Symbiotic interactions between soybean and competing strains of Bradyrhizobium japonicum

Selected symbiotic characteristics of five Brady rhizobium japonicum strains were assessed in association with 'Ransom' soybean plants (Glycine max [L.] Merr.). In the first of two greenhouse experiments, relative nodulation competitiveness of the strains was examined. Strains were grouped into pairs, and corresponding cells were applied to surface-disinfected seeds so as to provide seven ratios of cell numbers between the two strains. Tap root nodules were harvested 28 days after sowing and serotyped by means of an enzyme-linked immunosorbent assay. Strains differed considerably in nodulation competitiveness, and these differences were successfully quantified using relationships previously proposed in the literature. A second experiment involved assessment of the reproducibility of this technique and characterization of the symbiotic response to single-and double-strain inocula. Differences in relative nodulating abilities of strains were apparent between experiments and were possibly related to observed variations in greenhouse temperatures. Plant shoot weight and total N content were not significantly correlated with nodule number or weight when evaluated across inoculation treatments, but these correlations were often significant within inoculation treatments. Certain double-strain inocula produced either positive or negative effects on shoot weight, N content, and nodulation, when compared with values predicted from corresponding controls receiving single-strain inocula.