Differential sensitivity of plant-associated bacteria to sulfonylurea and imidazolinone herbicides

Abstract

The side effects of sulfonylurea and imidazolinone herbicides on plant-associated bacteria were investigated under pure culture conditions. Eighteen isolates, belonging to the generaAzotobacter, Azospirillum, Bacillus, Enterobacter Pseudomonas and Serratia, were exposed to four active compounds at concentration ranges similar to those in field soil. The sulfonylureas chlorsulfuron and rimsulfuron inhibited the growth of one of two Azospirillum and one of four Pseudomonas strains, while the imidazolinones imazapyr and imazethapyr were effective on two out of five Bacillus isolates. Surfactants in commercial formulation significantly enhanced rimsulfuron toxicity. With the exception of one Azospirillum strain, the differential tolerance of rhizobacteria to these herbicides was related to a differential sensitivity of their target, the activity of the first enzyme in branched-chain amino acid biosynthesis, acetohydroxyacid synthase (AHAS).Greenhouse pot studies were performed to assess the occurrence of inhibitory effects on bacterial growth in field conditions. Maize seedlings were bacterized with the two strains which had shown in vitro sensitivity to sulfonylureas. Following the application to the soil of a commercial formulation of rimsulfuron at rates of 0, 0.2 and 0.5 #mol a.i. kg-l, significative differences in the resulting degree of bacterial root colonization were found. Moreover, upon co-inoculation with two strains, one tolerant and one sensitive to the herbicide, the presence of rimsulfuron significantly enhanced root occupancy by resistant bacteria, suggesting that shifts in the microbial community structure of crop rhizosphere could indeed result as a consequence of weed control by AHAS inhibitors.Abbreviations: AHAS-acetohydroxyacid synthase, CETAB-cetyltrimethylammonium bromide, IDs0-concentration causing 50% inhibition of enzyme activity, LDso--concentration causing 50% decrease of growth constant value.

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

“…It was found that the sensitivity to inhibition of the herbicides varied greatly among strains. Previously, it has been shown that sulfonylurea herbicides can have a toxic e¡ect on microorganisms [14,15]. Chlorsulfuron has been reported to be toxic in concentrations ranging from 3 WM [16] to 2.8 mM [14] to di¡erent microorganisms.…”

Yield of maize hybrids: Is there any association among nitrogen rate, Azospirillum inoculation and fungicide treatment?

“…However, these authors recognize the lack of studies addressing this important issue and that most of them were performed under in vitro conditions. Research on the effects of agricultural pesticides on Azospirillum species are available, focusing on herbicides (Jena et al, 1990;Salmeron et al, 1991;Omar et al, 1992;Rivarola et al, 1992;Forlani et al, 1995) and insecticides (Langenbach et al, 1991;Buff et al, 1992;Sánchez et al, 1994).…”

Herbicides: the Face and the Reverse of the Coin. An in Vitro Approach to the Toxicity of Herbicides in Non-Target Organisms

“…If, on the one hand, the biodegradation constitutes an important factor favoring the removal of herbicides from the environment, on the other hand, the elimination of sensitive microorganisms and the proliferation of the most tolerant ones, due to the selection pressure exerted by those compounds, may cause changes to the existing microbiological balances with potential adverse consequences for the ecosystem. An example of differential sensitivity to herbicides has been found for sulfonylurea herbicides among fresh-water and marine micro-algae (Nystrom et al, 1999), microbial aquatic communities (Thompson et al, 1993) and soil bacteria (Forlani et al, 1995).…”