Winter-Hardiness

the ability of plants to withstand low above-zero temperatures (1°–10°C) for a long time. Winter-hardiness is to be distinguished from frost resistance, by which is usually meant resistance to sub-zero temperatures.

Winter-hardiness is unique to plants occurring in the temperate zone. Tropical and nonwintering subtropical plants are damaged when exposed to a temperature a little above O°C; the plants gradually die, sometimes after only a few minutes of exposure. Examples of winter-hardy plants are barley, oats, vetch, and flax. The degree of winter-hardiness varies from plant to plant and from organ to organ in the same plant. For example, the stems are the least winter-hardy organs in corn and buckwheat, and the roots in peanut. Leaf damage results in the loss of turgor and change in color owing to the destruction of chlorophyll. However, external signs of injury are not manifested immediately. “Invisible” changes occur much earlier and are not discovered until the chilled plants are moved to a warmer place and apparently healthy ones begin to die some time later.

The main cause of death of thermophile plants exposed to low temperatures is probably a disturbance of metabolism whereby the process of decomposition begins to prevail over that of synthesis. It is also possible that toxic compounds accumulate and the structure of the protoplasm is destroyed. It seems that various thermophiles die of different and still not completely understood causes. Winter-hardiness is determined by a plant’s capacity to preserve the normal structure of its protoplasm and adjust its metabolism appropriately during the period of chilling and subsequent warming.

Winter-hardiness largely depends on external conditions that can be changed to increase resistance to cold. It can be enhanced, for example, by adding potassium fertilizers and by maintaining a low temperature, high humidity, and conditions of good illumination. The most promising method is temporarily exposing plants to the lowest temperature at which they do not sustain injury. In doing so, it is advisable to control the pathogenic soil microflora, which at low temperatures damages the roots of thermophile plants.

Although hardening vegetable seedlings increases winter-hardiness, it also slows subsequent growth. It is best, therefore, to harden germinating seeds. The temperature is matched to the degree of plant winter-hardiness (ranging from 0° to –5°C) and continued for short periods of time (12 hours each) in order not to damage germinating seeds. The rest of the day the seeds are kept at 15° to 20°C. The alternation of cold and heat is continued for a month or somewhat longer. This method has made it possible to grow tomatoes, watermelons, and other thermophile plants farther north than ever before.

Presowing seed dressing with saline solutions is another means of increasing winter-hardiness. Another method—grafting—was proved successful in the cultivation of watermelons and muskmelons in Kirov and Moscow oblasts. Plants in the germinating phase are grafted onto squash plants on which are left not only the root system but also some leaves. Breeding for winter-hardiness is a promising approach. Several generations of seeds are hardened by exposure to cold just after the onset of germination. Hardier tomato varieties have been bred in this way. New winter-hardy varieties of tomato and buckwheat have been obtained by sowing seeds in the ground just before winter for several generations.