Isolation and Characterization of Rhizobium Strains for Enhancing Plant Growth and Stress Tolerance under Drought and Salinity

Rhizobium strains play a crucial role in sustainable agriculture by enhancing plant growth and stress tolerance. In this study, twelve bacterial isolates were obtained from root nodules of Vigna radiata and Cajanus cajan , with 10 confirmed as Rhizobium spp. through 16S rRNA sequencing. Isolate RZ5 showed the highest IAA production (38.6 ± 1.2 µg/mL), phosphate solubilization (2.1 ± 0.1 cm halo), and ACC deaminase activity (28.4 ± 1.1 nmol α-ketobutyrate/mg protein/h). Greenhouse experiments revealed that Rhizobium inoculation significantly improved plant performance under drought and salinity stress. Inoculated plants under drought exhibited a shoot length of 16.5 ± 1.1 cm and fresh biomass of 5.5 ± 0.4 g, compared to 12.7 ± 0.9 cm and 3.8 ± 0.3 g in uninoculated controls. Under salinity stress, inoculated plants had a shoot length of 17.1 ± 1.2 cm and fresh biomass of 5.8 ± 0.5 g, outperforming controls (13.2 ± 1.0 cm and 4.0 ± 0.4 g). Physiologically, inoculated plants maintained higher chlorophyll content (39.5 ± 2.1 SPAD units under drought; 42.1 ± 2.2 under salinity) and relative water content (78.4 ± 2.0% and 80.2 ± 2.1%), while exhibiting lower electrolyte leakage. Moreover, soil nutrient properties improved, with inoculated drought-stressed soils showing increased total nitrogen (0.16 ± 0.01%) and available phosphorus (10.5 ± 0.7 mg/kg). These findings highlight the potential of Rhizobium-based biofertilizers to enhance plant resilience and soil health under adverse environmental conditions, supporting sustainable agriculture practices.