Physiological responses of symbiotic rhizobium pea to exogenous calcium under salt stress

To clarify the effects of exogenous calcium on salt tolerance of nodule symbiotic pea, a pot experiment was conducted under salt stress (170 mmol·L-1) to examine the effects of exogenous CaCl2(0, 5 and 15 mmol·L-1) on physiological parameters of two pea cultivars, 'Dingwan 8' (salt tolerant) and 'Longwan 6' (salt sensitive), inoculated with rhizobium strains 15657, 15735 and Ca66. The results showed that plant biomass, the activities of superoxide dismutase (SOD) and peroxidase (POD), and the contents of proline (Pro) and soluble sugar (SS) were enhanced, but malondialdehyde (MDA) content was decreased, with rhizobium inoculation, CaCl2 application or CaCl2 application after rhizobium inoculation. Plant biomass and activities of SOD and POD and Pro content were increased significantly by the application of 15 mmol·L-1 CaCl2 after rhizobium inoculation. The application of CaCl2 after inoculation of rhizobium strain 15735 suitable for pea had limited effects on physiological parameters of pea plants, whereas the application of CaCl2 after inoculation of rhizobium (15657, Ca66) unsuitable for pea had obvious effects. The comprehensive analysis of membership function showed that pea treated with CaCl2 after rhizobium inoculation exhibited stronger salt tolerance, and 'Dingwan 8' plants treated with 15 mmol·L-1 CaCl2 after inoculation of rhizobium strain 15735 showed the strongest ability of salt tolerance with the highest membership function value of 0.814. In summary, compared with CaCl2 application or rhizobium inoculation alone, CaCl2 application after rhizobium inoculation could effectively enhance the activities of antioxidant enzymes and osmotic regulation ability, reduce the damage of membrane lipid peroxidation, and consequently improve the salt tolerance of pea.