Effect of 2,4-D on the Hydrolysis of Diclofop-Methyl in Wild Oat (Avena fatua)

The basis for 2,4-D [(2,4-dichlorophenoxy)acetic acid] antagonism of diclofop-methyl {methyl 2-[4-(2,4-dichlorophenoxy) phenoxy] propanoate} toxicity to wild oat ( Avena fatua L.) was investigated by studying changes in the metabolism of diclofop-methyl in vitro. An esterase from wild oat, which hydrolyzes diclofop-methyl to the acid diclofop, was extracted, partially purified, and the reaction characterized. The rate of hydrolysis of 14 C-diclofop-methyl was 0.14 ηmoles/2 h at standard assay conditions of 0.25 mg lyophilized enzyme preparation (19.6% protein) in 0.1 ml phosphate buffer (0.1 M, pH 7.0), substrate 5 μM. The addition of 2,4-D to this reaction did not inhibit 14 C-diclofop formation. Higher levels of 2,4-D stimulated enzymic hydrolysis. 14 C-diclofop-methyl was rapidly metabolized to 14 C-diclofop and polar 14 C-conjugates when vacuum-infiltrated into wild oat leaf segments. The addition of 2,4-D caused small increases in the rates of both 14 C-diclofop-methyl de-esterification and 14 C-diclofop conjugation. It is concluded that 2,4-D does not inhibit the in vitro de-esterification of diclofop-methyl.