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into cold storage; (1.8 × °C) + 32 = °F. Cuttings were taken from each tree on days 0, 3, 7, 14, and 20 for the cold hardiness study. On each day, 11 cuttings ≈2 to 3 inches in length were taken from each tree. The cuttings were divided into one control

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Tree and fruit losses from cold injury are important problems in growing citrus. Severe losses from the freezes of 1894-95, 1957-58, 1962, and 1970-71 in Florida; 1949, 1950, and 1962 in Texas; and 1913, 1937, 1949, and 1950 in California, have stimulated research on cold hardiness of citrus. One method of reducing losses from freezes is the production of cold hardy cultivars by breeding and selection. Citrus physiologists and breeders with the USDA at Orlando, Florida; Indio, California; and Weslaco, Texas, have coordinated their research to develop more cold hardy citrus cultivars (2, 3). This paper summarizes some recent efforts to develop methods for screening citrus hybrids for cold hardiness. The glossary (Table 1) of the citrus types and names used here include cold hardiness ratings. Common names or a designated number will be used for simplicity of discussion.

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Cold hardiness was studied in two interspecific Populus hybrids (P. trichocarpa × P. deltoides, and P. trichocarpa × P. maxomowiscii), using laboratory freezing tests of mid-winter dormant tissues and fully expanded leaves in the autumn. These laboratory measurements were compared to field observations. Hybrids having one parent from southern-source populations and the other parent from northern sources were compared to hybrids in which both parents were from southern-source populations. Populus hybrids with one parent of northern origin were generally hardier than hybrids from parents of southern sources; however, significant differences in cold hardiness were detected between hybrids having the same genetic parents. Field observations generally supported laboratory measurements and showed clonal differences in mid-winter cold hardiness and autumn leaf frost tolerance. Fully expanded leaves of different clones from the same parent also exhibited differences in frost tolerance.

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throughout the course of the winter and is influenced by a number of factors ( Quamme, 1978 ). During active growth, tissues have little cold tolerance but in the fall, growth stops and cold-hardiness develops. This process is called “cold acclimation” and

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Abstract

The need to breed for cold hardiness (where opportunities permit) is obvious to anyone who grows fruit, under temperate or subtropical conditions, where plants or bloom and young fruit are liable to frost damage or winter injury. The present report will be confined to progress to date with avocados, mangos and passion fruit, and to discussion of some of the possibilities inherent in 2 other fruits, the guanabana and acerola.

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To examine injuries caused by freezing temperature, six woody plants were placed under temperatures ranging from 0 to 20C. Control plants were placed at 0 or –2C, depending on the field sampling period. Freezing tests were done three times (September, October, and November) during the fall. In 1992, six species were tested: Genista tinctoria `Lydia', Parthenocissus `Veitchii', Weigela × florida `Variegata', Spiraea japonica `Shirobana', Spiraea japonica `Coccinea', and Arctostaphylos uva-ursi. After testing, all plants were stored at –2C for the remainder of the winter. The following May, plants were repotted into containers. Effects of freezing temperatures on plant growth were recorded at the end of the following summer. Preliminary results indicate that the most sensitive species to cold temperatures were Parthenocissus `Veitchii' and Arctostaphylos uvaursi. Plants of these two species did not survive the summer. However, for the third sampling period, Parthenocissus `Veitchii' (–18C) had better cold hardiness than A. uva-ursi (–9.5C). Genista tinctoria `Lydia' appeared to have the same cold hardiness (–10C) for the three sampling periods. The last three species had shown increasing cold hardiness beginning at around –8C in September to about –18C in November.

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Stem sections of 31 filbert genotypes were collected, artificially frozen, and evaluated by visual browning of cambium and other tissues to determine cold hardiness during 5 sample dates in 1984 and 1985. Corylus heterophylla Fish. ex. Trau. was the most cold-hardy filbert tested, but it deacclimated sharply before the end of February. The tested filberts were divided into 3 temporal groups of acclimation to maximum cold hardiness—early, midwinter, and late. C. avellana L. ‘Butler’, ‘Tombul’, ‘Barcelona’, ‘Ennis’, and ‘Casina’ acclimated early; ‘Gasaway’, acclimated in midwinter season; ‘Daviana’ and ‘Hall’s Giant’ acclimated late. The genotypes tested also were separated into very hardy, hardy, and least hardy groups for cortex-cambium, pistillate bud, and staminate bud tissues. The general order of tissue hardiness from least to most was pith, xylem, cambium, and cortex. Vegetative buds in midwinter were as hardy or hardier than the cambium. Staminate flowers were hardier than pistillate in October, but most pistillate flowers were hardier than staminate by January. Several filberts had fully blooming pistillate flowers that were uninjured at −30°C in December and −40° in January. Filbert flower buds demonstrated maximum cold hardiness during nondormancy.

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Twelve-week-old Malus seedlings were induced to cold harden by exposure to low temperature and freezing environments. The effectiveness of induced acclimation by exposure to stimuli such as low temperature (3 to 5 °C), frequency of exposure to freezing temperatures (-3 °C), storage time before and after induction and the effects of different screening temperatures (-20, -30, and -40 °C) were investigated with seedlings grown in a greenhouse from open-pollinated `Golden Delicious' apple (Malus pumila (Mill.), `Antonovka' apple (M. baccata (L.) Borkh. × (M. pumila) and `Rescue' apple (M. baccata) × (M. pumila). Differentiation of the seedling populations with respect to cold hardiness was not achieved until after acclimation at cool temperatures (3 to 5 °C) for 6 weeks. Further population differentiation was achieved by exposure to one or more frosts (-3 °C). Once the acclimation response had been initiated the seedlings could be held for up to 11 days, under the same conditions, with no significant decrease in hardiness. Hardiness levels of acclimated and nonacclimated open pollinated seedlings coincided with known inherent hardiness responses for all three maternal cultivars evaluated. A binomial form of regrowth data collection, percent seedling survival, was determined to be the most efficient and most precise measure of evaluation. Induction of cold hardiness in very young seedlings and the use of a controlled freeze testing protocol should facilitate rapid screening of large progenies and improve the rate of progress in breeding for cold hardiness.

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Bermudagrass [Cynodon dactylon (L.) Pers.] is widely used along its northern limit of adaptation. However, cold hardiness and winter survival are common concerns facing turfgrass managers in these areas. The objective of this study was to determine the effects of moderate salinity applications on bermudagrass cold hardiness. Two trials were conducted in Summer 2002. The cultivar Princess was seeded into pots in a glasshouse at a rate of 24 kg·ha-1. Pots received a weekly solution of 20-20-20 at a rate of 4.9 kg·ha-1 N. Bi-weekly salinity treatments began ≈2 months after germination and consisted of 0, 5, 20, and 40 dS·m-1 in the form of NaCl. These treatments continued for ≈8 weeks. Weekly quality ratings and chlorophyll fluorescence measurements showed similar results, with the high salinity treatments having the poorest quality. Soil electrical conductivity measurements showed a significant increase for the high salinity rates over the lower rates at the end of the trials. Proline concentrations increased with increasing salinity treatments in Trial 1 and were highest with the 20 dS·m-1 rate in Trial 2. Plants were acclimated in a growth chamber, and artificial freezing tests revealed that the 5 and 20 dS·m-1 treatments had the highest percentage of regrowth after freezing. These results indicate that moderate applications of salt or the use of effluent water prior to hardening may be an important way to increase bermudagrass cold hardiness.

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Differential thermal analysis, electrolyte leakage, tetrazolium stain test, and the “feeder plate” tissue culture regeneration technique were used to determine cold hardiness of passion fruit and maypop. The “feeder plate” technique showed that yellow passion fruit did not regenerate at 0C, -3C, and -6C while purple passion fruit showed callus formation at all temperatures. The remaining tests gave similar lethal temperatures for the two species. Lethal temperatures were -9C to -10C, -10C to -I2C, and -11C to -13C for yellow and purple passion fruit and maypop, respectively.

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