Competition between strains of Rhizobium meliloti (Dangeard) for nodulation of lucerne

In order to ensure that strains of rhizobia that are efficient nitrogen fixers nodulate cultivated legumes in the field, it is important to be able to identify those that can compete effectively with indigenous strains in the fonnation of nodules. To this end, experiments have been conducted to investigate whether the timing of infection of legume plants by rhizobia is related to the ability of the strains to compete for nodule occupancy. The periods that 16 strains of R. meliloti in stationary growth phase took to infect lucerne (Medicago sativa, L.) plants were evaluated by estimating the time interval between inoculation of plants and initiation of the uppermost tap root nodule. Subsequently, nodulation competitive ability of each strain was evaluated in 29 paired-strain experiments. The results from each pair were compared to the periods to infection by the constituent strains. Strains which infected plants significantly faster (p < 0.05) were more competitive in 8 of 11 pairs; the same strain was more competitive than could be predicted on the basis of timing of infection in the remaining 3 of the 11 pairs. However, when the strains showed no difference in periods to infection, they had similar nodulation competitiveness in only 2 of 18 pairs. The effects of inoculating plants with exponential-phase cells, and of incubating strains in rhizosphere or in luteolin solution before inoculation of plants, were investigated. Exponential phase cells infected plants more quickly than stationary phase cells, but incubation of cells in luteolin solution did not affect timing of infection. Cells that had been incubated in rhizosphere, i.e. in plant nutrient solution containing growing lucerne roots, were slower at infecting plants than cells that had been incubated in plant nutrient solution alone. In the treatments where different periods to infection were observed in these experiments, strains which were quicker to infect plants in single-strain inoculation experiments were also more competitive in paired-strain experiments. Antagonism between the strains was evaluated to find out whether it was a factor that could influence competitive ability. The results showed that antagonism was of minor importance compared with timing of infection as strains showing a higher degree of antagonism were more competitive in nodule formation in only 4 of 15 pairs, and all the four pairs were in the category (consisting of 16 pairs) that showed no differences in timing of infection between component strains of each pair. Prior inoculation of plants with one strain reduced the number of nodules formed by strains inoculated subsequently by 39 to 68 %, but the strains inoculated subsequently did not differ in competitive ability from those without prior inoculation. These results suggest that strains of rhizobia which infect plants more rapidly are often more competitive because they form the first nodules, which induce the autoregulatory system of the plant to limit further nodulation, thereby reducing the opportunities of slower strains to form nodules. The results of this investigation indicate that it is possible to rank strains of rhizobia for nodulation competitiveness by screening them for time taken to infect plants, but these results need to be tested in the soil environment.