Deep Undercooling in Woody Taxa Growing North of the −40°C Isotherm

The freezing of deep undercooled water in cold-hardened 3-year-old stems of 16 woody taxa was studied in mid-January by differential thermal analysis. The initiation temperature and the size of the low temperature exotherm (LTE) were compared for nonthawed, thawed, and freeze-killed stems. In general, the initiation temperature of the LTE for nonthawed stems occurred at a lower temperature than for thawed stems and freeze-killed stems. In some cases, no LTE was detected in nonthawed stems although a LTE was detected after thawing. The size of the LTE increased after thawing the stem and also after the stem was freeze killed. The LTE observed in one species disappeared upon exposure to continuous low sub-zero temperatures. Results suggest that undercooling which subsequently results in the LTE in woody stems is due to the cell wall and the plasma membrane. During periods of prolonged freezing, cellular water migrates from the cells which undercool to extracellular ice. This results in a concentration of cell solutes which lowers the homogeneous nucleation temperature of the cell sap. The cold hardiness of nonthawed and thawed stems was compared by a controlled freeze test. In general, thawing had little effect on the survival temperature whereas it had a marked effect on the initiation of the LTE.