INITIAL METABOLISM OF DIMETHENAMID IN SAFENED AND UNSAFENED WHEAT SHOOTS

The metabolic fate of the chloroacetamide herbicide dimethenamid was e xamined in excised shoots of unsafened and safened wheat seedlings. Co mpared to unsafened controls, fluxofenim treatment increased the rate of metabolism of dimethenamid to water-soluble compounds. Thin layer c hromatography revealed that similar dimethenamid metabolites were pres ent in extracts from both safened and unsafened wheat shoots. However, three metabolites were more abundant in extracts from fluxofenim-trea ted wheat relative to unsafened wheat. Reversed-phase highperformance liquid chromatography and mass spectrometry were used to isolate and c onfirm the identity of dimethenamid metabolites extracted from safened wheat shoots 1 h after treatment with dimethenamid. In addition to th e presence of the glutathione conjugate of dimethenamid, gamma-glutamy lcysteine and cysteinylglycine dipeptide conjugates, as well as a cyst eine conjugate and an oxygenated glutathione conjugate of dimethenamid , were detected. On the basis of time course analysis of dimethenamid metabolites by thin layer chromatography and structural identification by mass spectrometry, it appears that from 5 to 100 min after treatme nt with dimethenamid metabolism occurs exclusively via the glutathione conjugation pathway in wheat. Thus, the protection conferred to wheat by fluxofenim treatment is most likely due to increased glutathione-m ediated detoxification of dimethenamid.