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: 1) HTE occurs a few degrees Celsius below 0 °C and is associated with freezing of extracellular water and is nonlethal; and 2) LTE occurs at a lower temperature and is associated with freezing of deep supercooled water and lethal damage to plant

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al., 1977 ; Quamme et al., 1995 ). The freezing of apoplastic water, especially that in the xylem, creates an exotherm called the HTE. This freezing does not harm the plant ( Burke et al., 1976 ). Other observable exotherms are called LTE. The

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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.

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. In apple, pear, and azalea, a low temperature exotherm (LTE) of dormant woody stem sections, detected by DTA, correlated with injury to both xylem and pith that occurred during cooling ( Graham and Mullin, 1976 ; Montano et al., 1987 ; Quamme et al

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Abbreviations: BBI, budbreak index; DTA, differential thermal analysis; LTE, low-temperature exotherm; MHL, minimum hardiness level; XVE, xylem vessel element. I thank E.L. Proebsting, Jr., for his helpful comments and Rhoda Burrows for her skillful

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Abbreviations: DTA, differential thermal analysis; HTE, high-temperature exotherm; LTE, low-temperature exotherm; SEM, scanning electron microscopy. 1 Associate Professor, Dept. of Horticulture. 2 Associate Professor, School of Natural Resources. 3

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Abbreviations: DTA, differential thermal analysis; FAA, formalin-acetic acid-alcohol; LTEs, low temperature exotherms. Contribution from the Missouri Agr. Expt. Sta., J. Ser. no. 11577. We gratefully acknowledge the technical assistance of Milon

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Abbreviations: DTA, differential thermal anaylsis; HTEs, high temperature exotherms; LTEs, low temperature exotherms; XVE, xylem vessel elements. 1 Research Associate. To whom reprint requests should be addressed. 2 Horticulturist. Scientific Paper

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(physiological germination) was the criterion of germination used. For the low temperature emergence test (LTE), seeds (four replicates of 50 seeds/lot) were sown 3 cm deep in the experimental field of the Department of Horticulture, Faculty of Agriculture

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Abstract

Seasonal changes in the temperature of the median low-temperature exotherm (LTE50) of dormant sweet cherry (Prunus avium L. cv. Bing) flower buds were significantly correlated with the preceding minimum air temperature in the orchard and the water content of the flower primordia. When buds were exposed to temperatures just below the high temperature exotherms, water migrated from the primordia to the bud scales. Under these conditions, the LTE50 decreased almost 5°C during the first day, but only 1°/day thereafter. The minimum LTE50 was near −32° after the buds were frozen for 10 days. Thawing buds at 0° or above increased the LTE50 about 1°/hr, to −20° to −21°. The LTE50 did not increase above these temperatures until the buds were exposed to 20° for 1-2 days following the completion of rest. During deacclimation, both the LTE50 and temperature range of the low temperature exotherms (LTEs) increased. These changes were accompanied by fluctuations in the capacity of the flower buds to exhibit deep supercooling, expressed as the fraction of an LTE produced per flower primordium (LTE/primordium). Even on days when the LTE/primordium was low, the temperature required to injure 50% of the flower primordia was similar to the LTE50.

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