Index >> Rhizobium and Legume Root Nodulation >>Ecology

In the absence of legumes, the soil population of rhizobia declines. However, rhizobia are known to survive for 19 to 45 years despite the fact that they are non-spore formers. Rhizobium lupini and R. japonicum have been found to be comparatively resistant to high soil temperatures unlike R. trifolii and R. meliloti. It is remarkable that in spite of high temperature, tropical rhizobia nodulate Acacia, Lotus and Psorales, probably by adaptation to such temperature regimes.

Rhizobium is preferentially stimulated in the rhizosphere of legumes than in that of non-legumes. A given legume tends to promote the multiplication of bacteria able to infect it more than others, although evidence is available in literature to show that strains of different cross-inoculation groups are stimulated to the same degree by one and the same legume.

The inhibitorv or stimulatory effects of soil microorganisms such as bacteria, fungi and actmomycetes on Rhizobium are known. Culture filtrates of fungi isolated from soil and those isolated from washed nodules often inhibit the growth of rhizobia. The failure Of nodulation in certain parts of Western Australia has been attributed to the presence in soil of microorganisms antagonistic to rhizobia. Among the antibiotics, aureomycin, terramycin and ledermycin and ledermycin are most effective in inhibiting the growth of rhizobia the slow-growing ones less susceptible than fast growing strains.

The effect of soil fungi on rhizobia. The figures denote the number of isolates whose culture filtrates inhibited Rhizobiumand those in parenthesis denote the total number of isolates tested (from Sethi and Subba Rao, 1968)