Amitrole inhibits diclofop metabolism and synergises diclofop-methyl in a diclofop-methyl-resistant biotype of Lolium rigidum

Biotype SLR 31 of Lolium rigidum has developed resistance and cross-resistance to a wide range of herbicides. Herbicide resistance in SLR 31 has been demonstrated to be the result of multiple mechanisms of resistance. Despite much speculation, the mechanisms endowing resistance to the acetyl-coenzyme A carboxylase (ACCase)-inhibiting herbicide diclofop-methyl have not been fully elucidated. Two subsets of this population were isolated based on root length of individuals germinated on agar containing 3 mu M of the ACCase-inhibiting herbicide fluazifop-P-butyl. Those individuals with normal root length were highly resistant to all ACCase-inhibiting herbicides and contained a herbicide-resistant target enzyme, ACCase. Those individuals with highly stunted roots were resistant to diclofop-methyl and fluazifop-P-butyl, but not to sethoxydim, and contained a herbicide-sensitive ACCase. Biotype SLR 31 also has enhanced metabolism of diclofop acid. Amitrole, a non-ACCase-inhibiting herbicide, was demonstrated to be an inhibitor of diclofop acid metabolism in both SLR 31 and the susceptible biotype and synergised the effect of diclofop-methyl an both biotypes. This demonstrates that enhanced diclofop acid metabolism can confer substantial resistance to diclofop-methyl in L. rigidum. In contrast amitrole was unable to synergise the effect of chlorsulfuron, an acetolactate synthase-inhibiting herbicide, to which SLR 31 is resistant also due to enhanced herbicide metabolism. Amitrole also did not inhibit chlorsulfuron metabolism in SLR 31. Resistance to diclofop-methyl in biotype SLR 31 is due to at least two mechanisms: enhanced metabolism, which provides 26-fold resistance, and a less sensitive target site, which provides 9-fold resistance. When the two mechanisms are combined, individuals are 239-fold resistant compared to the susceptible population. (C) 1998 academic Press.