Variability in the response of six genotypes of N2-fixing Medicago ciliaris to NaCl

Abstract

Genotypic variability was assessed within six Medicago ciliaris genotypes growing symbiotically with Sinorhizobium medicae in order to identify physiological criteria (growth, ion content, and plant health) associated with salt tolerance. Response to salt stress depended on the line and the level of salt. Two lines with lower dry biomass under non-saline conditions (TNC 1.8 from a semi-arid area and TNC 10.8 from a sub-humid area), were more tolerant to NaCl, whereas the most productive lines (TNC 11.5 and TNC 11.9 from a humid bioclime) were more sensitive in terms of growth and nitrogen fixation. Susceptibility of symbiotic nitrogen fixation to saline stress was not associated with a higher accumulation of Na+ in nodules, since the most tolerant lines TNC 1.8 and TNC 10.8 accumulated the highest Na+ amount in nodules. Leaf area and net photosynthate assimilation rate were conserved in line TNC 1.8 and to a lesser extent in line TNC 10.8 potentially owing to a greater ability to protect aerial organs and nodules from Na+ damage and to insure a better supply of leaves with nitrogen. Our results suggest that nodule growth and number and nodule Na+ content should not be used as selection tools for tolerance or susceptibility, since two of the tested lines maintained consistent growth in spite of reduced nodule and high Na+ content. Instead, the most reliable physiological indicators for tolerance appear to be consistent growth (i.e., no growth changes) and reduced leaf Na+ accumulation with increasing concentrations of NaCl.