INHERITANCE AND MECHANISM OF RESISTANCE TO HERBICIDES INHIBITING ACETOLACTATE SYNTHASE IN SONCHUS-OLERACEUS L

A biotype of Sonchus oleraceus L. (Compositae) has developed resistance to herbicides inhibiting acetolactate synthase (ALS) following field selection with chlorsulfuron for 8 consecutive years. The aim of this study was to determine the inheritance and mechanism of resistance in this biotype. Determination of ALS activity and inhibition kinetics revealed that K-m and V-max did not vary greatly between the resistant and susceptible biotypes. ALS extracted from the resistant biotype was resistant to five ALS-inhibiting herbicides in an in vitro assay. ALS activity from the resistant biotype was 14 19, 2, 3 and 3 times more resistant to inhibition by chlorsulfuron, sulfometuron, imazethapyr, imazapyr and flumetsulam, respectively, than the susceptible biotype. Hybrids between the resistant and a susceptible biotype were produced, and inheritance was followed through the F-1, F-2 and F-3 generations. F-1 hybrids displayed a uniform intermediate level of resistance between resistant and susceptible parents. Three distinct phenotypes, resistant, intermediate and susceptible, were identified in the F-2 generation following chlorsulfuron application. A segregation ratio of 1:2: 1 was observed, indicative of the action of a single, nuclear, incompletely dominant gene. F-3 families, derived from intermediate F-2 individuals, segregated in a similar manner. Resistance to herbicides inhibiting ALS in this biotype of S. oleraceus is due to the effect of a single gene coding for a resistant form of the target enzyme, ALS.