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ABSTRACrThe in vivo rates of uptake and detoxification of alachlor and metolachlor were determined using Pioneer corn 3320 seedlings. Equal amounts of the radiolabeled herbicides were applied to etiolated coleoptiles and, at various intervals after treatment, the unabsorbed radioactivity was removed and quantified. Analysis of 80% methanol extracts by reverse phase liquid chromatography showed no significant differences in the rate of uptake of metolachlor and alachlor. However, the rate of glutathione conjugation of alachlor in vivo was two-to threefold greater than the rate for metolachlor at 2 and 4 hours after herbicide application.Since the initial step in detoxification is conjuption of the chloroacetanilide to glutathione, the activities of the enzymes responsible for conjuption, the glutathione-S-transferases (GST) were also analyzed in vitro, using crude extracts and the purified GST enzymes. The specific activities of the extracts were consistent with the results in vivo. Using alachlor as a substrate, the specific activity for glutathione conjugation was almost threefold higher than that for metolachlor. Kinetic analysis of purified GST III indicates that the enzyme has a higher affinity for alachlor (K,,,app = 1.69 millimolar) than for metolachlor (K",app = 8.9 millimolar).

The initial metabolism of acetochlor [2-chloro-N-(ethoxymethyl)-N-(2-ethyl-6-methylphenyl)acetamide] in tolerant and susceptible plants was examined to determine if metabolism is a factor in the selective phytotoxicity of this herbicide. Six crop and ten weed species were used. In every case acetochlor was converted initially to thioether conjugates. In thirteen of the species, initial conjugation was with glutathione (GSH). The homoglutathione (hGSH) conjugate was formed in three legumes. Tolerant plant seedlings metabolized acetochlor more readily than susceptible plant seedlings. Based on these results, acetochlor detoxification is a primary factor in the selective phytotoxicity of this herbicide.

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