Identification of stress tolerant rhizobial strains inhabiting Gliricidia sepium in Polonnaruwa district, Sri Lanka

Abstract

Rhizobia are well-reputed biological nitrogen fixing organisms due to their proficiency in symbiotic root nodule formation with leguminous plants. Gliricidia sepium is a widely distributed legume which hosts a wide range of rhizobia. Few studies have been carried out to identify the stress tolerant rhizobial populations inhabiting G. sepium in Sri Lanka. This study was focused on the characterization and identification of stress tolerant rhizobial strains residing in the root nodules of G. sepium growing in seven selected locations of Polonnaruwa district, Sri Lanka. Isolated rhizobial strains were separately subjected to stress conditions (pH, temperature, salinity, and drought) and to a combination of given stress conditions. Most of the strains were tolerant to 25-45 ºC temperature range, extreme drought conditions and 1.5 % and 2.0 % salinity levels. However all the strains were adversely affected by acidic pH. Among the fifteen strains that were selected from the statistical analysis, fourteen displayed higher survival capacity when the combinations of physiological conditions were applied. There was great genetic diversity within these isolates and they belonged to 10 clusters. Cross inoculation of these isolated stress tolerant strains into crop legumes can be used as an ideal alternative for chemical nitrogen fertilizers.