Effect of water stress and diclofop-methyl on photosynthesis, carotenoid and abscisic acid content of leaves of Avena byzantina and Avena fatua

Abstract

The effect of combined water stress and diclofop-methyl treatment on photosynthesis and carotenoid and abscisic acid (ABA) content of leaves of A. byzantina and A. fatua was investigated. Sublethal doses of diclofop-methyl caused a transient decline in net assimilation rate and a decrease in β-carotene and zeaxanthin in leaves of both species. The decline in carotenoid levels occurred concomitant with a substantial but transient increase in ABA. A similar but less dramatic trend was observed for water-stressed plants. Recovery of photosynthesis in seedlings exposed simultaneously to diclofop-methyl and water stress, was associated with an increase in β-carotene and zeaxanthin contentand a return to basal ABA levels in leaves of A. byzantina. By comparison, substantial accumulation of zeaxanthin was observed in leaves of A. fatua following combined water stress and herbicide treatment, apparently at the expense of ABA. Similar findings were made regarding levels of zeaxanthin when diclofop-methyl was applied to already water-stressed plants of A. byzantina and A. fatua. It is proposed that herbicide- and/or water-stress-induced alterations in acetyl-coenzyme A carboxylase activity coupled with reduced demand for fatty acid synthesis, facilitate channelling of photosynthetically fixed carbon into isoprenoids and that alterations in the capacity for terpenoid synthesis forms part of the mechanism by which drought stress antagonizes the activity of aryloxyphenoxypropionic acid herbicides.