Cold hardiness, one of the key components of the winter hardiness complex, is a significant problem confronting agronomists concerned with crop production in cold northern regions of the world. Ever since the earliest reports of the agricultural implications of low temperatures were recorded in 1127 (see Vasil'yev, 1961), considerable attention has been devoted to the problem. However, as is characteristic of most biological phenomena, that which initially appears to be a seemingly simple problem becomes an increasingly complex situation composed of many paradoxical facts. For example, although low temperature is responsible for the potentially lethal stresses that are imposed on a plant, it is also the primary environmental cue responsible for eliciting the plant's potential to survive freezing temperatures. Although an understanding of freezing injury and cold acclimation has steadily evolved, the final answers have not yet been realized. A significant amount of information exists in regards to the physicochemical events associated with the freezing process, but the manner in which injury is effected is not fully understood. The importance of several environmental cues in the cold acclimation process is well established; however, the manner in which these cues are translated into increased resistance remains to be resolved. Although numerous biochemical changes occur during cold acclimation, the significance of most can only be speculated. Insufficient information on what constitutes freezing injury at the molecular level precludes the final integration of the many, already known facts. In addition, the lack of an appropriate conceptualization of the freezing process and cold acclimation can hinder our usage and interpretation of the known facts. Too often, individual aspects of both processes have been considered as mutually exclusive. Rather than searching for any one specific event to explain freezing injury, we might better view the freezing process as a sequential series of potentially lethal stress barriers. In turn, the cold acclimation process can be envisioned as a sequential series of events that enable the plant to avoid, mitigate, or tolerate the stress barriers as they arise. In such a conceptualization, any one single freezing stress would only become the limiting factor at a particular moment in time, depending on the immediate conditions and the successful swnounting of prior stress barriers. Similarly, whether any one particular resistance mechanism would become the primary factor depends on the immediate conditions. Thus, differences in hardiness between species may be due to distinctly different stress barriers that arise during freezing, whereas differences in hardiness between cultivars within a species may be due to differences in the extent of resistance to a given stress barrier. The improvement of agronomic crops with respect to cold hardiness will require considerable input and coordination of numerous disciplines and individuals. And, although an extreme range of diversity in the ability to survive freezing temperatures exists in the plant kingdom-between 0° and - 196°—only relatively small increases (5°C) in the hardiness of a particular agronomic crop need be achieved in order to have a significant impact on world food production. © 1978, Academic Press Inc.
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