Basis of imazethapyr selectivity in cowpea (Vigna senensis), interactions between insecticides and cytochrome P450 activities, and cytochrome P450 monooxygenase metabolism of imazethapyr

Greenhouse and laboratory experiments were conducted to determine the basis of imazethapyr tolerance in cowpea. Cowpea was greater than 10-fold more tolerant than corn of foliar applied imazethapyr than corn. Extractable acetohydroxy acid synthase activity from cowpea was inhibited 24 after foliar imazethapyr treatment, then recovered to levels similar to the untreated control by 72 HAT. Parent imazethapyr decreased from 62 to 40% of the recovered ;We used in vitro assays to determine if organophosphate, carbamate, and pyrethroid insecticides affected the P450 catalyzed hydroxylations of nicosulfuron, bentazon, cinnamic acid, or lauric acid in corn microsomes. Terbufos moderately inhibited P450 hydroxylation of nicosulfuron. Terbufos-sulfone inhibited metabolism of nicosulfuron and imazethapyr, but not bentazon, in excised corn shoots. Metabolism of several pesticides, but not cinnamic acid, were strongly inhibited by terbufos-sulfone in corn microsomes. The insecticides that were the most potent inhibitors of nicosulfuron hydroxylation in microsomes are not necessarily the ones that cause the most nicosulfuron injury to corn in the field. However, inhibition of P450 hydroxylation of nicosulfuron by terbufos sulfone can explain the corn injury from this herbicide when used in conjunction with terbufos.;Imazethapyr conversion to the hydroxyethylpyridine metabolite was assayed in vitro using microsomes isolated from 3.5 day old naphthalic anhydride treated corn shoots. The addition of up to 4 mg ml...