Abstract
The nodules formed on white clover by a viomycin-resistant mutant of Rhizobium trifolii were ineffective, and addition of glycine or vitamin B12 did not promote nitrogen fixation by these plants. Nitrogen was also not fixed by clovers inoculated with these bacteria when the plants were treated with excretions of the wild type rhizobia or extracts of the cells, but the excretions increased the number and altered the distribution of the nodules. No correlations were found between the growth factor requirements or extracellular metabolites detected by gas chromatography and effectiveness of several strains of R. trifolii and R. meliloti, and no apparent relationship was evident between the amount of polysaccharide produced and effectiveness of R. trifolii strains. Some isolates derived from an effective R. trifolii treated with acridine orange had diminished effectiveness or were noninfective, but isolates derived from an ineffective mutant treated with acridine orange did not regain competence for nitrogen fixation.
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