Stress caused by low temperatures is greatly increased in the case of frost due to freeze-desicca-tion. Secondary strains such as high light intensity play, of course, an additional important role at temperatures below 0°C. Frost also occurs in tropical climates, namely in tropical high mountains. As the temperature in these regions only falls below the freezing point for a matter of hours, freezing avoidance mechanisms are very important. In contrast to chilling, frost is characterised by seasonal hardening (acquisition of frost resistance or frost tolerance) and dehard-ening. These processes involve many physiologically important aspects. Particularly topical are the many so-called cold-related proteins (CORs) which have been discovered recently. These constitute an important starting point for introducing frost hardening into cold-sensitive crop plants, in addition to the genetic engineering of biomembrane frost hardening described in Chapter 1.1.6. It should be mentioned in this regard that 70% of the annual harvest losses in the USA was caused by cold and drought in the 1980s, while insects, weeds and pathogens together were responsible for only about 10%. Late frosts caused damage of more than US$ 1 billion during 1981-1985 in southern Florida (Yelenos-ky 1985).
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