Evaluation and Expression Analysis of Alfalfa Genotypes in Response to Prolonged Salt Stress.

Salinity is one of the most important abiotic stresses that adversely affect plant growth and productivity. In this study, we analyzed 12 cloned genotypes (from 9 different varieties) differing in levels of salt tolerance under long term salt treatment. Salt tolerance index for these genotypes ranged from 0.39 to 1.0. The most tolerant genotypes SISA14 (G03) and AZ-90ST (G10) were among top performers for total biomass and exhibited least effect on height and number of shoots under salt stress. SISA14 (G03) was among the genotypes with smallest increase in shoot Na concentration and stored least amount of shoot Cl under salt treatment. Decrease in shoot K concentration did not correlate well with salt tolerance. There was little effect of salinity on shoot Ca, Mg, P, Fe and Cu concentrations; however, there were significant increases in shoot Mn and Zn concentrations. Salinity reduced leaf foliar area, photosynthesis, transpiration and stomatal conductance. Interestingly, salinity increased the chlorophyll content in all genotypes. Genotypes Cuf101 (G02), SISA14 (G03), AZ-90ST (G10) and Salado (G12) showed significant upregulation in roots for genes (SOS1, SOS2 and SOS3) involved in ion efflux from root to soil. All five genes (NHX1, NHX2, ATPase, SKIP1 and AVP1) known to be involved in sequestration of Na in vacuoles were strongly induced by salt in roots of Salado (G12). Both the genes involved in retrieval of Na from xylem, HKT1 and AKT1, were induced by salt stress in roots of AZ-90ST (G10) and Salado (G12). Analysis of genes involved in increased tissue tolerance (AP2, ERF1, P5CS, HSP90, SGP29) showed upregulation in the leaves of Cuf101 (G02), SISA14 (G03), SW9720 (G05), SISA9 (G06), and SISA10 (G09). Expression analysis of genes allowed us to classify genotypes based on their ability to regulate different components of salt tolerance mechanism.