Enhanced sensitivity to cloransulam-methyl in imidazolinone-resistant smooth pigweed

Several populations of smooth pig-weed with resistance to imidazolinone (IMI) herbicides have been identified in recent years. One IMI-resistant population (R2) was 10-fold more sensitive to cloransulam-methyl in the greenhouse when compared with a susceptible (S) population. Laboratory studies were conducted to determine differences in the absorption, translocation, and metabolism of cloransulam-methyl existed between S and R2 populations and to determine if these differences could account for the whole-plant responses observed in the greenhouse. Enzyme assays were also conducted to determine if differences in acetolactate synthase (ALS) sensitivity to cloransulam-methyl existed between S and R2 populations. Absorption foliar-applied cloransulam-methyl was rapid and similar in both populations smooth pigweed. Translocation of the absorbed radioactivity out of the treated was symplastic and generally similar in both populations. Translocated radioactivity was detected primarily in shoots above and below the treated leaf with little movement to the roots. C-14-cloransulam-methyl metabolism was also similar in S and R2 in populations populations. Differential tolerance to cloransulam-methyl in S and R2 the greenhouse cannot be explained by differences in the absorption, translocation, and metabolism of this herbicide. However, ALS from R2 was 25-fold more sensitive to inhibition by cloransulam-methyl than ALS from S.