As a follow-up to greenhouse studies, acetolactate synthase (ALS) (EC 188.8.131.52) was extracted from one imidazolinone (IMI)-susceptible (S) and three IMI-resistant (R1, R2, and R3) smooth pigweed populations, and activity was assayed in the presence of imazethapyr, chlorimuron, thifensulfuron, and pyrithiobac. ALS inhibitor concentrations, required to reduce enzyme activity by a specified percentage compared with the untreated control (Ip), were determined for each herbicide using regression analysis, and resistance ratios were calculated from these values. An I50 value of >35 μM imazethapyr was calculated for all R populations compared with a value of 3.4 μM for the S population. With chlorimuron, thifensulfuron, and pyrithiobac data sets, pairwise comparisons of regression coefficients were used to determine significant differences between regression lines. Using this technique, it was established that ALS from R3 was more sensitive than ALS from S to inhibition by chlorimuron and thifensulfuron. Also, ALS from R2 and R3 displayed increased sensitivity to pyrithiobac compared with ALS extracted from the S population. We have confirmed enzyme-level resistance to imazethapyr in all R populations and have documented negative cross-resistance in some R populations to ALS inhibitors other than imazethapyr.
Nomenclature: Chlorimuron; imazethapyr; pyrithiobac; thifensulfuron; smooth pigweed, Amaranthus hybridus L. AMACH.