Symbiotic characteristics of Rhizobium meliloti: and appraisal of the systematic treatment of nodulation and nitrogen fixation interactions between hosts and Rhizobia of diverse origins

Abstract

The symbiotic properties of 42 isolates of Rhizobium meliloti collected from widely separated sites in the upper basin of the Darling River were tested in bacteriologically controlled culture with 18 species and varieties of Medicago, three species of Melilotus, and one species of Trigonella. Seven strains of Rh. Meliloti of commercial interest were included in the experiments for comparative purposes. Nodule formation occurred on all plants in 889 out of 974 symbiotic combinations examined. Only Medicago laciniata (L.) Mill, among the hosts and the isolate from M. laciniata among the rhizobia exhibited distinctive nodule formation behaviour. Nitrogen fixation occurred in 637 combinations. On the basis of nitrogen fixation, eight groups of Rh. Meliloti were recognized; there were nine host groups. The groups could be arranged in an interlocking pattern of increasing host and bacterial specificity. These results, which modified to a small degree the bacterial and host groupings previously reported, strongly suggested that the pattern will be widely applicable to Rh. meliloti of diverse origin. In general, the isolates examined were promiscuous and 24 were capable of effective symbioses in association with 10 or more of the hosts. M. laciniata, Trigonella suavissima Lindl., and Medicago rugosa Desr. were the hosts most highly strain-specific in their requirements for nitrogen fixation, and Medicago sativa L. was the most promiscuous. A system for classifying host species into compatibility groups is proposed.