Soybean metabolism of chlorimuron ethyl: Physiological basis for soybean selectivity

Chlorimuron ethyl (2-((4-chloro-6-methoxypyrimidine-2-yl)amino carbonyl]amino sulfonyl)benzoic acid, ethyl ester) is a highly active sulfonylurea herbicide for preemergence and postemergence use in soybeans. Excised soybean (Glycine max. cv. ‘Williams’) seedlings rapidly metabolized ¹⁴C]chlorimuron ethyl with a half-life of 1–3 hr. Common cocklebur (Xanthium pensylvancium Wallr.) and redroot pigweed (Amaranthus retroflexus L.), which are sensitive to chlorimuron ethyl, metabolized this herbicide much more slowly (half-life >30 hr). The major metabolite of chlorimuron ethyl in soybean seedlings is its homoglutathione conjugate, formed by displacement of the pyrimidinyl chlorine with the cysteine sulfhydryl group of homoglutathione. A minor metabolite is chlorimuron, the deesterified derivative of chlorimuron ethyl. Each of these metabolites is inactive against plant acetolactate synthase, the herbicidal target site of chlorimuron ethyl. Thus, soybean tolerance to chlorimuron ethyl results from its rapid metabolism in soybean seedlings to herbicidally inactive products.