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tissue lethal damage. Water in the floral bud scales and flower axis do not exhibit supercooling and usually do not result in damage when frozen ( Ashworth and Wisniewski, 1991 ; Levitt, 1980 ). DTA detects water freezing events in plant tissues by

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The objective of this study was to relate the lethal freezing temperatures of St. Augustinegrass [Stenotaphrum secundatum (Walt.) Kuntze] genotypes, as measured by differential thermal analysis (DTA), to winter survival observed in the field. DTA-predicted lethal temperatures of 14 St. Augustinegrass genotypes ranged from –7.7 to –4.7C. Regression of field winter survival vs. DTA-predicted lethal temperatures resulted in an r 2 = 0.57 for one field trial that evaluated cultivars with a relatively narrow range of expected freezing tolerance. In a second study evaluating cultivars with a greater range of freezing tolerance, r 2 was 0.92 when winter survival was regressed on DTA-predicted lethal temperatures. DTA was successful in measuring freezing avoidance of St. Augustinegrass cultivars.

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Abstract

Differential thermal analysis was evaluated as a means of determining the cold hardiness of excised dormant buds of Vitis vinifera L. cv. Chardonnay grapevines. The manner in which buds were excised and cooled affected the freezing characteristics of bud primordia. Buds excised with 1 to 2 mm of subjacent nodal tissue exhibited both high temperature exotherms (HTEs) and low temperature exotherms (LTEs). HTEs apparently resulted from freezing of supercooled moisture in bud scales and/or in the subjacent nodal tissue and occurred at inconsistent temperatures. Cooling similarly excised buds on a water-saturated substrate caused HTEs to occur at −4° to −8°C and did not affect the occurrence of LTEs, which were consistently associated with primordia death. Median LTEs associated with primary bud death were 1.5° to 2.0° warmer than LT50s derived from temperature/survival freezing tests of similar buds. Buds killed by freezing did not supercool appreciably when refrozen. Bud cold hardiness increased when single-node cuttings were exposed to a step-wise cooling cycle; however, the ability to detect exotherms decreased under these conditions. The decreased detection of exotherms was due to increased bud death and, presumably, a decrease of freezable (and thus detectable) moisture in the supercooled primordia of viable buds. DTA provides a useful and reliable means of determining grapevine bud cold hardiness when conducted in a standardized fashion.

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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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Flower buds of two sweet cherry (Prunus avium L.), 12 sour cherry (Prunus cerasus L.) and one ground cherry (P. fruticosa Pall.) were collected monthly from Aug. 1990 to Mar. 1991, and subjected to freeze tests to determine the level of cold hardiness. LT50 values (temperatures at which 50% of the flower buds were killed) summed over all months were significantly correlated (r = 0.6844, P ≤ 0.01) to the flower bud low temperature exotherms (LTEs). Correlation of LTEs to LT50 values was highest, r = 0.85, P ≤ 0.01 for the acclimation and midwinter period, November to February collections. During this period the average LT50 occurred before and within 2.5 °C of the LTE, indicating tissue injury before the LTE occurrence. During deacclimation, represented by the March collection, the LT50 began within 2.0 °C, on average, of the LTE, and in 11 of 12 cultivars and seedlings preceded the LTE. In March, the correlation of LTEs to LT50 values was less, r = 0.69, P ≤ 0.05, indicating possible changes flower bud deep supercooling. LTE values were selected as a measure of flower bud hardiness in sour cherry. Exotherms were not detected in the flower buds of all germplasm tested on all evaluation dates, but were the best means of separating selections. While LTE analyses expressed significant differences in November, December, and March at P ≤ 0.01, the LT50 analyses expressed differences only in December and February at P ≤ 0.05. The relationship between ambient temperatures and floral tissue hardiness indicated that November and March are two critical times for flower bud injury. November injury would occur in years when sudden low temperatures occur without sufficient pre-exposure to freezing temperatures. March injury would occur in years when sudden freezing temperatures follow warm days. This type of injury would be most pronounced in southern genotypes. Spring freeze injury could be significantly reduced by the selection of cultivars and seedlings that have delayed deacclimation. Exotherm occurrence and bud volume were correlated (r = 0.95, P ≤ 0.05). In January, when exotherms were least prevalent, they were generally present only in the five cultivars and seedlings with large bud volumes. The LTEs in midwinter, occurred within 3 °C of the reported average annual minimum temperature for the northern range of Prunus commercial production (Zone 6). The results of the principal component analysis of flower bud LTEs indicated that other selection criteria as flowering time might have played a more significant role in the hardiness range of sour cherry than simply geographic origin. The first principal component (PC1), which accounted for 77% of the total variance was used to separate among cultivars and seedlings. Selections at the positive end of PC1 had flower buds that were more cold susceptible than selections at the negative end of PC. This concurs with other research showing that flower bud hardiness is related more to commercial range (i.e., the range of commercial production) than to geographic distribution.

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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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methods of characterizing hardiness may improve the recognition of critical limitations and influence decisions of cultivar release and deployment. Differential thermal analyses (DTA) have been used to imply cold hardiness in woody tissues of some species

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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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Cold acclimation of `Tifblue' rabbiteye blueberry was determined using differential thermal analysis (DTA). Electrolyte leakage and tissue browning test results were correlated with DTA. DTA showed that three exotherms occurred: exotherm 1 (ET) associated with extracellular freezing, exotherm 2 (CT) associated with tissue injury, and exotherm 3 (LTE) not associated with tissue injury. Maximum hardiness (–20C) occurred in January. The LT50 measured by electrolyte leakage and tissue browning was about –17C and –15C, respectively. Acclimation began in November and deacclimation in mid-Feb. 1994.

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Abstract

More than 4500 accessions of eight genera including Pyrus at the National Clonal Germplasm Repository, Corvallis, Ore., require testing for cold hardiness. Since pear xylem deep supercools (7), differential thermal analysis (DTA) would be a suitable test if large numbers of samples could be examined simultaneously. The object of this study was to produce a method of multichannel DTA for defining cold hardiness of pear accessions. Visual browning was also examined to confirm cold hardiness values.

Open Access