MECHANISM OF RESISTANCE TO CHLOROTOLURON IN 2 BIOTYPES OF THE GRASS WEED ALOPECURUS-MYOSUROIDES

Possible mechanisms of resistance to the photosystem II (PSII)-inhibit ing herbicide chlorotoluron (3-(3-chioro,p-tolyl)-1,1-dimethylurea) we re examined in two resistant biotypes (Peldon Al and Lines El) of Alop ecurus myosuroides from the United Kingdom. The resistance indices (th e ratio of ED,, values relative to a susceptible standard, Rothamsted) were found to be 28 for Peldon Al and 2.6 for Lines El biotypes, base d on whole plant responses. There were no significant differences betw een resistant and susceptible biotypes in rates of herbicide uptake an d translocation. Similarly, the herbicide target site, PSII, was found to be equally sensitive to inhibition by chlorotoluron in all biotype s, indicating changes at the herbicide active site were not responsibl e for resistance. Both resistant biotypes had a markedly enhanced abil ity to metabolize chlorotoluron. Following a 24-hr herbicide pulse, 81 % of chlorotoluron was metabolized by resistant Peldon Al compared wit h 37% by Lines El and 6% by susceptible Rothamsted. Twenty-four hours later, 94, 48, and 14% of the chlorotoluron was metabolized by Peldon Al, Lines El, and Rothamsted, respectively. Chlorotoluron metabolism w as decreased, and herbicide phytotoxicity correspondingly increased by treatment in the presence of the cytockrome P450 inhibitor 1-aminoben zoiriazole, suggesting that increased activity of cytochrome P450 mono oxygenses may be responsible for herbicide degradation and, therefore, herbicide resistance in two resistant biotypes. (C) 1995 Academic Pre ss. Inc.