Rhizobium inoculation alleviates separate and combined toxicities of Na+ and Cl– in alfalfa

Although both sodium and chloride in high concentrations have toxic effects to plant growth, some plant species are more tolerant to Na + stress than Cl − and vice versa. Until now, few studies compare the effects of separate and combined toxicities of Na + and Cl − on nodulated leguminous plants. In this study, Na + -dominant, Cl − -dominant, and NaCl treatments were applied to two alfalfa cultivars with or without rhizobial inoculation. The results showed that high Na + was more toxic than high Cl − to alfalfa growth. Shoot dry weight reduction was found in Na + but not Cl − -dominant treatments, and the reduction in root dry weight was caused by the additive effect of Na + and Cl − toxicities in uninoculated plants. Rhizobial inoculation significantly promoted plant growth by the improvement in both nitrogen nutrition status and root growth. At NaCl treatment, the promoted growth was also associated with the lower Cl − concentrations in the inoculated plants relative to the uninoculated plants. Additionally, rhizobial inoculation induced the differential expression of the salt-response genes, MsHSP81.2 and MsCLC1 . In conclusion, the above results indicate that both Na + and NaCl toxicities reduced alfalfa growth, and the reductions were greater under NaCl toxicity conditions and were mainly caused by the additive effects of Na + and Cl − toxicities. Rhizobial symbiosis can alleviate separate and combined toxicities of Na + and Cl − through enhanced nitrogen nutrition and root growth, the change of the accumulation and partitioning of K + and Cl − , and accordingly the differential expression of some salt-response genes. The above results offer an economical measure in the future application of biofertilizers for alfalfa salt tolerance.