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.
Die invoed van gekombineerde waterstremming en diklofopmetiel-behandeling op die karotenoïed- en ABA-konsentrasie van blare van A. byzantina en A. fatua is ondersoek. Subletale dosisse van dikloiopmetiel het tot ’n tydelike verlaging in die netto assimilasietempo en tot ’n verlaging in β-karoteen en zeaxantien in blare van albei spesies gelei. Die afname in karotenoïedvlakke het plaasgevind saam met ’n sterk maar tydelike verhoging in ABA. ’n Soortgelyke maar minder opval-lende neiging is waargeneem in plante wat aan waterstremming onderhewig was. In saailinge van A. byzantina wat aan beide diklofopmetiel en waterstremming blootgestel is, het die herstel in fotosintese plaasgevind saam met ’n verhoging in β-karoteen en zeaxantienvlakke en ’n herstel van die ABA na die basale vlak. In die geval van A. fatua wat aan diklo-fopmetiel en waterstremming blootgestel is, is ’n aansienlike verhoging in zeaxantien waargeneem, skynbaar ten kostevan ABA. Soortgeiyke waarnemings is gemaak met betrekking tot zeaxantienvlakke nadat diklofopmetiel toegedien is aan reeds watergestremde plante van A. byzantina en A. fatua. Daar word voorgestel dat herbisied- en/of waterstremming-geïnduseerde veranderings aan ACCase-aktiwiteit, gekoppel aan ’n verlaagde aanvraag vir vetsuursintese, tot die omskakeling van fotosintese-gefikseerde koolstof na isoprenoïede lei. Wysigings in die vermoë om isoprenoïede te vorm, kan ’n deel uitmaak van die meganisme waarvolgens droogtestremming die aktiwiteit van AOPP herbisiede beïnvloed.