Hardening of Plants

the acquisition by plants of resistance to unfavorable conditions, such as frost, cold, drought, or salinization. The effects of hardening are caused by changes in metabolism. Plant resistance to frost develops in the fall when plants have ceased to grow owing to the short days and become dormant and also in winter when frosts are mild or moderate. That is why trees that can withstand frosts as low as −60°C (larch, spruce, pine) may die in summer at temperatures of −7° to −8°C. The first phase of hardening takes place at a temperature of about 0°C during exposure to light, when plants accumulate carbohydrates because of a decrease in the rate of respiration. The second phase takes place during mild or moderate frosts and is accompanied by the loss of water in the cells owing to the formation of ice. At this time the protoplast becomes isolated and lipid-protein layers form on its surface. The plasmodesmata are drawn into the cells, and the living cellular contents become insensitive to the pressure of ice in the interstices.

Hardening is also used to increase the hardiness of such crops as cucumbers, tomatoes, cotton, and corn. The Russian market gardener E. A. Grachev was the first to employ hardening (1875); he kept corn seeds at a temperature of 0°C (in snow) for two weeks before planting them and by this method was able to obtain mature ears of corn in St. Petersburg. It has also been proposed that seeds (tomato) be exposed alternately to low and high temperatures. Seeds are made resistant to drought by soaking and then drying them before planting. Hardening may also take place in growing plants during a drought, but with a resulting decrease in yield. Methods have also been developed for making seeds resistant to soil salinization—chloride, sulfate, or carbonate (soda) salinization—by keeping them in the respective salt solutions.