Different toxic effects of the sulfonylurea herbicides metsulfuron methyl, chlorsulfuron and thifensulfuron methyl on fluorescent pseudomonads isolated from an agricultural soil

A total of 77 strains of genetically different fluorescent Pseudomonas strains were isolated from an agricultural soil. In pure culture growth experiments the strains were screened for their ability to grow in various concentrations of the sulfonylurea herbicides metsulfuron methyl, chlorsulfuron and thifensulfuron methyl. We found that the presence of the herbicides resulted in a reduction of the growth of the fluorescent pseudomonads. Metsulfuron methyl was shown to be toxic to a major proportion of the strains in low concentrations. Chlorsulfuron was found to be less toxic in low concentrations but toxic in high concentrations. Thifensulfuron methyl was toxic only to a minority of the strains. Indirectly, the growth-reducing effect of the sulfonylurea herbicides was shown to be caused by an inhibition of the enzyme acetolactate synthase. The enzyme is involved in the synthesis of the branched amino acids valine, leucine and isoleucine, and we demonstrated that the toxic effects of the sulfonylurea herbicides could be neutralized when the strains were grown in the presence of an excess amount of the three amino acids.