COMPETITIVE ABILITY AND SYMBIOTIC EFFECTIVENESS OF DOUBLY LABELED ANTIBIOTIC-RESISTANT MUTANTS OF RHIZOBIUM-TRIFOLII

Doubly labelled mutants of Rhizobium trifolii, resistant to streptomycin and spectinomycin, were studied in respect of nodulating competitiveness and symbiotic effectiveness relative to the ‘wild‐type’ parent strains using Trifolium repens cv. S184 as the host plant. A combination of antibiotic resistance, differential absorption of congo‐red and the fluorescent antibody technique permitted the rapid differentiation of all Rhizobium strains used, either from mixed inocula or from nodules. The doubly labelled antibiotic resistant mutants were inferior in terms of competitive ability for nodulation with an ineffective strain compared with the ‘wild‐type’ parent strains. A rapid method for evaluating effective antibiotic resistant strains for nodulating competitiveness is suggested. All the mutants examined were also found to be less symbiotically effective than the respective ‘wild‐type’ strains although these differences generally did not reach statistical significance. The reduced symbiotic effectiveness of the antibiotic resistant mutants was associated with an increase in magnitude of the variances for shoot dry weights, relative to that shown by the parent strains. A possible explanation for this phenomenon is presented. Copyright © 1979, Wiley Blackwell. All rights reserved