Metabolism of Abscisic Acid in Guard Cells of Vicia faba L. and Commelina communis L.

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Abstract
Metabolism of abscisic acid (ABA) was investigated in isolated guard cells and in mesophyll tissue of V. faba L. and C. communis L. After incubation in buffer containing [G-3H] .+-. ABA, the tissue was extracted by grinding and the metabolites separated by TLC. Guard cells of Commelina metabolized ABA to phaseic acid (PA), dihydrophaseic acid (DPA), and alkali labile conjugates. Guard cells of Vicia formed only the conjugates. Mesophyll cells of Commelina accumulated DPA while mesophyll cells of Vicia accumulated PA. Controls showed that the observed metabolism was not due to extracellular enzyme contaminants nor to bacterial action. Metabolism of ABA in guard cells suggests a mechanism for removal of ABA, which causes stomatal closure of both species, from the stomatal complex. Conversion to metabolites which are inactive in stomatal regulation, within the cells controlling stomatal opening, might precede detectable changes in levels of ABA in bulk leaf tissue. The differences observed between Commelina and Vicia in metabolism of ABA in guard cells, and in the accumulation product in the mesophyll, may be related to differences in stomatal sensitivity to PA reported for these species.