, Volume 92, Issue 3, pp 363-371

Effects of low pH and high Al, Mn and Fe levels on the survival ofRhizobium trifolii and the nodulation of subterranean clover

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Summary

The effects onRhizobium trifolii and the nodulation of subterranean clover of possible inhibitors associated with soil acidity were evaluated. In an inorganic salts solution in the presence of subterranean clover, the numbers ofR. trifolii increased, but the population declined at pH 4.2. Nodules were formed only at pH 4.8 and above. The bacterium grew after a lag period in a culture medium containing 75 μM Al or 100 μM FeSO4. At 150 μM Mn, the formation of nodules was not inhibited. At 200 μM Fe as an EDTA chelate,R. trifolii grew at the expense of plant excretions, but nodulation was scant. Rhizobial numbers did not increase and nodules were not formed at 500 μM Fe as the EDTA chelate or at 200 μM FeSO4. In the presence of 50 μM Al, rhizobial numbers declined around clover roots, and nodules were not formed. The data show that Al is toxic toR. trifolii growing on root excretions and that low pH and high Fe concentrations are detrimental to the formation of nodules on subterranean clover even in the presence of large numbers of rhizobia.