Phosphorus solubilizing bacteria: its effects on the symbiosis Sinorhizobium meliloti- Medicago sativa

Grassano A.*, Ronchi A.L.*, Scarone J.G.**, García P.*, Correa N***, Rosas, S***

*Departamento de Química. Facultad de Ciencias Exactas y Naturales. Universidad Nacional de La Pampa. Uruguay 151. (6300) Santa Rosa. La Pampa. Argentina. ** Facultad de Agronomía. Universidad Nacional de La Pampa. Ruta Nacional 35 Km 334. (6300) Santa Rosa. La Pampa. Argentina. ***Facultad de Ciencias Exactas Fco Qcas y Naturales, Universidad Nacional de Río Cuarto. Campus Universitario. 5800 Río Cuarto. alilopez@satlink.com.

The alfalfa is the most important forage specie in the semi-arid region of the Pampas. Since the available Phosphorous distribution is not the same in the different soils of this region, the possible promoter action of phosphorus Solubilizing bacteria over the alfalfa-Rhizobium symbiosis is studied. We hypothesized that inoculating with Phosphorus Solubilizing bacteria and Sinorhizobium meliloti we would find a solution for the problem (Devon et al, 1996). In the present work, strains of Phosphorus Solubilizing bacteria and of Sinorhizobium meliloti were used. The last ones were isolated from the own region and were evaluated as highly efficient (Grassano et al., 1996). Aiming to determine whether the typical infectiveness and effectiveness properties of the Sinorhizobium meliloti Lq22, Lq42 and Lq51 strains are maintained in presence of phosphorus Solubilizing bacteria SP21 and SP22, an controlled environment chamber essay with non-inoculated samples as reference and without Nitrogen limitations was carried out. Evaluation of nodules (appearance, location and aspect) and of aspect of seedlings was performed; dry weight and Nitrogen percentage of the aerial part were determined. No significant differences are found in between the samples inoculated with Sinorhizobium meliloti and the reference sample with Nitrogen. On the other hand, appreciable differences are found comparing the Lq51 treatment with SP21 with the same reference sample. However, the specific action of the Phosphorus Solubilizing bacteria is not proved at this level because the Jensen medium contains available Phosphorus (Pa). A greenhouse experiment was carried out, using soil with low levels of available Phosphorus (11 mg.Kg^-1) and strains Lq51, SP21 and SP22. A randomized design with five replications was used and compared to control treatments without inoculation, with N, with Pa and with N and Pa. Dry weight and N % in the aerial part, and Pa content of rhizosferic soils were analysed. 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 with Sinorhizobium meliloti and with the controls. An increase of 62% in the level of available phosphorus rhizosferic compared to its original content bases this enhancement effect.