INFLUENCE OF SOIL MOISTURE ON COLD TOLERANCE OF ALFALFA

Drought stress applied to alfalfa seedlings (Medicago media Pers.) grown at 20–22 °C by reducing soil moisture from 100 to 30% of soil water holding capacity prior to freezing increased their LT 50 by 4–7 °C. The tests were performed in 12-cm pots containing either a greenhouse sandy loam potting mixture or material sampled in the Ap horizon of several soils of varying texture. Similar increases were obtained when identical soil water treatments were applied to plants hardened for 2 or 4 wk at 1 °C. Both stresses, drought and low temperature, have additive influences on plant survival following freezing. Low soil moisture increased both the level of cold hardening achieved and plant survival to freezing, but it had a larger effect on the freezing process than on the hardening process. Freezing tests carried out with insulated pots, in order to simulate field conditions where frost comes from above, did not affect the survival of alfalfa in spite of a delay in the cooling of the soil. Differences in cooling rates during freezing could not be correlated with the higher mortality observed with moist soils.