Temperature - Agroecology: The Ecology of Sustainable Food Systems - page 64

microclimate, other temperature-related factors, and the
particular temperature responses of specific crops. In Cali-
fornia, for example, farmers shift to cool-season varieties
of crops such as broccoli for winter planting, plant cover
crops during the wet and cool time of the year when many
vegetable crops would not do well, plant avocado trees
close to the coast in areas that are frost-free because of
the maritime influence, and plant lettuce during the winter
in the interior desert valleys of southern California. Other
farming regions offer similar examples.
Temperature can also be used as a tool to cause desired
changes in plants. For example, farmers in central coastal
California chill strawberry transplants in order to induce
vegetative growth and good crown development.

• hairs (pubescence) on the leaves that insulate

leaf tissue

• small leaves with less surface area exposed to

sunlight

• leaves with a lower surface-to-volume ratio for

gaining less heat

• vertical orientation of leaves to reduce heat gain

• more extensive roots, or a greater root-to-shoot

ratio, for absorbing more water to offset water
loss from the leaves or to maintain more water
intake relative to leaf area

• thick, corky, or fibrous bark that insulates the

cambium and phloem in the plant trunk

• lower moisture content of the protoplasm and

higher osmotic concentration of the living tissue

A DAPTATIONS TO T EMPERATURE E XTREMES

These characters can be incorporated into farming
systems where water availability is limited and temper-
atures are high, either through the use of crop plants
with these characters, or the breeding of varieties that
show them.

Natural ecosystems are made up of plants and animals
that have been “screened” by natural selection. Periodic
temperature extremes is one of the factors that have elimi-
nated those species that are not tolerant of local conditions.
Therefore, we can expect the temperature range tolerances
of the species of local natural systems to give us an indi-
cation of the temperature extremes we might expect when
we try to farm in an area. Recognizing these indicators,
as well as selecting for adaptations to extremes in our crop
species, can help in the development of farming systems
that lower the risk associated with the natural variability
in temperature extremes.

Cold

When temperatures drop below the minimum required for
growth, a plant can become dormant, even though meta-
bolic activity may slowly continue. Chlorosis may occur,
followed eventually by death of the tissue. Death at low
temperature is due to protein precipitation (which can
occur at temperatures above freezing), the drawing of
water out of protoplasm when intercellular water freezes,
and the formation of damaging ice crystals inside the
protoplasm itself.
Resistance to extremes of cold depend greatly on the
degree and duration of the low temperature, how quickly
the cold temperature comes about, and the complex of
environmental conditions that the plant may have under-
gone before the cold event. Some specific structural adap-
tations provide resistance as well, such as coverings of
wax or pubescence that allow leaves to endure extended
cold without freezing the interior tissue, or the presence
of smaller cells in the leaf that resist freezing.
Temporary cold hardiness can be induced in some
plants by short-term exposure to temperatures, a few
degrees above freezing or withholding water for a few
days. Such plants undergo hardening , giving them limited
resistance to extreme cold when it occurs. Greenhouse-
grown seedlings can be hardened to cold by exposing them
to cooler temperatures in a shade house and cutting back
on irrigation for a few days before transplanting to the
field.
Many plants are adapted to extreme cold through
mechanisms that allow them to avoid cold. Deciduous
perennial shrubs or trees that lose their leaves and go

Heat

The effects of high temperatures on crops are the result
of a complex interaction between evaporative water loss,
changes in internal water status, and changes in other
physiological processes. Heat stress causes a decline in
metabolic activity, which is thought to come from the
inactivation of enzymes and other proteins. Heat also
raises the rate of respiration, which can eventually over-
take the rate of photosynthesis, halting plant growth, and
ultimately killing plant tissue.
Plants, native to temperate areas, generally have lower
limits to temperature stress than plants of more tropical
areas. In all cases, though, leaf functions become impaired
at about 42°C, and lethal temperatures for active leaf tissue
are reached in the range of 50 to 60°C.
Common morphological adaptations of plants to
excess heat include the following:

•a h CO 2 compensation point for the photo-
synthesis/respiration ratio, often aided by
changes in leaf structure

• white or gray leaves that reflect light and thus

absorb less heat