Characterization of Rhizobium from root nodules of leguminous trees growing in alkaline soils

Abstract

High temperature, pH, and salt stresses in tropical alkaline soils limit nodulation and dinitrogen fixation by strains of Rhizobium from the root nodules of nitrogen fixing trees (NFTs). This study was conducted to determine the variability among Rhizobium strains isolated from different NFTs in growth response to high temperature, pH, and salt concentrations. Variable response to increases in temperature, pH, and salt concentrations was observed. Rhizobium strain isolated from Albizia lebbek survived at 50 °C, while Rhizobium strains isolated from Sesbania formosa, Acacia farnesiana, and Dalbergia sissoo were well adapted to grow on pH 12.0. All the Rhizobium strains tolerated salt concentrations up to 5.0%. Strains were further characterized with respect to utilization of 27 carbon sources and for their effectiveness in substrate utilization at pH 7.0 and 9.0. Generally higher rates of O₂ consumption were observed at pH 7.0 compared with pH 9.0.Key words: Rhizobium, leguminous trees, root nodules, stress tolerance.