Variability in Effectiveness of Rhizobia during Culture and in Nodules

The ability of three strains of Bradyrhizobium sp. (Vigna) to fix dinitrogen in symbiotic association with siratro (Macropitilium atropurpureum) was measured after culture in broth and after isolation from nodules. Seven transfers were made between the initial broth culture and the final broth culture. A total of 40 single-colony isolates were obtained from cultures 1 and 7 to test effectiveness. Variation in dinitrogen-fixing effectiveness of the population of one strain did not change on culturing, whereas there was considerable variation in effectiveness of populations of the other two strains. Generally, single-colony isolates from individual nodules had similar levels of effectiveness, but some exceptions occurred. Isolates from different nodules formed by the same Bradyrhizobium strain often differed in their effectiveness.