Differential effects of the sulfonylurea herbicides chlorsulfuron and sulfometuron methyl on microorganisms

The sulfonylurea herbicides exert their effects on cells via their inhibition of the acetohydroxy acid synthase (AHS) enzymes. Although chlorsulfuron and sulfometuron methyl often affected microbial growth differently their effects on the AHS activities of toluenised cells were similar. Sulfometuron methyl was always a more potent inhibitor than chlorsulfuron. We have postulated that sulfometuron methyl penetrated into microbial cells more readily then did chlorsulfuron. The effect of the herbicides on microbial growth was altered by the composition of the medium and in particular by valine or valine plus isoleucine. Different microorganisms had different complements of AHS isoenzymes which together with differences in permeability were the most likely explanations for the different responses observed. It was pointed out that application of these sulfonylurea herbicides would have significant effects on the microbial ecological balance of soil, and particularly so in alkaline soils. The consequences would be most evident in agricultural situations where the microbial population played an important role in maximising the productivity of crops.