Mitigating salt stress in Lens culinaris: The protective mechanism of Rhizobium in enhancing growth, photosynthesis, and antioxidant defense system

Agricultural soil contamination, particularly salinization, poses a significant and growing threat to global crop productivity, thereby necessitating the development of sustainable strategies to mitigate its adverse effects. This study investigates the potential of Rhizobium inoculation to alleviate the detrimental impact of salinity stress on Lens culinaris. Rhizobium plays a crucial role in legume development through symbiotic nitrogen fixation and may also enhance plant growth under stress conditions by engaging in complex biochemical interactions. However, research on its ability to activate abiotic stress signaling pathways and induce physiological changes in plants remains limited. To address this, L. culinaris plants were subjected to varying concentrations of NaCl (50 mM and 150 mM) at 20 days post-germination, with and without Rhizobium inoculation. For inoculation, 100 g of jaggery or sugar was dissolved in 500 ml of water, boiled, cooled, and then used to mix the Rhizobium culture. This mixture was used to coat lentil seeds, which were air-dried in the shade before sowing. Salt stress significantly reduced plant growth, physiological parameters, and yield. In contrast, Rhizobium inoculation improved reactive oxygen species (ROS) balance and enhanced the activities of key antioxidant enzymes, including superoxide dismutase (SOD) by 21.0 %, catalase (CAT) by 11.34 %, and peroxidase (POD) by 10.43 %. Additionally, photosynthetic rate and chlorophyll content were increased by 10.08 %, stomatal conductance was improved, and leghemoglobin content rose by 15.90 %. Protein and proline levels were elevated by 18.31 % and 32.57 %, respectively. Moreover, membrane stability was enhanced, sodium accumulation was reduced, and overall yield and stomatal behavior were optimized under saline conditions. Rhizobium inoculation also directly supported ACC deaminase (ACCD) activity and increased indole-3-acetic acid (IAA) production in both plant tissues and bacterial isolates. These findings underscore the potential of Rhizobium in mitigating salt-induced damage in L. culinaris by modulating antioxidant enzyme systems, enhancing photosynthetic performance, and promoting redox homeostasis.