Competition for nodule occupancy between introduced and native strains of Rhizobium leguminosarum biovar trifolii

The aim of this work was to evaluate the competitive ability between Rhizobium leguminosarum bv trifolii strain U204 used as commercial inoculants in Uruguay for Trifolium repens L. and Trifolium pratense L. and two native strains isolated from inoculated pastures of T. pratense. T126 is an efficient nitrogen fixer and a melanin producer strain; T70 is inefficient and a melanin non-producer strain; and U204 is very efficient in both hosts but is a melanin non-producer strain. Competitiveness between the strains was determined in experiments in pots and in growth pouches under controlled conditions. In the last experiment, we evaluated pH of plant nutrient solution and inoculum ratios. Plant dry weight was determined, and the identification of nodule bacteria was done using melanin production and DNA fingerprinting (GTG(5)-PCR). The U204 symbiotic efficiency was not affected by the co-inoculation with the others two native strains. The T70 strain was a poor competitor when was co-inoculated with one of the effective strains in both experiments. Our results confirmed a selective nodulation because an effective symbiosis occurred preferentially over an ineffective one in Trifolium species. The native effective strain competed with U204 for nodule formation in both clovers species, but the nodule occupancy depended on the inoculum ratio. The pH of nutritive solution did not affect competition ability of the studied strains. It may be possible to isolate efficient, competitive, and genetically different native rhizobial strains to be used as inoculant strains for clover pastures in Uruguay. Both (GTG)(5)-PCR and melanin production were useful methods to identify nodulating bacteria in competition studies.