Ak. Cowan et al., EFFECT OF WATER-STRESS AND DICLOFOP-METHYL ON PHOTOSYNTHESIS, CAROTENOID AND ABSCISIC-ACID CONTENT OF LEAVES OF AVENA-BYZANTINA AND AVENA-FATUA, South African journal of botany, 61(1), 1995, pp. 29-34
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.