DROUGHT-INDUCED AND ABA-INDUCED CHANGES IN PHOTOSYNTHESIS OF BARLEY PLANTS

The changes caused by drought stress and abscisic acid (ABA) on photos ynthesis of barley plants (Hordeum vulgare L. cv. Alfa)have been studi ed. Drought stress was induced by allowing the leaves to lose 12% of t heir fresh weight. Cycloheximide (CHI), an inhibitor of stress-induced ABA accumulation, was used to distinguish alterations in photosynthet ic reactions that are induced after drought stress in response to elev ated ABA levels from those that are caused directly by altered water r elations. Four hours after imposition of drought stress or 2 h after a pplication of ABA, the bulk of the leaf's ABA content measured by enzy me-amplified ELISA, increased 14- and 16-fold, respectively. CHI fully blocked the stress-induced ABA accumulation. Gas exchange measurement s and analysis of enzyme activities were used to study the reactions o f photosynthesis to drought stress and ABA. Leaf dehydration or ABA tr eatment led to a noticeable decrease in both the initial slope of the curves representing net photosynthetic rate versus intercellular CO2 c oncentration and the maximal rate of photosynthesis; dehydration of CH I treated plants showed much slower inhibition of the latter. The calc ulated values of the intercellular CO2 concentration, CO2 compensation point and maximal carboxylating efficiency of ribulose 1,5-bisphospha te (RuBP) carboxylase support the suggestion that biochemical factors are involved in the response of photosynthesis to ABA and drought stre ss. RuBP carboxylase activity was almost unaffected in ABA- and CHI-he ated, non-stressed plants. A drop in enzyme activity was observed afte r leaf dehydration of the control and ABA-treated plants. When barley plants were supplied with ABA, the activity of carbonic anhydrase (CA, EC 4.2.2.1) increased more than 2-fold. Subsequent dehydration caused an over 1.5-fold increase in CA activity of the control plants and a more than 2.5-fold increase in ABA-treated plants. Dehydration of CHI- treated plants caused no change in enzyme activity. It is suggested th at increased activity of CA is a photosynthetic response to elevated A BA concentration.