HOST PLANT EFFECT ON COMPETITION AMONG STRAINS OF RHIZOBIUM-LEGUMINOSARUM

Analysis of plasmid profiles was used to type Rhizobium leguminosarum biovar viciae strains isolated from nodules of peas, lentils and faba beans grown in two different soils. One soil was from a native pasture with no previous history of cultivation, the other was from a plot in a rotation study which included lentils every 2 years. The results indicated a strong preference of both peas and faba beans for strains having certain specific plasmid profiles. Strains belonging to one plasmid profile group (group 2) formed over half the nodules on peas grown in soil from the rotation plot but were never found on faba beans grown in the same soil, while strains from another group (group 5) formed nearly all of the nodules on faba beans grown in soil from the rotation plot, but no nodules on peas. Competitiveness for pea nodulation was correlated with an ability to catabolize homoserine, an amino acid found in large quantities in pea root exudate. Strains having plasmid profiles corresponding to those of strains that have been used in commercial inoculants over the last few years were isolated only rarely, regardless of the soil and host plant studied.