Tomato and pepper differ in their whole-plant tolerance to sulfonylurea (SU) herbicides rimsulfuron and halosulfuron despite both being members of the Solanaceae family. This study examined whether tomato's tolerance to SU herbicides rimsulfuron and halosulfuron was due to insensitivity of the target enzyme acetolactate synthase (ALS). Rimsulfuron and halosulfuron inhibited ALS from both tomato and pepper leaves. Enzyme inhibition and kinetic analyses showed that extractable ALS from tomato was more sensitive to rimsulfuron and halosulfuron than ALS from pepper. ALS from both species were inhibited with a mixed inhibition pattern. Thus, results indicate that enzyme insensitivity is not the reason why tomato is more tolerant than pepper to these herbicides. Tomato tolerance to rimsulfuron at the whole-plant level was reduced in the presence of terbufos, a known inhibitor of cytochrome P450 enzymes. Rimsulfuron applied at 0.018 and 0.035 kg ha⁻¹ with 1.1 kg ha⁻¹ of terbufos reduced tomato shoot weight 69 and 66%, respectively, compared with a 4 and 29% reduction when rimsulfuron was applied alone. The reduction of tomato tolerance to rimsulfuron by terbufos suggests that the sensitivity differences between these species may reflect their differences in SU herbicide metabolism.

Nomenclature: Halosulfuron; rimsulfuron; pepper, Capsicum annuum L. ‘X3R-Camelot’; tomato, Lycopersicon esculentum Mill. ‘FL 47’.