The cambium of both ‘Early Amber’ peach and ‘Sungold’ nectarine, Prunus persica L. Batsch gained cold hardiness from November through January even though day and night temperatures were above 15°C. Cultivars attained the greatest hardiness in January, surviving temperatures of -12°C. ‘Early Amber’ maintained its hardiness until just prior to bud break (February 8) and decreased thereafter. ‘Sungold’ remained somewhat resistant to cold until February 15 and gradually lost hardiness until March 8. Characteristically the low chilling cultivars reached an acclimation base higher than reported for cultivars adapted to higher latitudes.
A method of exotherm analysis is described for determining the cold hardiness of dormant peach flower buds (Prunus persica (L.) Batsch). Twig pieces with intact flower buds were placed into thermos bottles and cooled at a constant rate in a programmed cold chamber. A thermocouple was inserted in the node of each twig piece to measure the temperature at which sudden heat release occurred. Thermocouples connected in series made it possible to record exotherms of 5 twig pieces from a cultivar on a single channel of a multipoint recorder. The exotherm method was compared to the LT50 method (the temperature at which 50% of the flower buds are killed) using 10 cultivars on one collection date. The mean temperatures at which the exotherms were initiated were significantly correlated with the LT50 (r = +.880). In addition, the mean temperatures at which exotherms were initiated and the LT50 for these cultivars were significantly correlated with the percentage of flower bud mortality averaged over 4 winters (r = −.667 and −.797, respectively). The exotherm method gives a direct measurement of the temperature at which peach flower buds are killed and offers an opportunity for studying the cold hardiness of individual flower buds not possible with other methods.
Cooling treatments of 2, 4, and 6C/hour or warming at 25, 4, or 0C influenced the cold hardiness estimates of x Cupressocyparis leylandii (A.B. Jacks. and Dallim.) Dallim. and A.B. Jacks. (Leyland cypress), Lagerstroemia indica L. (crape myrtle), and Photinia ×fraseri Dress `Birmingham' (redtip photinia) at four times during the year. New growth from all taxa, especially spring growth, was injured or killed at higher temperatures by the fastest cooling rate and/or by warming at 25C. Cold hardiness of Leyland cypress was unaffected by the cooling and warming treatments. Crape myrtle had a significantly higher lowest survival temperature (LST) when warmed at 25C than at 4 or 0C. Photinia leaves and stems cooled at 6C/hour or warmed at 25C generally resulted in a higher LST than those cooled more slowly or warmed at lower temperatures. Cooling rates of 14C/hour and warming at 0 to 4C should be used in freeze tests with Leyland cypress and crape myrtle. For leaves and stems of photinia, 2C/hour cooling and warming at 0 to 4C should be used.
`Redcrest' plants were renovated at 2, 4, 6, 8, or 10 weeks after harvest (WAH) from July 1 to Aug 26, 1992 and July 7 to Sept 1, 1993, plus an unrenovated control. Data on fall yield, maximum cold hardiness, and summer yield and berry weight were collected. For maximum cold hardiness, crowns were subjected to controlled freezing (-8, -11, -14, -17 or -20°C) and then evaluated by oxidative browning. Fall yield in 1993 was greater than in 1992. In 1992, fall yield was comparable for all renovation dates except the latest, 10 WAH. Unrenovated plants tended to have a lower fall yield than renovated plants. In 1993, plants renovated 2, 4, or 6 WAH had higher yields than control or late-renovated plants. Fall yield was not correlated with summer yield in 1993. Plants renovated 4 WAH had a higher summer yield in 1993 than unrenovated plants or those renovated at other times, which all had similar yields. Date of renovation had no effect on berry weight or percent fruit rot. Unrenovated plants and those renovated 2 or 4 WAH were hardier in winter 1992/93 than those renovated later.
Air temperature on crop productivity is a complex topic in environmental physiology. Blueberry growers in the Southeastern United States experience major crop losses due to late spring frosts. A 2-year study was conducted on `Tifblue' rabbiteye blueberry flower flushes to determine the effect of abscisic acid (ABA) at 0, 20, 40, and 60 mg·L–1 and paclobutrazol at 0, 100, 200, and 300 mg·L–1 on cold hardiness of floral parts. Two types of flower flushes were identified in `Tifblue'. Critical freezing temperatures and the effectiveness of the treatments were determined by differential thermal analysis (DTA), electrolyte leakage (EL), oxidative browning and tetrazolium staining. Floral parts frozen to –40 °C produced only one exotherm, confirming that rabbiteye floral parts do not supercool. Both growth regulators were more efective in the induction of hardiness in floral parts at second flush than at first flush. Floral parts developed in April were more prone to freezing injury than the floral parts of March. Ovaries were the hardiest, followed by calyx, stamen, style, and corolla. Air temperature had a profound influence on cold hardiness as influenced by ABA and paclobutrazol. The sequence of exotherms of DTA and the LT50 of the viability tests were air temperature-dependent.
Black currant (Ribes nigrum L.) cultivars with heavy, light, and no gooseberry mite (Cecidophyopsis grossulariae Collinge) infestation levels (MIL) were tested for cold hardiness by visually determining the bud injury rating (BIR) after laboratory freezing in Jan. 1998. Lightly mite-infested cvs. Blackdown and Risager, usually thought of as less cold hardy than Nordic cultivars, survived -35 °C, while mite-infested buds of the Finnish cv. Brödtorp were injured at -35 °C. Heavily mite-infested buds of the Swedish R. nigrum L. cv. StorKlas from Corvallis, Ore., were injured at -20 °C while lightly infested buds were injured to -25 °C. Noninfested `StorKlas' buds from Pennsylvania and British Columbia survived laboratory freezing to -35 °C. Heavy mite infestation lowered the bud cold hardiness of `Brödtorp' and `StorKlas' by 10 °C, as estimated by a modified Spearman-Karber T50, relative to the hardiness of lightly mite-infested buds of these cultivars. Heavily mite-infested buds contained unusual tissues forming what appeared to be spherical blisters or eruptions, ≈100 μ in diameter. Other tissues in the region of heavy mite infestation appeared to be more turgid than their noninfested counterparts. Abiotic and biotic stresses can have a combined impact on field-grown black currants.
Deacclimation response is an important part of reproductive success in woody perennials because late winter or early spring thaws followed by hard freezes can cause severe injury to dehardened flower buds. There is a need to develop more spring-frost tolerant cultivars for the blueberry (Vaccinium L.) industry. The identification of later or slower deacclimating genotypes could be useful in breeding for more spring-frost tolerant cultivars. This study was undertaken to investigate cold hardiness and deacclimation kinetics under field conditions for 12 Vaccinium (section Cyanococcus A. Gray) genotypes (the cultivars Bluecrop, Duke, Legacy, Little Giant, Magnolia, Northcountry, Northsky, Ozarkblue, Pearl River, Tifblue, and Weymouth; and a population of V. constablaei Gray) with different germplasm compositions and expected mid-winter bud hardiness levels. Examination of bud cold hardiness (BCH) vs. weeks of deacclimation over a 7-week period in 2 consecutive years (2002 and 2003) revealed clear genotypic differences in cold hardiness and timing and rate of deacclimation. Among cultivars, `Legacy' was the least cold hardy at initial evaluation, even less so than `Tifblue'. Regarding deacclimation kinetics, the weekly intervals with the largest losses (i.e., high rates of deacclimation) also varied among genotypes. For `Duke', the largest losses in BCH were detected at weeks 2 and 3, making it the earliest deacclimator. For `Bluecrop', `Ozarkblue', `Weymouth', `Tifblue', and `Legacy', the greatest losses in BCH were observed at weeks 3 and 4. For `Little Giant', `Magnolia', `Northcountry', `Northsky', and `Pearl River', losses in BCH were greatest at weeks 4 and 5, while for V. constablaei, losses were greatest at weeks 6 and 7, making it the latest deacclimator. Deacclimation kinetics were not correlated with mid-winter hardiness or chilling requirements in any fixed pattern. On the other hand, a strong positive correlation was found between BCH and stage of bud opening (r = 0.84). A comparison of timing of deacclimation with germplasm composition indicated that V. constablaei was particularly late to deacclimate. `Little Giant', a 50:50 hybrid of V. constablaei and V. ashei Reade, was nearly as late to deacclimate as the 100% V. constablaei selections. Thus, V. constablaei may be useful in breeding programs to contribute genes for late deacclimation, which should translate into greater spring frost tolerance, in addition to genes for mid-winter hardiness.
The effects of rootstock, pruning, and preplant soil fumigation on floral bud dormancy status and shoot cold hardiness of `Redhaven' peach [Prunus persica (L.) Batsch] trees were monitored. Dormancy status, expressed as percent floral budbreak, was significantly affected by rootstock and pruning, although differences were small. In late January, significant interactions occurred between rootstock and pruning treatments, as well as between pruning and soil treatments. Pruning of trees on Lovell rootstock resulted in significantly lower budbreak as compared to trees on Nemaguard and unpruned trees on Lovell. Also, for trees pruned in December, higher budbreak was associated with those growing in fumigated vs. nonfumigated soil. Treatment effects on dormancy status did not correspond with treatment effects on hardiness. In fact, differences in hardiness were minimal and probably not biologically meaningful.
Fruit buds of 5 peach cultivars—‘New’, ‘Daroga’, ‘Redskin’, ‘Mayflower’, and ‘Loring’—grown in Kentucky and exhibiting varying degrees of cold hardiness, were compared biochemically. Fruit bud analysis for total and reducing sugars, starch, total protein, and total and individual free amino acids indicate some correlation between the degree of hardiness and the biochemical make-up of these cultivars. Generally, a high sugar and protein content, and a low total free amino acids were associated with increase in hardiness. Specifically, significant correlation was found between hardiness and a high sugar and protein content when buds were frozen at −2½° F. Significant correlation was also found between 2 amino acids (arginine and γ-NH2 butyric) and hardiness at both −2½° and −5°.
The root system of Taxus cuspidata Sieb, and Zucc. exhibited zones of varying cold hardiness. Root hardiness was greatest in sections close to the stem while the root tips were least hardy. The light-colored young roots did not develop hardiness under any of the environmental treatments. Hardiness of the dark mature roots was usually similar to that of the leaves. Under natural conditions mature root hardiness was greater than −29.0°C. Daily application of Alar as a soil drench, increased the hardiness of young roots to the hardiness level of mature roots. Amo 1618 was less effective and significantly decreased the hardiness of mature roots. Mature roots from girdled plants did not develop hardiness, indicating their dependence on the top of the plant for development of root hardiness. Roots dehardened more slowly than leaves in the spring.