Nitrogen fixing bacteria and phosphate solubilizers isolated in lettuce (Lactuca sativa L.) and evaluated as plant growth promoters

The urgent need for sustainable and low environmental impact strategies in agriculture requires the development of microbial preparations that improve plant nutrition. The present work was carried out with the purpose of obtaining promising growth promoter bacteria from the rhizosphere and seeds of lettuce (Lactuca sativa L.) plants. At least seven strains belonging to the genus Rhizobium were detected as rhizosphere colonizers, identified with the nodulation test using Phaseolus vulgaris as host plant and by their growth in GPA, YEM containing bromotimol blue media and NaCl 2, 3 and 5% media. These rhizobia were hydroxyapatite solubilizers, and in some cases they were able to grow at >= 2% NaCl. Endophytic bacteria were isolated from ground seeds and characterized by their microaerobic growth and acid production, as revealed when turning from green to yellow the Nfb-glucose medium. The inoculation into lettuce seeds of two potential rhizospheric Rhizobium spp., strains 33 and 45, and the endophytic strain M, showed that strain 33 and the consortium 3345 exhibited higher (p <= 0.05) dry weight of 30 day plantlets than the non-inoculated control. Even though all bacterial strains produced IAA with and without tryptophane, this is not the only parameter implicated in growth promotion of lettuce plantlets.