Physiological characterization of rhizobial strains nodulating alfalfa (Medicago sativa) isolated from soils of Southeastern Morocco

The legume-rhizobia symbiosis provides the necessary nitrogen for plants growth and contributes to the improvement of soil nitrogen level. However, environmental constraints prevailing in many Moroccan regions constitute the limiting factors for this symbiotic interaction. For this reason, the present work aims to select some rhizobial strains nodulating alfalfa (Medicago sativa L.) for their performance under environmental stresses. Rhizobial strains nodulating alfalfa were isolated from soils of Tafilalet region (Arfoud 1, Arfoud 4, Aoufous and Rich) and purified in yeast extract mannitol (YMA) medium. After nodulation test, 20 strains were able to induce root nodulation in alfalfa and they were tested for their tolerance to high salinity, water deficit, acidity and temperature. These strains were also assessed for their abilities to solubilize inorganic phosphorus using Pikovskaya (PVK) medium. The results indicated that the studied strains developed different behaviors in response to all considered abiotic stresses. The positive correlations between strains behaviors and origin were noted. In general, strains from Arfoud1 and Arfoud 4 were qualified as the most tolerant to salt stress and can grow under extreme pH levels (4 to 10). Whereas, strains from Rich were the most tolerant to water deficit. Results recorded on PVK medium mentioned that some strains (Arf1RL1, RYRL9 and Arf4RL16) showed a high potential of inorganic phosphorus solubilization. Some strains such as Arf1RL1 and Arf4RL13 both high tolerance level to considered environmental constraints and high potential of inorganic phosphorus solubilization.