 and -9 bars within 25 min and remained constant for the next several hours. This treatment led to considerable variation between leaves in both abscisic-acid (ABA) content and Rs, although the two were well correlated after a 5-h treatment. There was an apparent 7-fold increase in leaf ABA levels necessary to initiate stomatal closure when plants were exposed to a -3.0 bar treatment, but when plants were exposed to a -5.0 bar stress Rs values increased prior to any detectable rise in ABA levels. To explain these seemingly contradictory results, we suggest that the rate of ABA synthesis in the leaf, rather than the total ABA content, determines the status of the stomatal aperture.