Association of phosphorous solubilizing bacteria and Sinorhizobium meliloti: its effect on the rhizobium-legume symbiosis

PATRICIA GARCÍA ARHEX; ALICIA ESTER GRASSANO; ANA LÍA RONCHI; CARLOS ALBERTO MOLDES; JORGE G. SCARONE

Arequipa

Encuentro; XX Reunión Latinoamericana de Rhizobiología y defensa del Medio Ambiente; 2000

Instituto de Defensa del Medio Ambiente

This paper deals with partial results within an integral study of alfalfa leading to the optimization of this legume in the pampas region. Due to the fact that in the region available phosphorous (Pd) distribution is erratic we studied the promoting effect of phosphorous solubilizing bacteria on the rhizobium-legume symbiosis. We hypothesized that inoculating whit phosphorous solubilizing bacteria and Sinorhizobium meliloti we would find the solution for the problem (Devon et al, 1996). To carry out the tests we used Sinorhizobium meliloti strains isolated in the region and evaluated as highly efficient ones(Grassano et al, 1996) and strains of phosphorous solubilizing bacteria provided by the Departament of Plant Physiology, National University of Rio Cuarto. Experiments were carried out in controlled emvironment chamber whit Sinorhizobium meliloti LQ22, LQ42 and LQ51 and with two Phosphorous solubilizing strains (SF21 and SF22) according to a randomized design whit eight replications. Evaluation of nodules (appearance, location and aspect) and of aspect of seedling was performed; dry weight and nitrogen percentage of the aerial part were determined by the end of the test. Qualitative and quantitative results of all combinations showed that in comparison whit other cases without inoculation nitrogen restriction, there were no deleterious effects, bacteria coexisted and there were no significant differences between treatments inoculated whit Sinorhizobium meliloti and the control treatment with nitrogen, what confirms its efficiency in the symbiosis. Significant differences are registered in relation to the same control treatment in the case of LQ51 and SF21, through at this level the specific action of phosphorous solubilizing strains cannot be proved since the Jensen medium had Pd. In view of these results, a greenhouse experiment was carried out, using soil little Pd and strains LQ51, SF21 and SF22. A randomized design whit five replications was used and compared to control treatments without inoculation, with N, with Pd and with Pd and N. Dry weight and nitrogen percentage in the aerial part and Pd content of rhizospheric and non rhizospheric soils were analyzed. The results from the statistical analysis show a promoting effect in those treatments in which both bacteria were used as compared with the ones carried out only with Sinorhizobium meliloti, this could be due to the fact that the rhizospheric Pd had been increased and possibly to some other effects that should be analyzed according to what was suggested by the results of the controlled environment chamber test.