Salt tolerance of nitrogen fixation in Medicago ciliaris is related to nodule sucrose metabolism performance rather than antioxidant system

In this study, the effect of 100 mM NaCl on physiological and biochemical responses were investigated in nodules of two Medicago ciliaris lines differing in salt tolerance (TNC 1.8 and TNC 11.9). Results showed that, on the basis of growth and nitrogen fixation, the line TNC 1.8 proved more salt tolerant than TNC 11.9. The salt-induced oxidative stress (membrane lipid peroxidation, leghemoglobin degradation, antioxidant activities reduction) occurred similarly in nodules of both lines. The tolerant line TNC 1.8 showed a better capacity to preserve higher sucrolytic activities and maintained higher nodule malate concentration, although total organic acids decreased in both lines. The higher amount of organic acids in the tolerant line seems to be related to its capacity to maintain higher NH4 nodule concentration in comparison with the sensitive line. Although salt stress reduced concentrations of the majority of amino acid in both lines, the decrease of the most preponderant amino acids glycine, valine, aspartate and glutamate was more accentuated in the sensitive line TNC 11.9. However, alanine concentration increased in the nodules of this sensitive line, suggesting a higher incidence of stress-induced hypoxia. The present study provides further evidence that salt tolerance of nitrogen fixation in the tolerant line is linked to a more effective supply of malate to bacteroids which allows the synthesis of amino acids required to maintain both plant and nodule growth.