Screening of alfalfa (Medicago sativa) cultivars for salt tolerance in West Texas

Alfalfa (Medicago sativa) is a very important crop commodity in the Trans-Pecos region of Texas and New Mexico where salinity problems occur in the soil and irrigation water resources. There has been significant research in the area of salt tolerance of alfalfa but there is need for screening current and experimental alfalfa cultivars to assist growers with variety selection. Due to the complexity of salinity tolerance in plants, it is also necessary to compare laboratory and greenhouse screening methodologies and results to field conditions. These experiments were designed to evaluate commercial and experimental alfalfa cultivars at different growth stages and varying salinity concentrations and assess correlations between laboratory, greenhouse and field experiments. In addition to salinity tolerance, there is a need to evaluate alfalfa cultivars for potential quality differences between varying fall dormancy (FD) ratings to assist growers in cultivar selection.

The first experiment evaluated plant germination under increasing levels of salt concentrations conducted in the laboratory. Statistical differences were observed for percentage germination among cultivars which could influence stand establishment of alfalfa in saline conditions. These differences in turn are exhibited in the IC(50) values which reflect osmotic potential required to inhibit 50% of seed from germination. The second experiment evaluated forage production under two levels of salt concentration conducted in the greenhouse. There were no significant differences in raw or transformed SCR values among the 32 cultivars at Cut 2, Cut 3 or Cut 4 when subject to analysis of variance. SCR values were then evaluated for statistical differences using the three harvests as replications over time. Significant differences in raw and transformed SCR values were found between cultivars with Cut 2, Cut 3 and Cut 4 serving as replications over time. The third experiment evaluated forage production under saline field conditions that exist at the Texas AgriLife Research Station west of Pecos, TX. Throughout the two-year project significant differences among cultivar yields were observed only 4 out of the 13 (31%) harvests at alpha = 0.05 (6 out 13 exhibited significant differences when using alpha = 0.1). There were no significant differences when two-year totals or when two-year averages were subjected to ANOVA. It should be noted that all 12 of the cultivars in this experiment were selected for the potential to tolerate saline conditions (based upon breeding or adaptation of the cultivar) and therefore all were expected to perform well under saline conditions. However, the differences seemed to become more apparent later in the growing season, perhaps indicating salt stress has a more severe impact over the long term (i.e., salt loading during the year, salt loading during the life of the stand, etc).

When comparing data from the three experiments several principles become evident. First, as percentage germination increases regrowth potential decreases indicating a negative correlation. However, as percent germination increases potential yield under saline conditions increases. Second, as percent germination increases the potential field production and yield also increases. Third, as Salt/Control Ratio (SCR) values increase production potential in the field decreases indicating a negative correlation. However, as average saline yields in the greenhouse increase potential yield under saline conditions in the field also increased.

The fourth experiment compared fall dormancy (FD) ratings and forage quality data and no significant differences between FD8 and FD10 alfalfa cultivars evaluated were found to exist over the course of a growing season. The data does, however, show that differences can exist at individual cuttings during the growing season although it seems to be more the exception than the rule. Therefore, forage quality differences between FD8 and FD10 alfalfa cultivars may occur, but in general should not be a factor for producers when selecting cultivars.