LIGHT-DEPENDENT ENHANCED METABOLISM OF CHLOROTOLURON IN A SUBSTITUTEDUREA HERBICIDE-RESISTANT BIOTYPE OF LOLIUM-RIGIDUM GAUD

A population of Lolium rigidum Gaud. displays resistance to the herbic ide chlorotoluron endowed by enhanced metabolism of this herbicide. Th e level of resistance in intact plants of this population is light dep endent. Resistance is about 4-fold at 110 mu mol photons m(-2). s(-1), but increases to 11-fold at 600 mu mol photons m(-2). s(-1). For seed lings grown in the dark, the rate of chlorotoluron metabolism is ident ical between biotypes; however, seedlings of the resistant biotype gro wn in the light display enhanced chlorotoluron metabolism compared to the susceptible biotype. Specifically, light with blue wavelengths ind uces chlorotoluron metabolism in the resistant biotype. An analysis of the metabolites produced indicates that two routes of chlorotoluron m etabolism occur in L. rigidum. These are characterised by initial reac tions leading to ring-methyl hydroxylation or N-demethylation of the h erbicide. The ring-methyl hydroxylation pathway is increased greatly i n light-grown resistant seedlings compared to susceptible seedlings, w hereas the N-demethylation pathway is only slightly increased. The dif ferential induction of these two pathways in resistant L. rigidum by l ight suggests that enhanced activity of two different enzymes may be i nvolved in chlorotoluron resistance.