Inheritance of fenoxaprop-P-ethyl resistance in a blackgrass (Alopecurus myosuroides Huds.) population

A blackgrass population has developed resistance to fenoxaprop-P-ethyl foll owing field selection with the herbicide for 6 consecutive years. Within th is population, 95% of the individuals are also resistant to flupyrsulfuron. Both the inheritance(s) and the mechanism(s) of resistances were investiga ted by making crosses between the resistant and a susceptible biotype. The inheritance was followed through the F-1 and F-2 generations either by spra ying the herbicide on seedlings at the three-leaf stage or using a seedling bioassay, based on coleoptile length. No maternal effects were evident in the fenoxaprop-P-ethyl responses of the F-1 plants, suggesting that the inh eritance was nuclear. Some F-1 families treated with fenoxaprop-P-ethyl seg regated in a 3:1 (resistant: susceptible) ratio, indicating that the resist ance was conferred by two dominant and independent nuclear genes. This was confirmed by the 15:1 (R:S) ratio observed in the F-2 generation treated wi th fenoxaprop-P-ethyl. The use of selective inhibitors of herbicide detoxif ying enzymes (aminobenzotriazole, pyperonylbutoxide, malathion and tridipha ne) with the F-2 plants suggested that each of the two genes may govern two different mechanisms of fenoxaprop-P-ethyl resistance: the ACCase mutation previously postulated and an enhanced herbicide metabolism, mediated by cy tochrome P 450 mono-oxygenases (P 450) susceptible to malathion. The P 450 activity may also confer resistance to flupyrsulfuron. This study clearly i ndicates that two distinct mechanisms of resistance may co-exist in the sam e plant.