Growth, nitrogen fixation and ion distribution in Medicago truncatula subjected to salt stress

This study compared the growth, nodulation, N-2 fixation, and ion distribution in three Medicago truncatula lines, in response to salt in nutrient solution. Two local lines (TN8.20 and TN6.18) and a reference line (Jemalong 6) were inoculated with a reference strain Sinorhizobium meliloti 2011, a very tolerant strain to salinity (700 mM NaCl) and grown in a controlled glasshouse with or without 75 mM NaCl. A genotypic variation in tolerance to salt was found: TN6.18 was the most sensitive line whereas TN8.20 was the most tolerant. The relative tolerance of TN8.20 was concomitant with the lowest leaf Na+ concentration and the highest nodule biomass production. However, nodule efficiency (amount of nitrogen fixed per g dry weight nodule) decreased in all lines. Results suggest that the tolerance to salt seems to depend on the host plant ability to protect its leaves against an excessive Na+ (and Cl-) accumulation, and its ability to maintain the development of an abundant nodular system, which in turn determines an important rate of nitrogen fixation and allows the plants to conserve their growth potentialities. The loss of the nodular efficiency under salt stress seems to be compensated by a large nodule biomass.