Effect of monooxygenase inhibitor piperonyl butoxide on the herbicidal activity and metabolism of isoproturon in herbicide resistant and susceptible biotypes of Phalaris minor and wheat

Experiments were carried out under controlled environmental conditions to compare the dose response of isoproturon to resistant (R) and susceptible (S) biotypes of Phalaris minor and wheat and the effect of the cytochrome P-450 inhibitor, piperonyl butoxide (PBO), on the efficacy of isoproturon. Qualitative and quantitative assessments were undertaken on the influence of PBO on metabolism of [14C]isoproturon applied to the base of cut leaves. Resistance to isoproturon in the R biotype was found to be circa 12-fold of that occurring in the S biotype. Addition of PBO lowered the GR50 (growth reduction by 50%) values of the R biotype by 14 times compared to isoproturon alone; in the S biotype of P. minor and wheat comparable reductions were only 2 times. Over 48 h the rate of degradation of [14C]isoproturon was more rapid in the R biotype of P. minor and wheat; the major metabolites were hydroxy isoproturon, monodesmethyl isoproturon, carboxylic acid metabolite, didesmethyl isoproturon, conjugates, and two unidentified metabolites. The hydroxy metabolite was present in a greater proportion in the R than the S biotype. In all the test plants the addition of PBO significantly inhibited demethylation and hydroxylation as 80-94% of the applied [14C]isoproturon was found to be unchanged after 24 h; only 45 to 56% recovery was obtained without PBO. Hydroxylation and demethylation seem to be the major breakdown pathways of isoproturon in P. minor biotypes and wheat. The rapid degradation of [14C]isoproturon in the R biotype of P. minor in the absence of PBO indicates that resistance is governed by increased activity of cytochrome P-450 monooxygenase enzymes. This study illustrates the important role which monooxygenases play in conferring differential sensitivity of P. minor and wheat to the action of isoproturon