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Plasmid Profiles of Rhizobia Used in Inoculants and Isolated from Clover Fields

Dep. of Soil and Crop Sciences, Texas A&M Univ., College Station, TX 77843

^* Corresponding author.

The persistence of inoculant strains of Rhizobium is a factor determining the need for inoculation of legumes. Four pastures in east Texas, with known histories of arrowleaf [Trifolium vesiculosum Savi] and crimson [T. incarnatum L.] clover planting and inoculation with R. leguminosarum bv. trifolii, were used to investigate the prevalence of inoculant strains in clover root nodules. Three pastures had mixed stands of the two clover species, and the fourth had only arrowleaf clover. Using plasmid profiles as a means of identifying rhizobial strains, we analyzed 50 rhizobial isolates made from root nodules of each clover species in each of the pastures. Sixty-six plasmid profile types were obtained from the 350 isolates. For the first three sites, only 2 of 300 isolates had plasmid profiles matching that of an inoculant strain, and 5 others had profiles similar but not identical to that of another inoculant strain. More than one-half of the plasmid profile types corresponded to variations of a single type, distinctly different from that of any inoculant strain. At the site where only arrowleaf clover had been grown, 70% of the isolates had a plasmid profile identical to that of an inoculant strain not used at the site. The instability of the plasmid profile of the latter, when grown in vitro, and the similarity of its plasmid and total soluble protein profiles to those of the inoculant strains used at this site, indicate that the prevalent type probably originated from one of them. The inoculant strains failed to persist as such in the field, regardless of the frequency of inoculation.

Received for publication April 19, 1993.

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