A number of herbicides have been reported, over the years, to inhibit lipid metabolism. The effects of many of these compounds are believed to be secondary. However, three classes of herbicides may have a more specific effect on fatty acid (and lipid) synthesis. These groups of compounds are the substituted pyridazinones, thiocarbamates and a rather diverse class of graminaceae-selective herbicides which include the oxyphenoxy propionic acids and cyclohexanediones. These three groups form the substance of this brief review.
The overall topic of herbicides and lipid metabolism has been reviewed by several authors (Duke, 1985; Fedtke, 1982; Rivera & Penner, 1979; St John, 1982).
The mode of action of pyridazinone herbicides has been reviewed recently and appears to involve several target sites in plants (St John, 1982; Duke, 1985). Depending on the exact structure of the compound, and the plant test species, the effects have been noted to include inhibition of photosynthetic O2 evolution, of pigment synthesis and changes in fatty acid composition.
Two substituted pyridazinones which have excited particular interest with regard to their effects on fatty acid formation are San 9785, (BASF 13 338; 4-chloro-5-(dimethylamino)-2-phenyl-3(2H) pyridazinone) and San 6706 (metflurazon); 4-chloro-5-(dimethylamino)-2-(α,α,α-trifluoro-m-tolyl)-3(2H)-pyridazinone)(Figure 1). It was noted by St John (1976) that San 9785 altered the proportions of linoleic and α linolenic acids in monogalactosyl- and digalactosyldiacylglycerol. In contrast, San 9774 merely reduced the proportion of α-linolenate in monogalactosyldiacyl-glycerol alone.