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AbstractAttempts to improve the symbiotic nitrogen fixation with effective (Brady) rhizobiumstrains do not always succeed under field conditions due to the lower nodulation competitiveness of the introduced strains than that of the indigenous rhizobia (Triplett and Sadowsky 1992). An introduced strain needs to be marked for monitoring its nodule occupancy under competitive nodulation conditions.

Soybean bradyrhizobia, Bradyrhizobium japonicumand Bradyrhizobium elkaniidiffer in various traits such as DNA fingerprint, rhizobitoxine production, indole-3-acetic acid production and uptake of hydrogenase. In this communication, we investigated whether the differences between both species extend to host preference in multistrain environments. Nodule occupancy of B.japonicumand B. elkaniisignificantly depended on host plants. B. japonicumand B. elkaniipreferentially nodulated Glycine maxand M. atropurpureum, respectively. Both bacterial species were shown to nodulate G. sojawith similar efficiency. There was a significant divergence of DNA sequences in and around nodulation genes between the B. japonicumand B. elkaniifield isolates. However, flavonoid and lipo-chitin nodulation signals were not involved in the difference in host preference of B. japonicumand B. elkanii. The ecological implications of host preference are discussed.