Competitiveness of KIM 5 and VIKING 1 bean rhizobia: Strain by cultivar interactions

Competitiveness, which in rhizobia refers to the relative ability of a strain to inject a legume and cause nodule formation in the presence of other strains, is critical to biological N₂-fixation. The mechanisms for strain competitiveness are unknown, but are probably affected by biotic and abiotic factors. The competitiveness of two strains of rhizobia (Rhizobium leguminosarum biovar phaseoli) KIM 5 and VIKING 1 that are known to be highly competitive were evaluated when added together as inoculant to 12 bean (Phaseolus vulgaris L.) host cultivars grown in Leonard jars in sterilized sand. Strain identification of bacteria from harvested nodules was determined by fluorescent antibodies. KIM 5 was generally more competitive than VIKING 1, but competitiveness of both strains was highly dependent on strain by cultivar interactions. With Tendergreen, no nodules contained VIKING 1 alone, whereas with Jampa, 46.3 of the nodules contained only VIKING 1. Comparisons of parent cultivar and sib progeny lines showed that even small differences in the host genotype affected the competitiveness of both strains. All symbioses were effective in fixing N₂. Based on these data, studies evaluating the competitiveness of rhizobia should utilize several host genotypes to avoid misinterpretations of the competitiveness of a given strain.