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Although differential thermal analysis has been routinely used to evaluate cold hardiness, the relationship of deep supercooling ability and plant survival are not well understood. In this study, we compared the seasonal profiles of changes in low-temperature exotherm (LTE) occurrence with visually determined cold hardiness of Acer rubrum L. 'Armstrong', Fraxinus americana L. 'Autumn Purple' and Zelkova serrata (Thunh.) Mak. 'Village Green' growing in three locations representing plant cold hardiness zones 8b, 7b, and 5a. Between December and February, LTEs in Acer rubrum 'Armstrong' and Fraxinus americana 'Autumn Purple' occurred at temperatures around 10 to 25C lower than the lowest survival temperatures. The mean difference between LTEs and lowest survival temperature was not significant for Zelkova serrata 'Village Green' from January to April and for Acer rubrum 'Armstrong' and Fraxinus americana 'Autumn Purple' in March. Data indicated that LTEs could be used as an estimate of lowest survival temperature in Zelkova serrata 'Green Village' but not in Acer rubrum 'Armstrong' and Fraxinus americana 'Autumn Purple'. This study demonstrated that LTEs may not reliably estimate cold hardiness in all species that deep supercool. Factors other than freeze avoidance ability of xylem may limit stem survival at temperatures above the LTE. Deep supercooling of water in certain stem tissues such as axial and xylem ray parenchyma is a freezing-avoidance mechanism characteristic of many woody plants, especially deciduous hard- woods (Quamme, 1985, 1991). The deep supercooled water in these tissues freezes intracellularly as temperature decreases be- low a nucleation point, and subsequently causes lethal injury. The freezing of deep supercooled water releases heat of fusion that can be detected by thermal analysis and is observed as a low- temperature exotherm (LTE) (Quamme, 1991). Occurrence of a LTE can be determined by differential thermal analysis (DTA). Although this technique has been routinely used to evaluate cold hardiness, the relationship between freeze tolerance and freeze avoidance of tissues and their effect on plant survival are not well understood.

Low-temperature Exotherms and Cold Hardiness in Three Taxa of Deciduous Trees