Available young hybrid trees of Eremocitrus glauca with ‘Valencia’ orange (Citrus sinensis (L.) Osbeck), Sicilian sour orange (C. aurantium L.), ‘Nagami’ kumquat (Fortunella margarita (Lour.) Swing.), and Koethen sweet orange (C. sinensis) were more cold hardy than the Citrus or kumquat parent in natural and controlled freezes. Eremocitrus may be a useful source of cold hardiness for breeding cold-hardy citrus hybrids.
`Maurin Makea', `Muskoka', ` Ottawa', and `Preussen' red raspberry (Rubus idaeus L.) canes were collected from the field and subjected to different hot water treatments (20, 35, 40, 45, and 50 °C) to determine if endodormancy could be removed by a near lethal stress. Estimation of days for 50% budbreak (DD50) was found useful for describing the state of bud dormancy in the samples. Bud dormancy was broken in `Ottawa' by immersing the canes in 45 °C water for 2 hours, in `Maurin Makea' by treating the canes in 40 °C water, and in `Preussen' by both 40 and 45 °C treatments. The influence of this treatment on dormancy and cold hardiness at different times of the winter was further examined using `Ottawa' raspberry. The treatment removed bud dormancy most effectively in October, when the samples were in deepest dormancy. A slight effect was observed in November, but no effect in January. During ecodormancy in February the treatment delayed budbreak. Hot water treatment reduced cold hardiness of `Ottawa' canes by 8 to 15 °C, and that of buds by 9 to 13 °C during both endo- and ecodormancy. Based on the capacity of buds and canes to reacclimate, recovery from the stress treatment was possible at temperatures ≥4 °C. Loss of cold hardiness was caused by high treatment temperature itself and was not related to breaking of dormancy in samples. This finding suggests that dormancy and cold hardiness are physiologically unconnected in raspberry.
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.
The effect of water stress on cold hardiness was examined in evergreen azaleas, `Coral Bell' (CB), `Hinodegiri' (HD), and `Red Ruffle' (RR). Plants were well-watered between 8 Aug. and 1 Nov. (wet) or were subjected to 3 weeks of reduced water supply starting on one of three dates, 1 Aug. (dry 1), 29 Aug. (dry 2), and 19 Sept. (dry 3). Cold hardiness of leaves and lower, middle, and upper stems was tested on 29 Aug., 19 Sept., 10 Oct., 1 Nov. By the end of each 3-week period, water potential of water stressed plants reached –1.5 to –1.8 MPa compared to around –0.8 MPa of well-watered plants. Reducing the water supply significantly increased cold hardiness of all tested plant parts in all cultivars regardless of timing of watering reduction, with two exceptions, CB middle stems on 29 Aug. and HD leaves on 19 Oct. Three weeks after rewatering cold hardiness of water-stressed plants did not differ significantly from well-watered plants, except for HD plants under dry three treatment, which continued to be 1.0 (middle stems) to 4.3 (upper stems) more cold hardy.
Temperature is a major environmental factor governing the distribution of both wild and cultivated plant species. During acclimation and deacclimation plants undergo a series of metabolic changes that lead to cold hardiness or loss of hardiness. One of these changes is the accumulation of certain lipids. This research was conducted to compare hardiness among three pecan cultivars: `Desirable', `Jackson', and `Owens' growing under Mississippi condition and to determine the relationship between fatty acid levels and cold hardiness of pecan shoots. Differential thermal analysis (DTA), electrical conductivity, and tetrazolium tests were used to determine cold hardiness. Pecan stems were collected from September to March in 2002 and 2003 to determine cold acclimation and deacclimation. Fatty acid composition of pecan stems during this time period was determined by gas chromatography. DTA indicated that pecan stems acclimated in October and deacclimated in March. During cold acclimation, there was a shift in the fatty acid composition to more unsaturated fatty acids. The percentage of linoleic and linolenic fatty acids increased, while the percentage of palmitic and stearic fatty acids decreased. The correlation between unsaturated fatty acids and cold hardiness suggests that unsaturated fatty acid may play a role in membrane fluidity.
Cold hardiness and cryogenic survival of micropropagated pear (Pyrus cordata Desv.) shoots were evaluated after pretreatments with ABA and sucrose. Shoot cold hardiness increased by 3 °C, and cryopreserved shoot tip growth increased by 17% after a 4-week 150 μm ABA pretreatment. Low temperature (LT) pretreatments improved the recovery of cryopreserved P. cordata shoot tips. Six to 10 weeks of LT were required for reaching high cryopreservation recovery. ABA and LT treatments produced significant synergistic effects on both cold hardiness and cryopreservation recovery. ABA shortened the LT requirement for high cryopreservation growth from 10 to 2 weeks. The optimal treatment for recovery of cryopreserved shoot tips was a 3 week culture on 50 μm ABA followed by 2 weeks of LT, while the maximum cold hardiness (-22.5 °C) was obtained with 150 μm ABA and 2-week LT. A 4 week culture on 150 μm ABA at 25 °C induced dormancy in 74% of shoot tips, but had little effect on cryopreservation growth unless combined with LT. Control and ABA-treated shoot tips, lateral buds, and leaves had similar cold hardiness (-10 to -12 °C), but LT and LT+ABA-treated shoot tips survived the lowest temperatures (-17 to -23 °C), lateral buds next (-15 to -20 °C), and finally leaves (-14 to -18 °C). An increase in the preculture-medium sucrose concentration from 2% to 7% combined with 2-week LT significantly increased cryopreserved shoot tip growth (0% to 75%) and decreased the LT50 from -7.8 to -12.4 °C. The optimal shoot pretreatment for successful recovery of cryopreserved P. cordata shoot tips was a 3 week culture on either 50 μm ABA or 5% to 7% sucrose medium followed by 2 weeks of LT, and increased shoot tip growth from zero to >70%. Chemical name used: abscisic acid (ABA).
Whole, half, and quarter leaves and leaf disks were used to make laboratory estimations of the cold hardiness of Magnolia grandiflora. The effects of ice nucleation temperatures, length of exposure to nucleating temperatures, rates of temperature drop, thawing regimes, and methods of injury analysis were investigated for each leaf type in the fall and midwinter. In general, whole and half leaves responded more consistently to freezing tests than did quarter leaves and leaf disks. The most critical factors in the freezing procedure are the temperature at which the samples are nucleated with ice crystals and the regime in which the samples are warmed. These data suggest that whole and half leaves can effectively be used to reliably predict the cold hardiness of southern magnolia leaves.
In previous work, we have shown that near-lethal heat stress can overcome dormancy in Red-osier dogwood, Cornus sericea L. The objective of this study was to determine the effects of premature breaking of dormancy on the development of cold hardiness. Plants at three stages of dormancy (early, deep, and late) were exposed to 47C for one hour and then placed into 3 post-treatment environments (0C, 23C, and natural conditions). At periodic time intervals, the plants were evaluated for bud break, cold hardiness, and stem injury. These studies suggest that premature breaking of dormancy at the early stage had no effect on hardiness development, whereas at the deep and late stages of dormancy, premature breaking of dormancy caused a faster rate of deacclimation at the warmer post-treatment environments. In addition, we observed that the heat-treated plants died during storage at 0C, and survived at 23C storage and natural conditions.
The influence of root temperature on whole-plant water relations and cold hardiness in seedlings of 2 citrus rootstocks—rough lemon (Citrus jambhiri Lush.) and Carrizo citrange [C. sinensis (L.) Osbeck × Poncirus trifoliata (L.) Raf.]—and ‘Valencia’ scions on both rootstocks was examined. Plants were exposed to root temperatures of 5°, 10°, or 15°C for 5–8 weeks, while shoots were exposed to a nonacclimating air temperature of 30°. Root temperatures of 5° decreased leaf xylem water potential and increased cold hardiness. Statistical differences in diffusive resistance and transpiration were observed only at the 5° root temperature. Root temperature did not significantly alter leaf relative water content in either seedlings or budded plants. A decrease in soil and root temperature alone, without a simultaneous reduction in air temperature, can provide an effective cold-acclimating environment for citrus.
Flower buds and apical shoots of ‘Redhaven’ peach (Prunus persica (L.) Batsch) were shown to be slightly more cold hardy in the autumn and winter when propagated on seedlings of Siberian C than on those of Harrow Blood. Apical shoots consistently had higher levels of total carbohydrates, reducing sugars, and other carbohydrate fractions on Siberian C than on Harrow Blood seedlings from winter to spring. The cold hardiness of flower buds was closely correlated with the hardiness of apical shoots. In addition, both flower bud and shoot hardiness were closely correlated with total sugars, sucrose, and reducing sugars in the shoots from autumn to spring. However, hardiness of flower buds and apical shoots was not correlated with total carbohydrates or starch. The TI50, a new method of expressing the hardiness of apical shoots was an objective index of cold hardiness and somewhat analogous to the T50 method for expressing hardiness of flower buds.