Plant specificities and deoxyribonucleic acid homologies were studied among 122 strains of . Some strains were assigned to species on the basis of their source of isolation and present nodulation capabilities, but many did not fit into one of the six currently recognized species of the genus . Among those strains assigned to species were many which also nodulated plants outside their species-specific, cross-inoculation group. Conversely, isolates from a wide variety of plants could be designated since they were capable of nodulating . Acid production and growth rate on yeastmannitol agar were tested for all strains. Some strains grew rapidly but did not produce an acid reaction; these were grouped with the fast growing acid producers. Deoxyribonucleic acid homology was used to identify four genetic groups of fastgrowing, acid-producing rhizobia. Group 1 included strains of (except strains obtained from (obtained from ), and two strains obtained from . Group 2 comprised six American strains obtained from crown vetch (), sainfoin (), and spp. Species status for this group should remain tentative until further strains have been studied. Group 3 corresponded with as presently defined. Group 4 included fast-growing rhizobia, two strains obtained from , and a wide variety of previously unclassified strains. Nine fastgrowing strains could not be included in any of these groups. The nine slowgrowing, non-acid producing strains included in this study showed < 10% homology with DNAs from seven fast-growing reference strains. The relationships between subgroups in group 1 are discussed, and the genetic diversity of strains obtained from is examined. It is proposed that fast-growing rhizobia comprise at least four species corresponding with the four genetic groups described.


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