the fall, thereby reducing potential coldhardiness. Linden (2002) reported that a longer growing season resulting from high precipitation levels in fall is an important factor that increases apple tree sensitivity to winter injury.
Forsythia mandschurica Uyeki, F. ovata Nakai, and F. × intermedia Zab. ‘Lynwood’ flower buds were frozen from 0 to –35°C during January to mid-April 1978, then forced under intermittent mist. In field and laboratory tests, ‘Lynwood’ flower buds were significantly less hardy than those of F. mandschurica or F. ovata. The hardiness of the latter 2 species was not significantly different throughout the test period. In May, 50 to 60% of F. mandschurica and F. ovata flower buds bloomed; 5% of the ‘Lynwood’ bloomed. Regression equations at 20% flower bud survival indicated that during mid-winter F. mandschurica survived temperatures 3.6° colder than did F. ovata.
Effects of temperature on cold acclimation and deacclimation of red-osier dogwood (Cornus sericea L. syn. Cornus stolonifera Michx.) plants were determined at different stages of plant development. Results were used to develop models for predicting stem hardiness. Acclimation and deacclimation rates were related to temperature and plant developmental stage (expressed as degree growth stage, °GS). Decreasing temperature promoted increasing acclimation. Maximum acclimation rates in the temperature range of 5° to 20°C occurred at maximum rest (270°GS). During the decreasing rest phase (270 to 315°GS), deacclimation occurred at temperatures from 7° to 20°C. At earlier stages of development (315° and 335°GS) during the quiescent phase (315 to 360°GS), 5°C was the only temperature that promoted hardiness, whereas at a later stage (341°GS) all temperatures tested caused deacclimation. The models, using bihourly temperatures and accumulating °GS, predicted hardiness within an average deviation of 4.7°C.
Cold hardiness of 101 Rhododendron genotypes was assessed. In hardy rhododendrons, the difference in hardiness between floral primordia and other tissues or organs was greater than in the less hardy species. All rhododendrons tested were divided into 4 hardiness divisions: very hardy, semihardy, hardy, and tender. In all hardiness divisions, the vegetative buds, leaves, and stem cortex were hardiest, whereas the floral primorida were least hardy, with a lowest survival temperature (LST) of −35°C or higher. The majority of the very hardy division of rhododendrons, such as R. brachycarpum D., R. catawbiense Michx., and R. maximum L., belong to the ponticum series. In general, rhododendrons from cold regions were hardier than those from warm regions. The hardiness of the floral primordia of rhododendrons growing near the timberline (3900 m alt.) of East Himalaya and the high altitudes of the northwestern Yunnan area were similar (−20° to −23°). These were less hardy than those found in the subalpine forests of Japan and eastern North America, probably due to moderate temperature and high humidity during the winter in eastern Himalaya and northwestern Yunnan.
In preparation for winter, woody perennial plants of the temperate zone have evolved mechanisms to first enter a state of dormancy in late summer or early fall and then, as fall progresses, to develop coldhardiness. The development of cold
In an assessment of natural hazards on world crop production, an average of more than $ 100 million per year is lost in the continental United States due to freeze damage to 22 major crops. The greatest loss is in citrus production and amounts to about one-third of the national average. A severe freeze occurred in 1962 which cost Florida more than one-third of its total production, in excess of 10 million boxes (41 kg.) of fruit and onefourth of its 52 million trees. Similar losses occurred again in 1977 and, most recently, in 1981 and 1982. These losses impact negatively not only on the present and future economic stability of an industry, but also in meeting the dietary needs (supply and nutrition) of an expanding world population, which is facing ever-increasing uncertainties in adequate nutrition and subsistence levels.
Flower buds of 20 Prunus species representing 4 subgenera were collected during winter and spring of 1989-90. Buds were preconditioned at +3° or 7°C to test their minimum hardiness level (MHL) or the rate of hardiness increase. DTA revealed that most of the prunus species have flower primordia that supercool. The subgenus Padus have racemose inflorescences and do not deep supercool during dormancy. P. besseyi, P. nigra and P. americana had small exotherms between -22° and -27°C while P. davidiana and P. subhirtella had larger exotherms at higher temperatures. Exposure of flower buds to -7°C shifted LTES to lower temperatures and/or reduced the size of LTE, which became undetectable for many species including P. nigra and P. americana. P. davidiana and P. subhirtella increased hardiness by 6°/day at -7° while dormant. Deacclimation coincided with an increase in LTE50 and the development of xylem vessel elements in the bud axis, calyx and filaments as indicated by dye movenent. P. davidiana was the least hardy species and required only 700 chill units to satisfy the chilling requirement, while P. nigra and P. americana had LTE average of -26°C at MHL and required over 1000 chill unit accumulation.
Although citrus is grown in the southern regions of the United States, it is exposed to periodic freezes. Freezes are a serious production problem, as evidenced by the enormous loss of fruit and trees in recent years in the major citrus-growing areas (8). In 1962 in Florida, approximately 50 million boxes of fruit were lost, and substantial wood damage and loss of bearing surface occurred in approximately two-thirds of the citrus-producing areas (12). The monetary loss was estimated to exceed 500 million dollars. In Texas in 1951, 85,000 acres of trees were killed, and in 1962, 40,000 acres. Surviving trees lost their entire bearing surface. Many trees were killed back to 2-inch wood (27). Severe freezes also occur periodically in California and Arizona. In 1963, for instance, temperatures as low as 20°F occurred in some areas in California for eight consecutive nights. Injury was primarily to the fruit and very young trees (12). Studies of long-term weather records have indicated that freezes of 22°F minimums and lower occur in all major citrus-growing areas in the United States every 8 to 13 years on the average (5). Thus, it is expected that citrus-growing areas will be exposed to damaging freezes periodically.
On the basis of investigating pollination habits of 45 cultivars of Mei Hua, interspecific hybridization between Mei Hua and apricot (Prunus armeniaca). David's peach (P. davidiana) as well as siberian apricot (P. sibirica) were made from 1982 to 1991. With total number of pollination flower 17,050, 168 hybrid seed and 75 hybrid seedlings were obtained. Embryo culture in vitro was used for undeveloped young hybrid embryos. Test of freezing resistance both in artificial freezing and in overwintering for the hybrid seedlings showed that there were 5 hybrids with double and nice flower to be very hardy to low temperature. They were able to tolerate as low as -35C for 30 days in open ground, and now they were planted in northwest China's Gansu province and northeast China's Liaonin province without cold injury.
Stem critical temperatures for September showed that `Hughes' was later in acclimating than `Jackson'. Maximum hardiness for all cultivars occurred in January and deacclimation in February. Bud critical temperatures for September and October also showed that `Hughes' acclimated later than the other cultivars. Maximum hardiness for buds occurred in January and deacclimation in March. In December, the LT50 for the tetrazolium test, the electrolyte leakage test, and the tissue browning test were –18, –20, and –20C, respectively, as shown by differential thermal analysis of `Desirable'.