Identifying base temperature for alfalfa germination: Implications for frost seeding

Frost seeding alfalfa (Medicago sativa L.) can be convenient and economical for establishing or renovating forage stands; however, premature seedling emergence triggered by unusually warm temperatures followed by fatally cold temperatures can lead to seedling mortality and stand failure. Delaying germination could improve establishment success in frost-seeded stands. Our objective was to measure the effect of temperature and water potential (Ψ) on germination across a range of alfalfa varieties. Germination rate (1 divided by days to 50% germination) was estimated for 11 varieties at nine constant temperatures (−1.1 to 10°C) and three Ψ (0, −0.2, and −0.6 MPa). Linear regression between temperature and germination rate was tested for all variety–Ψ combinations. Base temperature (T_b; minimum temperature for 50% germination) and thermal constant (DD; time to 50% germination in growing degree-days) were determined by calculating the intercept on the temperature axis and the inverse of the slope parameter from each regression, respectively. The T_b ranged from −0.55 to 0.49°C across varieties and was greater in low Ψ conditions. The DD was negatively correlated with T_b, which supports the hypothesis that seeds with lower T_b require more degree-days to germinate. Selecting alfalfa varieties with higher T_b could delay germination and reduce the risk of frost mortality; however, these varieties are also likely to have higher DD, which would expedite seedling emergence and potentially offset delays from high T_b. Based on small variations in germination parameters compared with field temperatures, no variety was identified as superior for frost-seeding success.