Fatty acid elongation is important in the activity of thiocarbamate herbicides and in safening by dichlormid

The thiocarbamates, such as pebulate (S-propyl butyl (ethyl) thiocarbamate) are a well-estabilished class of herbicides. They inhibit fatty acid elongation, which is necessary for the biosynthesis of constituents of surface waxes and suberin and this has been proposed to be important for their toxicity. In this study lipid metabolism was investigated in herbicide-treated barley (Hordeum vulgare) and a pernicious weed, wild oats (Avena ludoviciana), to test the hypothesis that inhibitory effects on fatty acid elongation could be counteracted by the safer, dichlormid. Pebulate and its sulphoxide derivative (thought to be the active metabolite in vivo) were tested against lipid metabolism in barley or wild oat shoots. In both plants there was a significant inhibition of very long chain fatty acid (VLCFA) synthesis at herbicide concentrations greater than or equal to25 muM. The extent to which safener dichlormid could prevent the inhibition of VLCFA synthesis was different in the two species. Previous treatment of barley with dichlormid (N,N'-diallyl-2,2-dichloroacetamide) enabled fatty acid elongation in the presence of pebulate or pebulate sulphoxide, but had no effect on wild oats. The effects on fatty acid elongation mimicked the differential safening action of dichlormid observed on shoot elongation and growth in the two species. These data provide further evidence that inhibition of VLCFA formation is important for the mechanism of action of thiocarbamates.