Metabolism of diclofop-methyl in root-treated wheat and oat seedlings

Abstract

Diclofop-methyl [methyl 2-[4-(2,4-dichlorophenoxy)phenoxy]propanoate] was hydrolyzed initially to diclofop [ 2-[4-(2,4-dichlorophenoxy)phenoxy]propanoic acid] in roots of resistant wheat and susceptible oat. Diclofop was detoxified rapidly in wheat roots by aryl hydroxylation of the 2,4dichlorophenyl moiety to at least three isomeric aryl-hydroxylated compounds and conjugation to acidic aryl 0-glycosides. Virtually no diclofop accumulated in wheat roots, but in oat roots, approximately 35% of the radioactivity recovered was phytotoxic diclofop. The primary pathway in oat roots produced a water-soluble metabolite that was identified by mass spectroscopy as a neutral glucose ester conjugate of diclofop. Small quantities of the three hydroxylated isomers of the acidic aryl 0-glycoside were products of a secondary metabolic pathway in oat roots. The ability of wheat roots to detoxify diclofop-methyl completely by aryl hydroxylation and subsequent conjugation is probably the basis for herbicidal selectivity.Diclofop-methyl is a selective herbicide that controls a number of annual grass weeds in cereal crops (Anderson, 1976; Miller and Nalewaja, 1974). Root and shoot growth in oat and wild oat is inhibited by diclofop-methyl, whereas wheat growth is unaffected (Shimabukuro et al., 1978;Hoerauf and Shimabukuro, 1979). Absorption and translocation of diclofop-methyl are not important factors in selectivity (Brezeanu et al., 1976;Shimabukuro et al., 1979;Jacobson and Shimabukuro, 1982). Gorbach et al. (1977) and Shimabukuro et al. (1979) found a water-soluble conjugate in diclofop-methyl-treated wheat shoots that was acid hydrolyzed to ringhydroxylated diclofop. They suggested that the conjugate was an aryl 0-glycoside. In wild oat shoots, the major water-soluble conjugate was hydrolyzed to the free acid, diclofop, suggesting that the conjugate was a neutral glucose ester (Shimabukuro et al., 1979). This difference in metabolism between resistant wheat and susceptible oat suggests that metabolism is the basis for herbicidal selectivity. This report examines the metabolism in diclofop-methyl in roots of resistant wheat and susceptible oat for two reasons: (1) to compare the metabolism in root and shoot tissues of both species and (2) to further characterize and identify the water-soluble metabolites found in both species.

Reciprocal Antagonism between the Herbicides, Diclofop-Methyl and 2,4-D, in Corn and Soybean Tissue Culture

“…In corn the radioactivity detected as diclofop increased substantially following hydrolysis (Table IV). This indicated that in corn roots DM was metabolized predominantly to water-soluble ester conjugates similar to the glucose ester conjugates identified previously (12). The results did not differ greatly between corn roots treated with DM alone or in combination with 2,4-D (Table IV).…”

“…Diclofop may be conjugated as a glucose ester (12) or ringhydroxylated (2,4-dichlorophenyl-OH) and conjugated as a phenolic conjugate (22). These water-soluble conjugates in tissue extracts may be quantified by HCI hydrolysis and subsequent quantification of diclofop and ring-OH diclofop as hydrolysis products (Tables IV, V).…”

“…Metabolism and detoxication of DM have been established as the basis for herbicide selectivity between resistant and susceptible plants (6,12,21). Aryl hydroxylation to ring-OH diclofop and subsequent phenolic glycosyl conjugation is the predominant detoxication mechanism in resistant plants.…”

Effects of Diclofop and Diclofop-Methyl on the Membrane Potentials of Wheat and Oat Coleoptiles

“…In both plants diclofop-methyl is rapidly hydrolyzed to the free acid by esterases present in the cell free space or in the cytoplasm. The acid may be conjugated to an inactive bound form (glucose ester) or detoxified by aryl glucoside formation (14,23). The lipid solubility of both the ester and the acid allow them to partition into the membrane phase where the acid-base dissociation characteristics (pKa = 3.57) (15), the pH gradient, and the charge difference across the membrane causes diclofop to function as a proton ionophore.…”

“…In both susceptible oats and resistant wheat, diclofop-methyl is rapidly hydrolized to the free acid, diclofop (24442',4'-dichlorophenoxy)phenoxy)propanoic acid. In wheat diclofop is subsequently metabolized by aryl hydroxylation and conjugation to form an acidic, aryl-glucoside; in oat, diclofop is metabolized to a neutral glucosyl ester (7,14,21,23 ofdiclofop-methyl is less well understood than is the metabolism. In susceptible plants such as oat, diclofop-methyl induced an inhibition of leaf, stem, and root elongation, chlorosis of leaves (11), and severe damage to cellular ultrastructure (2).…”