Rhizobia inhabiting Gliricidia sepium in Puttalam district of Sri Lanka: assessment of stress tolerance and genetic diversity

Legume-rhizobia symbiosis has received higher attention as it enhances soil nutrition through the biological nitrogen fixation. However, stress factors such as excess salts, drought, acidity, alkalinity, and temperature etc. suppress the growth and symbiotic characteristics of rhizobia. Identification of stress tolerant rhizobial strains and their genetic diversity which influence the efficiency of nitrogen fixation in Gliricidia sepium is poorly studied. This study mainly focused on the characterization and identification of stress-tolerant rhizobial strains that were isolated from five root nodules of G. sepium from seven selected locations (Anamaduwa, Chilaw, Vanathawilluwa, Saliyawewa, Etiyawala, Palaviya, Kurinchanpitiya) of Puttalam district in Sri Lanka. Isolates were separately subjected to four different stress conditions, namely, pH (3.0 9.0), temperature (25 °C 45 °C), salinity induced by NaCl concentrations ranging from 0.1% to 3.0%, and drought induced by polyethylene glycol 8000 (PEG-8000) concentrations ranging from 0.1% to 0.4%. Rhizobial strains isolated from Anamaduwa, Chilaw and Kurinchanpitiya such as An-3, An-4, An-5, Ch-1, Ch-4, Ch5, Ku-2 and Ku-5 showed tolerance for alkaline pH (8.0 and 9.0) and extreme drought conditions (3.0% and 4.0% of PEG-8000). But their growth was adversely affected by acidic pH conditions (pH 3.0 and 4.0). Most of the rhizobial strains except strains in Etiyawala were moderately tolerant to extreme salt concentrations (2.5% 3.0% of NaCl) and extreme temperature levels (4045 oC). In the graphical interpretation, no growth pattern was observed with respect to four physiological conditions. Thirteen strains that were selected from the statistical analysis displayed higher survival capacity when the combination of different stress conditions was applied. As 13 rhizobial strains showed different banding patterns in the Enterobacterial Repetitive Intergenic Consensus (ERIC) profile, they belonged to 09 clusters at the 66.67% similarity level. Furthermore, these stress-tolerant rhizobial strains can be used to further studies on cross-inoculation of crop legumes as an alternative substitution for the vast usage of chemical nitrogen fertilizers.