Abstract
Changes in abscisic acid and its metabolites were followed through two drought cycles in Pseudotsuga menziesii (Mirb.) Franco seedlings to determine the metabolic pathway of the hormone and its relationship to branch (stomatal) conductance. Three year‐old, intact seedlings were water‐stressed, watered, and restressed over a period of 30 days. Water potential was sampled with a pressure chamber and branch conductance with a steady‐state porometer. Needle content of abscisic acid and 2‐trans‐abscisic acid and their saponifiable conjugates were quantified with gas‐liquid chromatography. The typical water potential threshold in branch conductance, decreasing abruptly at ‐2.0 MPa, corresponded to an increase in abscisic acid content of 240 ng g−1. The relationship between abscisic acid and water potential was not definitive, though the general trend was an increase in the hormone with intensifying stress until water potential was ‐5.0 MPa, when concentration sharply declined. No adjustment to stress was observed in the relationships, but stress during the second cycle progressed more slowly. A linear relationship between abscisic acid and its conjugate indicated the importance of the interconversion of the two compounds for storage and supply of the free acid.
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