Halotolerant Nitrogen-Fixing Mesorhizobium ciceri Modulates Antioxidant Homeostasis and Growth Performance in Chickpea Cultivars Under Salt Stress

Soil salinity inhibits biological nitrogen fixation (BNF) in legumes, compromising nitrogen nutrition and crop productivity. This study evaluated whether two halotolerant Mesorhizobium ciceri strains (S1, S2) can sustain BNF and alleviate moderate salt stress (100 mM NaCl) in three Tunisian chickpea (Cicer arietinum L.) cultivars (Amdoun, Béja 1, and Nour). Inoculated and non-inoculated plants were grown under controlled conditions. Salinity reduced shoot dry weight by 37.5–42% and severely impaired nodulation (≈60% reduction) in non-inoculated plants. Bacterial inoculation significantly increased germination rate, shoot and root biomass, and nodule number compared to non-inoculated salt-stressed controls. Improved nodulation corresponded to better nitrogen nutrition, reflected by higher leaf chlorophyll content (a proxy for nitrogen status). However, direct measurements of nitrogenase activity (e.g., acetylene reduction assay) are needed to confirm enhanced BNF. Inoculated seedlings also exhibited lower oxidative stress markers (hydrogen peroxide and malondialdehyde) and enhanced antioxidant enzyme activities (superoxide dismutase and glutathione peroxidase), indicating reduced reactive oxygen species damage. Cultivar-specific responses were observed: Amdoun responded best to S1, Béja 1 to S2 for biomass recovery, while Nour showed strong antioxidant induction but limited growth gain. We conclude that halotolerant M. ciceri strains improve chickpea performance under salt stress primarily by sustaining BNF and nodulation, thereby maintaining nitrogen nutrition. Strain–cultivar compatibility is critical for optimizing this bio-inoculant strategy in saline agroecosystems. Our findings identify the combination of cultivar Béja 1 with strain S2 as the most promising for biomass recovery under moderate salinity, providing a practical, strain–cultivar matching framework that can guide the development of effective bio-inoculants for chickpea production in salt-affected areas of Tunisia and similar Mediterranean regions.