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.

Characterization of high temperature-tolerant rhizobia isolated from Prosopis juliflora grown in alkaline soil.

“…Michiels et al (1994) have reported enhanced cell survival at 50°C when bean nodulating Rhizobium strain CIAT899 cells were first exposed to a temperature of 40 or 45°C. Recently we have reported maximum survival temperature for Rhizobium NB8 from Albizia lebbek to be 50°C (Surange et al, 1997). In our present study, the maximum survival temperature recorded for the five strains at 60°C in YEB for 6 h, to our knowledge, is the highest reported for rhizobia (Table 1).…”

“…Rhizobium strains were isolated from the root nodules of P. juliflora growing in alkaline soils (pH 8.5 to 11.0) at Banthara Research Station, Lucknow, India. The soil temperature of the sites during summer varied from 48 to 52°C (Surange et al, 1997). Nodules were disinfected, crushed in a small amount of saline (0.85% NaCl), and suspension streaked on yeast extract mannitol (YEM) plates.…”

“…Rhizobium strains unless otherwise stated were grown on YEM plates and maintained on YEM slants that contain per liter 15 g agar, 10 g mannitol, 1 g yeast extract, 0.5 g K 2 HPO 4 , 0.2 g MgSO 4 · 7H 2 O, and 0.1 g NaCl with 0.025% (w/v) Congo red (Hahn, 1966). The pH of the medium was adjusted to 7.2, and the cultures were grown as described earlier (Surange et al, 1997). Rhizobium strains that grew on YEM plates (0% salt) were white, translucent, glistening, and elevated colonies with entire margins (Vincent et al, 1980).…”

“…The filter-sterilized solutions of the carbon sources (25% w/v stock solutions) were added to the final concentration of 1% YEM (devoid of mannitol) as described earlier (Surange et al, 1997).…”

“…In contrast, Rhizobium leguminosarum strains are saltsensitive (Zhang et al, 1991), and have lower T max (Lindstrom and Lehtomaki, 1988). Differences in the tolerance of high temperatures among species and strains of Rhizobium have been recognized, and the selection of strains for temperature tolerance has been suggested as a means of overcoming temperature stress (Graham and Parker, 1964;Pate, 1961;Surange et al, 1997;Weber and Miller, 1972). Temperature-tolerant rhizobia are likely to be found in environments affected by high temperature.…”