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The genetic structure of a population of Sinorhizobium meliloti (formerly Rhizobium meliloti) isolated from nodules of alfalfa (Medicago sativa cv. Europe) or isolated directly from the soil using an elective medium and colony hybridization was assessed by three molecular methods. Sixty‐seven S. meliloti isolates were obtained from alfalfa nodules and 61 directly from soil. Plasmid profiles of S. meliloti isolates were analyzed as well as insertion sequence (IS) fingerprints using probes corresponding to the two indigenous IS elements: ISRm1 and ISRm5. The presence of a gene involved in nodule forming efficiency (nfeA gene) was also determined by PCR using specific primers, among S. meliloti isolates. Plasmid profile analysis revealed the presence of 23 distinct plasmid types among the 128 isolates tested and the distribution of plasmid types was significantly different between isolates obtained from nodules and isolates obtained from soil. The distributions of plasmid types among S. meliloti nodule isolates were similar to those in a collection of nodule isolates obtained from the same field 8 years previously. We have thus demonstrated that the distribution of plasmid types in alfalfa nodules has not varied over long periods in the absence of the host plant. IS fingerprinting was more discriminative than plasmid profiling as a total of 65 distinct IS genotypes were found among the same isolates. The distribution of IS genotypes was different between isolates obtained from nodules and those isolated from soil. These results suggest that sampling S. meliloti populations using the host plant may not be representative of existing soil populations. Genes like nfeA were detected in neither the nodule nor soil isolates. The dominant plasmid type in nodule isolates (30% of the population), carrying a large plasmid of 230 kb, is poorly represented among soil isolates (8%). This result suggests that this plasmid might be involved in the competitiveness of S. meliloti strains.

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Four insecticides, carbofuran, chlormephos, terbufos and benfuracarb, currently used on maize (Zea mays) at sowing, were tested for their compatibility with Azospirillum lipoferum strain CRT1 used as an inoculant to improve maize growth and yield. The growth or survival of A lipoferum was studied in the presence of the insecticides: (1) in liquid and solid cultures of the bacteria, (2) when a commercial inoculant (Azogreen-m, Liphatech, Meyzieu, France) was inoculated directly on insecticide granules, (3) when inoculated Azogreen-m granules were mixed with insecticide granules and (4) when inoculated Azogreen-m granules were delivered separately to the seed bed. Of the four insecticides tested, only terbufos had a slight effect on growth of A lipoferum in solid cultures. All the insecticides decreased the survival of A lipoferum when the bacteria were inoculated directly on to the granules, or when inoculated Azogreen-m granules were mixed with an insecticide. We hypothesize that the discrepancies between bacterial culture tests and survival studies might be explained by the conditions of desiccation encountered during inoculation of the granules. Desiccation stress could increase the toxic effect of the insecticides. We therefore suggest including desiccation stress in the biotest used to assess inoculant-pesticide compatibility.

Commercial soybean inoculants processed with sterilised peat and stored at 20 degrees C for 1-8 years were used as experimental materials to assess the changes in the physiological activity of Bradyrhizobium japonicum after storage. Viable counts decreased and physiological characteristics of the bacterium changed during storage, with an increase in the time taken for colony appearance on a medium without yeast extract, an increase in the lag time for nodule appearance on soybean grown in glass tubes and a decrease in survival on seeds. All the inoculants produced a significant increase in grain yield in a field experiment. The percentage of efficient cells in the field (relative to the plate counts) decreased as the length of storage increased. These results suggest that the physiological activity of B. japonicum cells changes after storage. Practical implications for inoculant quality control are discussed.