A possible role for abscisic Acid in regulation of photosynthetic and photorespiratory carbon metabolism in barley leaves.

The influence of abscisic acid (ABA) on carbon metabolism, rate of photorespiration, and the activity of the photorespiratory enzymes ribulose bisphosphate oxygenase and glycolate oxidase in 7-day-old barley seedlings (Hordeum vulgare L. var. Alfa) was investigated. Plants treated with ABA had enhanced incorporation of labeled carbon from (14)CO(2) into glycolic acid, glycine, and serine, while (14)C incorporation into 3-phosphoglyceric acid and sugarphosphate esters was depressed. Parallel with this effect, treated plants showed a rise in activity of RuBP oxygenase and glycolic acid oxidase. The rate of photorespiration was increased twofold by ABA treatment at IO(-6) molar while the CO(2)-compensation point increased 46% and stomatal resistance increased more than twofold over control plants.