Genetic characterization of naturally occurring Rhizobium meliloti populations and their potential to form effective symbioses with lucerne

Genetic characterization by Randomly Amplified Polymorphic DNA (RAPD) fingerprinting was employed to study the status of Rhizobium meliloti populations inhabiting nodules of lucerne. Rhizobium strains were isolated from nodules harvested from plants growing in inoculated or uninoculated experimental plots, uninoculated commercial fields and from lucerne grown in pots containing soils of different origin. Dry matter analyses were recorded and rhizobia were assessed for relative genetic diversity between treatments. Inoculated and uninoculated soils did not differ in terms of dry matter production, and lucerne grew, and was adequately nodulated, in soils with no history of lucerne cultivation. These findings, and the demonstration that there is a rich genetic diversity of Rh. meliloti in these soils, show that it is not always necessary to apply a standard commercial inoculant.