plants, given sufficient time under suitable conditions, can adapt themselves to freezing temperatures, and
some cannot. This dichotomy is discussed in Sec. II.B.
- Freezing of Plant Tissues
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 (Pisum sativum) are particularly sus-
ceptible, is low-temperature stunting of young plants. When such peas and snap (wax) beans (Phaseolus
vulgaris) are growing side by side, immature pea plants may be permanently stunted by a brief chilly pe-
riod from which the beans usually recover.
- Microclimates
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 com-
pression 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 mi-
croclimate because they have a very short rest period, with consequent susceptibility to spring frosts,
which are virtually unknown in inland chinook areas. 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.
The writer’s master’s thesis [11], dated 1940, includes a map of a microclimate area once known as
the “fruit bowl of Canada.” Thirty-five miles (56 km) long at its maximum and varying in width from 5
to 14 miles (8–22 km), the fruit-growing area of the Niagara peninsula once produced most of Canada’s
peaches, plums, cherries, pears, and small fruits and virtually all the wine grapes of eastern Canada. A
high cliff (the Niagara escarpment) shelters this area on the south side. On the north, Lake Ontario mod-
erates the temperature of the north winds in midwinter. In spring, the escarpment protects the orchards
from unseasonable warm south winds that might induce too early a bloom, with consequent risk of a blos-
som freeze. Now, more than 50 years later, it is sad to return to the once overflowing “fruit bowl”: this
precious miracle of microclimate has been largely paved over with factories, shopping centers, and hous-
ing developments that could just as well have been located a few miles to the south, above the escarp-
ment. Such squandering of invaluable microclimates is all too common everywhere.
What might be termed “mini-microclimates” occur within any local microclimate, as indicated by
the surprising range of temperatures recorded within a single lemon orchard [12]. When studying such
fine details as individual leaf temperatures, even heat conduction along thermocouple wires must be con-
sidered [13].
But microclimate effects can also manifest themselves in far more subtle ways, often involving ver-
tical as well as horizontal temperature differences. When air temperatures are favorable for growth, it is
easy to forget that soil temperatures can also be limiting. Soil temperatures, both above and below opti-
mum range, have been shown to limit uptake of soil water by citrus trees to the extent that visible wilting
occurs even when soil moisture is adequate [14]. When water uptake is limited, obviously the uptake of
water-soluble ions can also be affected. Iron deficiency chlorosis of citrus trees has been reported to be
exacerbated by soil temperatures below 12.8°C [15]. Such ion uptake limitation can also be critical in nu-
TEMPERATURE IN THE PHYSIOLOGY OF CROP PLANTS 15