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TEMPERATURE IN THE PHYSIOLOGY OF CROP PLANTS

A more specific effect is the response to brief periods of freezing, or near-freezing, temperatures. The classical example, feared by fruit growers almost everywhere except in the tropics, is a freeze while the trees are in full bloom. This is much more drastic for deciduous fruit trees than for evergreen trees such as citrus. If the blossom-bearing wood is not damaged, such tropical or subtropical trees have a chance to replace fruit buds within the same bearing season, although yield and fruit quality may be impaired. As discussed later, this cannot happen with deciduous fruit trees.

A more subtle effect, to which green (English, garden) peas (Possum sativa) are particularly susceptible, is low-temperature stunting of young plants. When such peas and snap (wax) beans (Pharsalus vulgaris) are growing side by side, immature pea plants may be permanently stunted by a brief chilly period from which the beans usually recover. It is apparent to even the most casual observer that on a frosty night, cold air can drain into hollows, thereby sometimes limiting damage to such small “microclimate” areas. In addition, vegetation can be markedly different on the north and south sides of a steep valley because the exposures to sunlight are very different.

Foehn winds provide striking examples of rather larger microclimates utilized for the growing of specialized crops. A classic example is the chinook of the Rocky Mountains of Washington State and British Columbia. Strong winds off the Pacific Ocean are forced to rise on encountering the coastal range. As the air rises rapidly, moisture condenses, releasing great amounts of latent heat and forming a bank of clouds (the “foehn wall”) that drenches the western slopes.

This sequence of events provides a mild, moist area ideal for such crops as cane fruits, crucifers, and many ornamentals. By the time the air mass has crossed the coastal range, it is very dry, and on its leeward descent adiabatic compression warms it rapidly, providing a sudden spring. The resultant microclimate is (provided irrigation water is available) ideal for the growing of stone fruits. Apricots are particularly well served by this microclimate because they have a very short rest period, with consequent susceptibility to spring frosts, which are virtually unknown in inland chinook areas.

Conclusion

The chinook occurs on such a grandiose scale as to almost exceed definition as “microclimate.” But the eponymous foehn winds in the Austrian Alps, the ghibli in the Tripolitanian Mountains of Libya, and the zonda in the Argentine Andes produce the same effects on a much more local scale.

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