Symbiotic effectiveness and response to mannitol-mediated osmotic stress of various chickpea-rhizobia associations

Thirty-six symbiotic associations involving six chickpea cultivars against six rhizobial strains were evaluated for symbiotic performance and responses to osmotic stress applied by mannitol (50 mM) in aerated hydroponic cultures. Analyses in different symbioses were focused on biomass production, nodulation, nitrogen fixation, and their modulation under osmotic stress conditions, as well as expression of nodular antioxidant enzymes. Mesorhizobium ciceri reference (835) and local (CMG6) strains, as well as the local (C-11) M. mediterraneum allowed the best symbiotic efficiency for all chickpea cultivars. The osmotic stress induces severe decrease ranging 30-50% in aerial biomass and 50-70% for nitrogen fixation. Nevertheless, plants inoculated with M. ciceri (835) and M. mediterraneum (C-11) preserve a relatively high growth (4 g plant(-1)) with nitrogen-fixing activity (25 mu mols h(-1) plant(-1)). The bacterial partner was the most important factor of variance of the analysed parameters in osmotic stress or physiological conditions where it gets to 60-85%. The strains allowing the best competent symbioses were proposed for field assays. Under osmotic stress, nodular peroxidase (POX) and ascorbate peroxidase (APX) activities were significantly enhanced. The increase of POX and APX was inversely correlated with the inhibition of aerial biomass production (p = 0.05) and nitrogen-fixing capacity (p = 0.01), suggesting a protective role of these enzymes in nodules. Superoxide dismutase (SOD) was also activated in stressed nodules. However, the spectacular decrease in catalase (CAT) activity discounts its involvement in osmotic stress response.