Evaluating the Genetic for Germination Salt Tolerance in Alfalfa Using a Sodium‐Chloride Gradient 1

Progress made in breeding for salt tolerance at germination may be difficult to evaluate when only limited numbers of salinity levels are used for germplasm comparison. Patterns of response to salt stress may be better distinguished when seed is germinated at numerous osmotic potentials. A gradient of osmotic potentials can be easily created by making a series of small dilutions of a concentrated salt solution. Using this technique, alfalfa ( Medicago sativa L.) seed from the cultivar Mesa‐Sirsa and a Cycle 7 syn‐1 experimental developed from Mesa‐Sirsa were planted in 37 concentrations of NaCl ranging from 0 to −2.87 MPa. Germination responses across the gradient were sigmoid shaped for both germplasm sources. Natural logarithmic transformations of the data revealed relatively similar and constant germination rates for the two populations at osmotic potentials between 0 and −0.75 MPa. Below these osmotic potentials, percent germination declined abruptly in Mesa‐Sirsa while in Cycle 7 syn‐1, a stable germination rate was maintained until the salt level reached −1.76 MPa, a threshold concentration more than 2.3 times that of Mesa‐Sirsa. At salinities greater than these threshold values, the two populations exhibited similar rates of decline in seed germination. Cycle 7 syn‐1 seed germinated more than twice as fast as Mesa‐Sirsa at −1.90 MPa. Selection for salt tolerance has both extended the germination range of alfalfa by 1 MPa and decreased the time required for seed to reach peak germination speed by 5 days. An osmotic gradient provides an excellent method for evaluating germplasm performance in a salt tolerance breeding program.