BIOLOGICAL-ACTIVITY OF 2 STEREOISOMERS OF THE N-THIENYL CHLOROACETAMIDE HERBICIDE DIMETHENAMID

Due to the presence of an asymmetrically substituted C atom, dimethena mid -3-thienyl)-N-(2-methoxy-1-methylethyl)acetamide], a recently intr oduced N-thienyl chloroacetamide herbicide, exists as two stereoisomer s (S and R) having differing herbicidal activities as demonstrated wit h a selection of weeds and Lemna minor. The activity of the two isomer s was investigated in greater detail with the green alga Scenedesmus a cutus and compared to that of alachlor [2-chloro-N-(2, 6-diethylphenyl )-N-(methoxymethyl)acetamide]. As with alachlor, the S isomer (5 mu M) strongly inhibited algal growth and fatty acid desaturation while the R isomer had no effect. In short-term experiments (up to 5.5 h), the S isomer and alachlor (100 mu M) inhibited [C-14]acetate uptake and it s incorporation into fatty acids in the same manner, while the R isome r did not. Incorporation of [(14)]acetate into a non-lipid fraction of the algae was strongly inhibited by alachlor and the 5 isomer (100 mu M) and only slightly by the R isomer. A 50% inhibition of incorporati on of [(14)]oleic add into the same non-lipid fraction was attained wi th less than 10(-7) M of the S isomer while 10(-5) M of the R form of dimethenamid achieved only a 40% inhibition. The same stereospecificit y of the compound on growth, fatty acid desaturation, acetate uptake a nd oleic acid incorporation provides strong evidence that dimethenamid may act upon a primary, specific target in lipid metabolism. Furtherm ore, the comparable biological activities of dimethenamid and alachlor indicate that this target is common to both N-phenyl and N-thienyl ch loroacetamide herbicides.