A DICLOFOP-METHYL-RESISTANT AVENA-STERILIS BIOTYPE WITH A HERBICIDE-RESISTANT ACETYL-COENZYME-A CARBOXYLASE AND ENHANCED METABOLISM OF DICLOFOP-METHYL

An Avena sterilis biotype was found to be highly resistant to aryloxyp henoxypropionate (APP) herbicides, especially diclofop-methyl. At the enzyme level, this biotype contained a modified acetyl-coenzyme A carb oxylase (ACCase) with six-fold resistance to diclofop acid. Absorption and translocation of [C-14]diclofop-methyl applied to the leaf axil o f the two-leaf stage plants were similar in both susceptible and resis tant biotypes. However, the rate of metabolism of [C-14]diclofop was i ncreased 1.5-fold in this resistant biotype compared to the susceptibl e. Experiments with tetcyclacis, a cytochrome P450 monooxygenase inhib itor, indicated that inhibition of this enhanced diclofop metabolism i ncreased diclofop-methyl phytotoxicity in this biotype. Studies with t en individual families of the resistant biotype indicated that both me chanisms of resistance, an altered target site and enhanced metabolism , are present in each individual of the population. Hence, it is likel y that these two mechanisms of resistance both contribute to resistanc e in this biotype.