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Abstract

Differential thermal analysis (DTA) experiments, viability tests, and anatomical studies were conducted to investigate the biophysics of freezing in ‘Darrow’ blackberry (Rubus spp.) buds at selected stages of development from Nov. 1985 through Sept. 1986. As many as four low-temperature exotherms (LTEs) associated with the crystallization of supercooled water were detected in DTA experiments on buds collected 16 Nov. 1985. Anatomical observations revealed seven to nine floral initials present at that date. On 16 Jan. 1986, buds were morphologically similar to those examined in November, with four to 10 LTEs per bud. By 2 Mar., the size of the floral initials increased and distinct floral parts were evident. One to 10 LTEs were observed per bud at this date. Five to seven floral initials were observed in buds collected on 23 Sept. 1986, but floral parts were not evident. DTA experiments conducted in September revealed one or no LTE per bud. The median LTEs for November, January, March, and September buds were −20.5°, −28.0°, −22.0°, and −16.5°C, respectively. T50 values calculated from viability tests were within 4.5° of the median LTEs at all test dates. These results indicate that injury to the entire floral region is associated with a single freezing event when ‘Darrow’ blackberry buds are at an early stage of development. However, as floral differentiation progresses, individual primordia supercool, freeze, and are injured independently.

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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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Abstract

Low temperatures (LT) exotherms were found by differential thermal analysis (DTA) at −30°C in ‘Siberian C’ peach (Prunus persica [L.] Batsch) and −39° in ‘Starkrimson Delicious’ apple (Malus domestica Borkh. Nuclear magnetic resonance (NMR) spectrometry of intact stems and isolated bark and wood revealed that the LT exotherm was produced by freezing of deep supercooled water which was detected in the wood but not the bark. Freezing processes of the wood and bark appeared to be independent. In both species, xylem injury occurred at the same temperature as the LT exotherm and was closely, if not causally related to freezing of the supercooled water. Bark injury also occurred at the same temperature as the LT exotherm and may have been caused by dehydration stress or freezing of a small amount of supercooled water which remained undetected by NMR spectrometry. The dehydration resistance of apple wood on desiccation at 70 to 90% relative humidity was greater than that of the peach wood which in turn was greater than that of the bark of both species. The dehydration resistance of apple and peach wood may involve both nonliving and living elements of the wood because pulverizing the tissue destroyed the effect, whereas heat killing only lowered it. Both supercooling and dehydration resistance may be related to microcapillary pore structure which restricts heterogeneous nucleation and sublimation of supercooled water from the ray parenchyma cells.

Open Access

subzero temperatures: freeze tolerance (nonsupercooling) and freezing avoidance (deep supercooling) ( Burke et al., 1976 ; George et al., 1982 ). In nonsupercooling cells, ice formation is initiated in extracellular spaces and intracellular water is

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Abstract

Deep supercooling of stem tissue water was found in all the native species of rose (Rosa spp.) studied. Freezing of this supercooled water was associated with injury to the stems, indicating that maximum cold hardiness of these species is limited to about −40°C. Therefore, these species have some potential for use in breeding to develop cold-hardy cultivated roses, but their hardiness would be limited to −40°C by the supercooling characteristic.

Open Access

Abstract

Deep supercooling was found in the stem tissues of all the Pyrus species studied. There was more than 1 low temperature exotherm resulting from the freezing of supercooled water in stem tissue, and these exotherms were associated with the tissue injury. The supercooled water in the stems of P. nivalis Jacq., P. cordata (Desv.) Schneider and P. elaeagrifolia Pall, was found in both xylem and bark tissues. The supercooling characteristics of vegetative and flower buds are also described. The hardiest and least hardy species found were P. caucasica Fed. and P. pashia D. Don., respectively.

Open Access

Abstract

Seasonal changes in deep supercooling and cold-hardiness of stem tissue and apical buds of pecan [Carya illinoensis (Wang enh.) C. Koch] cultivars were studied. All the pecan cultivars showed supercooling in stem and apical buds. Supercooling in stem and apical buds was maximal in early January and least in early spring. A good correlation between killing temperatures and freezing of supercooled water was found in apical buds. Similar results were observed for stem samples collected during early spring. Apical buds appeared to be more prone to injury during spring than stem tissue in all the pecan cultivars. In early April, stem samples of pecan cultivars were killed at or below –20.1C, whereas apical buds were killed at –16C or above. Apical buds of ‘Posey’ showed greater cold-hardiness than those of other pecan cultivars in midwinter and early spring.

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Abstract

A study to determine the freezing pattern in overwintering flower buds of peach (Prunus persica (L) Batsch) revealed that masses of ice formed in the flower bud scales and flower bud axis, but not the flower primordium. Water appeared to be withdrawn from the bud axis to freeze in preferred sites in the scales, but not from the flower primordium. The flower primordium appeared to survive to below −20°C by supercooling. Below −10°C the supercooled flower primordium could be induced to freeze by inoculating it with ice or excising it from the flower bud at the base. It is proposed that 2 barriers operate below −10°C to prevent external nucleation of the supercooled tissue, 1) the cuticle or epidermis which prevents nucleation by ice on the surface of the flower primordium, and 2) a dry region at the base which prevents an ice boundary from spreading into the flower primordium from the interior of the bud axis. The dry region is formed by water withdrawal into the scales during the initial stages of freezing. Below some critical temperature the ice boundary spreads through the dry region or the supercooled tissue nucleates spontaneously. The sudden explosive growth of ice kills the flower primordium, probably by intracellular freezing.

Open Access

Abstract

The capacity of woody tissues to deep supercool has been associated with the presence of a marked dehydrative resistance (i.e., the capacity of a tissue to retain moisture against a vapor pressure gradient). Naturally occurring seasonal changes in the extent of deep supercooling and dehydrative resistance in xylem tissues of Cornus florida L., Prunus persica (L.) Batsch, and Salix babylonica L. were monitored to determine if such a relationship could be established. Results indicated that the relationship between these 2 parameters is quite complex and may be more qualitative than quantitative. Greatest seasonal fluctuation in dehydrative resistance occurred in xylem tissue of peach and dogwood equilibrated at 90% RH. Seasonal changes in willow were relatively small. The capacity of peach and dogwood to retain water at 90% RH generally increased as supercooling increased. At 86% and 81% RH the same general trend was present, but the degree of seasonal fluctuation was much less than at 90% RH. The predicted relationship was not present at 95%, 78%, and 58% RH. The capacity to withstand a desiccation stress equivalent to 90% RH (− 140 bars) may somehow be integrally related to the capacity to maintain a stable supercooled system.

Open Access

Thermal analysis of Forsythia × intermedia `Spectabilis' flower buds had previously detected the occurrence of low temperature exotherms (LTE) during freezing. The LTE apparently resulted from the freezing of supercooled water and corresponded to the death of the florets. The genus Forsythia encompasses a wide array of species and interspecific crosses ranging in flower bud hardiness and floret size. The ability of buds to supercool, the relationship between the LTE and flower bud hardiness, and the extent to which floret size affects both were studied in flower buds of the following Forsythia species: F. × intermedia `Spectabilis', F. × intermedia `Lynwood', F. `Meadowlark', F. suspensa var. fortunei, F. `Arnold Dwarf, F. europaea, F. giraldiana, F. × intermedia `Arnold Giant', F. japonica var. saxatilis, F. mandshurica, F. ovata, and F. viridissima. Flower buds used for thermal analysis were also used in subsequent size determinations. Hardiness evaluations were conducted using controlled freezing tests, and the sampling interval defined using the temperature range of the LTEs. Initial evaluation indicated a high degree of correlation (α>.50) between mean LTEs and mean killing temperatures. The Forsythia genus, with its broad range of bud hardiness and size provides an excellent system in which to study the mechanisms of supercooling. Thermal analysis of cultivars which exhibit LTEs can accurately assess bud hardiness with minimal plant material.

Free access