Variation within as well as between varieties of white clover (Trifolium repens L.) in their selection or preference for rhizobial strains in nodulation was studied using antibiotic-resistant mutant strains of Rhizobium trifolii.

Significant differences between varieties were found. Cv. S100 was nodulated almost exclusively by 75 str but there was more variation within S184 and, in particular, Pajbjerg smalbladet. In this experiment 93% of the total variance was genotypic, of which 68% was due to the difference between varieties and 25% due to variance between genotypes within varieties. Significant correlations were found between the preference for rhizobial strains of plants grown from seeds and of stolon lines vegetatively propagated from the former plants, indicating that the preference for rhizobial strains is genetically controlled by the host. A considerable drift in preference towards selection of more compatible rhizobial strains by the clonally propagated stolons compared with the original white clover plants was observed.

The results are discussed from the point of view of improving symbiotic nitrogen fixation by combining highly effective and competitive Rhizobium strains with a host variety with uniform preference for those strains.