Herbicide Resistant Weeds

Mechanism of inheritance of diclofop resistance in Italian ryegrass (Lolium multiflorum).

Betts, K. J., N. J. Ehlke, D. L. Wyse, J. W. Gronwald, and D. A. Somers. 1992. Mechanism of inheritance of diclofop resistance in Italian ryegrass (Lolium multiflorum). Weed Science. 40: 184-189.

A diclofop-methyl-resistant biotype of Italian ryegrass was characterized to determine the expression and inheritance of herbicide resistance and whether this trait was due to the presence of a diclofop-insensitive form of acetyl-coenzyme A carboxylase (ACCase). At the whole plant level, the resistant biotype was >93-fold more resistant to diclofop-methyl than the susceptible biotype. Crosses of diclofop-resistant and -susceptible plants were performed to produce F1 plants. No maternal effects were evident in responses of reciprocal F1 plants to diclofop. GR50 diclofop rates determined for resistant, F1 and susceptible plants were 15, 6.3 and 0.16 kg/ha, resp. F2 populations treated with 7.5 kg/ha diclofop exhibited 3 injury response phenotypes 3 weeks after treatment: a susceptible (S) phenotype which was killed, an intermediate resistance (I) phenotype with severe injury and a resistant (R) phenotype with little or no injury. Testcross progeny exhibited only I and S phenotypes. Observed segregation of phenotypes in F2 and testcross populations conformed to segregation ratios predicted for a trait with inheritance controlled by a single partially dominant nuclear gene. ACCase activity determined in crude cell-free extracts of resistant, F1 and susceptible biotypes exhibited I50 values of 50, 20 and 0.7 æM diclofop, resp. A positive relationship between the injury response phenotype and site of action (ACCase) response to diclofop was evident in both F1 and F2 populations. In extracts from R, I and S phenotype F2 plants, 20 æM diclofop acid inhibited ACCase-mediated incorporation of 14C by 27.1, 45.1 and 78.9%, resp. The ACCase data were consistent with the hypothesis that diclofop resistance in Italian ryegrass was conferred by a diclofop-insensitive form of ACCase.