EFFECT OF WATER-STRESS AND DICLOFOP-METHYL ON PHOTOSYNTHESIS, CAROTENOID AND ABSCISIC-ACID CONTENT OF LEAVES OF AVENA-BYZANTINA AND AVENA-FATUA

The effect of combined water stress and diclofop-methyl treatment on p hotosynthesis and carotenoid and abscisic acid (ABA) content of leaves of A. byzantina and A. fatua was investigated. Sublethal doses of dic lofop-methyl caused a transient decline in net assimilation rate and a decrease in beta-carotene and zeaxanthin in leaves of both species. T he decline in carotenoid levels occurred concomitant with a substantia l but transient increase in ABA. A similar but less dramatic trend was observed for water-stressed plants. Recovery of photosynthesis in see dlings exposed simultaneously to diclofop-methyl and water stress, was associated with an increase in beta-carotene and zeaxanthin content a nd a return to basal ABA levels in leaaves of A. byzantina. By compari son, substantial accumulation of zeaxanthin was observed in leaves of A. fatua following combined water stress and herbicide treatment, appa rently at the expense of ABA. Similar findings were made regarding lev els of zeaxanthin when diclofop-methyl was applied to already water-st ressed plants of A. byzantina and A. fatua. It is proposed that herbic ide- and/or water-stress-induced alternations in acetyl-coenzyme A car boxylase activity coupled with reduced demand for fatty acid synthesis , facilitate channelling of photosynthetically fixed carbon into isopr enoids and that alterations in the capacity for terpenoid synthesis fo rms part of the mechanism by which drought stress antagonizes the acti vity of aryloxyphenoxypropionic acid herbicides.