Emergence, Yield, Ion Relations and Physiological Parameters of Alfalfa (Medicago sativa L.) in Response to Salinity Stress.

Alfalfa is an important crop utilized in regions under irrigation commonly impacted by salinity. Four commercial non-dormant, purported salt tolerant cultivars ‘Salado’, ‘S&W8421s’, ‘S&W9720’ and ‘S&W9215’ were grown in outdoor sand tanks in Riverside CA, with irrigation water (sodium sulfate dominated) at EC_iw: 3.1, 7.2, 12.7, 18.4, 24.0 and 30 dSm^-1 (4 replicates) imposed at planting date.  Relative emergence (%) decreased significantly (p<0.01) above EC_iw 12.7 dSm^-1 with reductions of 47.6 % and 86.5% at EC_iw 18.4 and 24 dSm^-1 ; at these levels there were also significant differences among cultivars (p<0.01) with Salado having the least reduction. At 30 dS m^-1 only a few plants emerged (Salado).  Significant reductions in absolute and relative yield (average from 7 harvests) occurred when the EC_iw was higher than 12.7 dS m^-1 (p<0.01). No significant difference in cultivar yield was detected,  except at EC_iw 18.4 dS m^-1 in which the reduction was significantly lower for Salado. The physiological parameters (Pn, gs, Tr) decreased with increasing EC_iw.  Significant differences were observed among cultivars when these rates were analyzed as relative (p<0.05), with Salado having the least reduction. The Na, Cl and S content in shoots, increased significantly with increasing salinity. The interaction was significant with the lowest Na concentrations (40% less) in Salado at EC_iw 18.4 dS m^-1 and the highest concentrations in S&W 9720. The K/Na and Ca/Mg ratios decreased significantly with increasing EC_iw where Salado had the highest ratios and S&W 9720 the lowest. We conclude that EC_iw12.7 dSm^-1 was the critical level affecting both emergence and yield.  The salt tolerance observed in Salado cultivar may be attributed to either less Na uptake or less Na transport to the shoot.