The field distribution of strains of nodule bacteria from species of Medicago

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Abstract
Tests of nodulation and ability to fix nitrogen, together with serological reactions, are reported for localized isolates from Medicago hispida var. denticulata and M. laciniata. For a particular host species strains were distributed randomly within the area investigated, except that a particular somatic antigen found among M. laciniata isolates was limited to one locality. In reciprocal tests between isolates from these two hosts, nodulation has been mainly confined to the homologous host. Although some strains fail in laboratory tests to nodulate the homologous host and although separation into two groups is not complete, there are relatively few cases of discrepancy and many of these show intermediate characteristics. Medicago sativa and Melilotus alba are generally readily nodulated by isolates from both field hosts. The nodulation differences between strains from the two field hosts are paralleled to some extent by serological properties, but it is suggested that this correlation shows recent common origin of similar strains rather than any fundamental relationship between invasibility and antigenic constitution. Effective association was seldom, if at all, achieved with M. laciniata, whereas with other testing hosts effective responses were in the majority and even included a number of isolates originating from M. laciniata and ineffective with that species as host.