A MECHANISM OF CHLOROTOLURON RESISTANCE IN LOLIUM-RIGIDUM

Many biotypes of Lolium rigidum Gaud. (annual ryegrass) have developed resistance to herbicides; however, few have developed resistance to p henylurea herbicides. Two biotypes with different histories of herbici de selection pressure were six to eight times less sensitive to the ph enylurea herbicide, chlorotoluron, than a susceptible biotype. Resista nce was not due to differences in the herbicide target site as oxygen evolution by thylakoids isolated from resistant and susceptible biotyp es was similarly inhibited by diuron and chlorotoluron. There was no d ifference in the uptake and distribution of chlorotoluron into resista nt and susceptible plants. There was a twofold greater rate of chlorot oluron detoxification in resistant plants with N-demethylation being a major detoxification reaction. Resistant plants treated with a 3-h pu lse of 120 muM chlorotoluron recovered net carbon fixation after 42 h, half the time taken by susceptible plants. The mixed-function oxidase inhibitor 1-aminobenzotriazole (70 muM) intensified the effects of ch lorotoluron in resistant plants when applied in combination with the h erbicide for 7 d. 1-Aminobenzotriazole also inhibited the metabolism o f chlorotoluron in both resistant and susceptible plants. The cytochro me P-450 inhibitor, piperonyl butoxide piperonyl butoxide, interacted with chlorotoluron when applied to plants growing in soil. Chlorotolur on applied with reduced plant dry weight to a greater extent than chlo rotoluron alone. It appears, therefore, that enhanced detoxification i s the major mechanism of resistance to chlorotoluron in the resistant biotypes studied.