The ability of temperate zone plants to survive subzero temperatures. This characteristic is a predominant factor that determines the geographical distribution of native plant species and the northern limits of cultivation of many important agronomic and horticultural crops. Further, freezing injury is a major cause of crop loss resulting from early fall frosts, low midwinter temperatures, or late spring frosts. Problems of cold hardiness are of concern to farmers in diverse areas of agriculture. As a result, the development of varieties of cultivated plants with improved cold hardiness is of long-standing concern.
Within the plant kingdom there is a wide range of diversity in low-temperature tolerance—from low levels of hardiness in herbaceous species such as potatoes, to intermediate levels of hardiness for winter annuals such as wheat and rye, to extremely hardy deciduous trees and shrubs such as black locust and red osier dogwood that can withstand temperatures of liquid nitrogen. Within a given species, the range in hardiness can be substantial. Within a given plant there is a wide range in the cold hardiness of different tissues and organs. For example, roots are much less tolerant of subzero temperatures than shoots; flower buds are more sensitive than vegetative buds.
The cold hardiness of a given species is an inherent genetic trait that requires certain environmental cues for its expression. With the shorter days and cooler nights of autumn, temperate zone plants become dormant and increase their cold hardiness. This process is referred to as cold acclimation. In the spring, increasing daylength and warmer temperatures result in the resumed growth and development of the plant and a corresponding decrease in cold hardiness. Cold hardiness may be influenced by radiation, temperature, photoperiod, precipitation, and stage of development of the plant, with different optimum conditions for different species or cultivars and ecotypes within a species. The various environmental cues serve to synchronize plant development with the environment. This synchronization has taken centuries to evolve, and freezing injury in cultivated species can result from any factor that disrupts this synchrony.
Temperature is the key environmental parameter for increasing a plant's capacity to withstand freezing temperatures. Low, above-freezing temperatures are conducive to an increase in hardiness in the fall, and warm temperatures are responsible for the decrease in the spring. Generally, it is considered that most plants will acclimate as temperatures are gradually lowered below 50°F (10°C). However, during acclimation, the progressive decline in temperatures is extremely important. The development of cold hardiness may take 4 to 6 weeks.
Photoperiod is the second major factor influencing cold acclimation, but only in those species that are photoperiodically responsive in relation to growth cessation or induction of dormancy (a true physiological rest period). In other species, light is important only in providing sufficient photosynthetic reserves required for the cold acclimation process. In some cases (for example, germinating seeds), sufficient energy reserves are already present and acclimation can occur in the dark. See also Photoperiodism.
There are conflicting reports on the role of moisture in relation to cold hardiness. High soil moisture may reduce the degree of cold acclimation; however, severe winter injury of evergreens will occur if soil moisture levels are too low. Most often tissue moisture levels will influence the survival to a given freeze-thaw cycle rather than directly influencing the process of cold acclimation. Thus, whereas temperature and light effects on hardiness are probably mediated through the development of hardiness (cold acclimation), tissue moisture content directly affects the stresses that are incurred during a freeze-thaw cycle. In addition, various cultural practices can influence the cold hardiness of a given plant. For example, late fall applications of fertilizer or improper pruning practices may stimulate flushes of growth that do not have sufficient time to acclimate. Conversely, insufficient mineral nutrition can also impair the development of maximum cold hardiness.
The process of cold acclimation results in numerous biochemical changes within the plant. These include increases in growth inhibitors and decreases in growth promoters; changes in nucleic acid metabolism; alterations in cellular pigments such as carotenoids and anthocyanins; the accumulation of carbohydrates, amino acids, and water-soluble proteins; increases in fatty acid unsaturation; changes in lipid composition; and the proliferation of cellular membrane systems. Some of these are merely changes in response to slower growth rates and decreased photosynthate utilization; others are changes associated with growth at low, above-zero temperatures; and still others are associated with other developmental phenomena, such as vernalization or the induction of dormancy, that also occur during the period of cold acclimation.
Large increases in cellular solute concentrations are one of the most universal manifestations of cold acclimation. A doubling of the intracellular solute concentration, most notably sugars, is not uncommon. Such increases have several beneficial effects. First, they serve to depress the freezing point of the intracellular solution. More important, a doubling of the initial intracellular solute concentration will decrease the extent of cell dehydration at any subzero temperature by 50%. An increase in intracellular solutes will also decrease the concentration of toxic solutes at temperatures below 32°F (0°C), because less water will be removed. Following cold acclimation there are also substantial changes in the lipid composition of the plasma membrane. This includes an increase in free sterols with corresponding decreases in steryl glucosides and acylated steryl glucosides, a decrease in the glucocerebroside content, and an increase in the phospholipid content. The complexity of the plasma membrane lipid composition and the numerous changes that occur during cold acclimation preclude the possibility of any simple correlative analysis; however, studies have demonstrated that differential behavior of the plasma membrane observed in protoplasts isolated from nonacclimated and cold-acclimated leaves is a consequence of alterations in the lipid composition. See also Altitudinal vegetation zones; Plant physiology; Plant-water relations.
