Drought‐ and ABA‐induced changes in photosynthesis of barley plants

The changes caused by drought stress and abscisic acid (ABA) on photosynthesis of barley plants (Hordeum vulgare. L. cv. Alfa) have been studied. Drought stress was induced by allowing the leaves to lose 12% of their fresh weight. Cycloheximide (CHI), an inhibitor of stress‐induced ABA accumulation, was used to distinguish alterations in photosynthetic reactions that are induced after drought stress in response to elevated ABA levels from those that are caused directly by altered water relations. Four hoars after imposition of drought stress or 2 h after application of ABA, Ihe bulk of the leaf's ABA content measured by enzyme‐amplified ELISA, increased 14‐ and 16‐fold, respectively. CHI fully blocked the stress‐induced ABA accumulation. Gas exchange measurements and analysis of enzyme activities were used to study the reactions of photosynthesis to drought stress and ABA. Leaf dehydration or ABA treatment led to a noticeable decrease in both the initial slope of the curves representing net photosynthetic rate versus intercellular CO2 concentration and the maximal rate of photosynthesis; dehydration of CHI‐treated plants showed much slower inhibition of the latter. The calculated values of the intercellular CO2 concentration, CO2 compensation point and maximal carboxylating efficiency of ribulose 1,5‐bisphosphate (RuBP) carboxylase support the suggestion that biochemical factors are involved in the response of photosynthesis to ABA and drought stress. RuBP carboxylase activity was almost unaffected in ABA‐ and CHI‐treated, non‐stressed plants. A drop in enzyme activity was observed after 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 that increased activity of CA is a photosynthetic response to elevated ABA concentration.