Genetic and phenotypic relationships in response to NaCl at different developmental stages in alfalfa

The perennial forage alfalfa (Medicago sativa L.) may be affected by salinity at all stages of development. Selection for increased seed germination or seedling growth in saline environments has not resulted in improved forage yield under salt stress. The purpose of this study was to determine genetic and phenotypic relationships between plant performance in the presence of NaCl at three developmental stages in alfalfa. Understanding these relationships may improve the efficiency of breeding programs aimed at increasing crop survival and yields in saline environments. Fourteen half-sib families were randomly chosen from both an experimental alfalfa population produced from two cycles of mass selection for improved forage regrowth yield at 80 mM NaCl (A80), as well as from an unselected control population (AC1). In two separate experiments, individual plant performance was measured in these families at seed germination (radicle length at 7 days), and during seedling growth (forage yield at 40 days post-planting) and post-harvest regrowth (forage yield at 67 and 95 days post-planting) in the presence of 0 or 80 mM NaCl. Genetic, phenotypic, and family rank correlation coefficients, and broad-sense and narrow-sense heritability estimates were calculated within each growth stage, NaCl level, and population. Radicle length was not highly correlated with seedling or regrowth forage yield within a population or across NaCl levels. Phenotypic correlations between seedling and regrowth yields were also low. Heritability estimates were higher at 0 NaCl in AC1 between all growth stages, but were greater in A80 at 80 mM NaCl. Genetic correlations between seedling and regrowth yields were all positive. This suggests that selection for forage yield in saline environments at harvests-1,-2, or -3 should not decrease performance at other stages. Genetic correlations between seedling and regrowth yields were higher in A80 than in AC1 at 80 mM NaCl. The results indicate that selection for increased alfalfa forage yield in saline environments at germination may not be optimum. Family selection at germination or during seedling growth may be more effective than individual plant selection at any growth stage in saline environments. The results suggest than selection methods which include each critical growth stage may be required to develop alfalfa cultivars with increased forage yield in saline environments.