Nine strains of Rhizobium meliloti, ranging in competitive ability on Medicago sativa from excellent to poor in autoclaved soils, were paired in 29 combinations and used to inoculate M. sativa in a liquid rooting medium. A positive correlation (r = 0.545) between strain ratios in nodules after 28 days and root surface cell ratios after 7 days was determined. Two cell fractions from the root surface, representing loosely and firmly adhering cells, were investigated. Infectivity was linked to the more firmly adhering cells. A significant relationship was established between the cell ratios of competing strains in the two fractions. In another experiment, adherence of cells of both infective and noninfective Rhizobium strains to roots of M. sativa and Trifolium repens was demonstrated; the ratios of loosely to firmly adhering cells on the root surface were significantly narrower with the infective combinations than with noninfective strain-legume associations
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