THE MECHANISM OF ACTION OF BENSULFURON-METHYL ON HYDRILLA

Bensulfuron-methyl has been reported to specifically inhibit acetohydr oxyacid synthase (AHAS, E.C. 4.1.3.18) in susceptible terrestrial plan ts. AHAS is the first enzyme unique to the synthesis of the branched-c hain amino acids leucine, isoleucine and valine. Extraction of AHAS fr om the submersed aquatic plant hydrilla (Hydrilla verticillata (L.f.) Royle) and the assay conditions used are described. In vitro inhibitio n of AHAS by bensulfuron-methyl was significant. The addition of In M of herbicide resulted in a 25% inhibition of enzyme activity. Increase s in herbicide concentration caused significant increases in inhibitio n (1 mM = 93% inhibition). The I-50 was calculated to be 22 nM. The le vel of inhibition was also shown to be time-dependent. In vivo inhibit ion of hydrilla by bensulfuron-methyl was also investigated and assess ed as growth inhibition (I-50 = 110 nM). These results suggest that th e inhibition of hydrilla growth by bensulfuron-methyl is due to the in hibition of AHAS.