Soil acidity and the composition of an indigenous population of Rhizobium tripolii in nodules of different cultivars of Trifolium subterraneum L.

Three methods of strain identification were used to determine the composition of the Rhizobium trifolii population in nodules formed on four cultivars of Trifolium subterraneum L. grown in a soil unamended (pH 4.8) or amended with CaCO₃ (pH 6.4). Whole cell somatic tube agglutination placed 68–90% of the nodule occupants from each of the cultivars Mt Barker, Nangeela and Howard into four identifiable serogroups. Only in the case of cultivar Woogeneliup were the majority of the isolates unidentifiable with the antisera at our disposal. In unamended soil different serogroups dominated each cultivar whereas in limed soil serogroup 27 was the dominant identifiable serogroup on all of the cultivars. The latter serogroup was also the most promiscuous nodule occupant being found in seven of eight treatments. Gel-immune-diffusion analysis revealed three serotypes within serogroup 27; serotype 27-A was dominant in the nodules on cultivar Nangeela alone whereas serotype 27-B was dominant on both cultivars Mt Barker and Woogenellup. Separation of cellular proteins by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) revealed four and five different strains within serotypes 27-A and 27-B respectively. Only one strain from serotype 27-B could be considered a common nodule occupant on cultivars Mt Barker and Woogenellup. Two of three strains, representing 82% of the total isolates of serotype 27-B found on cultivar Mt Barker, were nodule occupants of that cultivar in both soil treatments. In contrast, only one of four strains, representing 38.5% of the total isolates of serotype 27-A found on cultivar Nangeela, could be considered a common nodule occupant in both soil treatments.