In spite of various adaptations to cold, plants may
be injured through exposure to cold temperatures in a variety of ways,
depending on the temperature range.
One type of injury, called chilling injury, occurs
at exposures to temperature of between 20 and 0 _C. Some injuries are
irreversible. Common chilling injuries
include interruption of normal germination,
flowering,and fruit development, which eventually adversely impact crop yield.
Stored products may also suffer chilling injury. A more severe low-temperature injury
is freezing injury, which occurs when temperatures drop below the freezing
point of water. Sometimes, ice crystals form in the protoplasm of cells,resulting
in cell death and possibly plant death.
Plants may be classified into
three groups according to the tolerance to low temperatures. Frost tender plants
are intolerant of ice in their tissues, and are hence sensitive to chilling
injury. The plant can be killed when temperatures fall just below 0 _C. Frost
resistant plants can tolerate some ice in their cells and can survive cold
temperatures of up to _40 _C. Cold-hardy plants are predominantly temperate
woody species. Most crops that originate in the tropics and subtropics are
sensitive to chilling temperatures. However,some temperate fruits are also
susceptible to chilling injury. The temperature at which chilling injury starts
varies among species and depends on where they originate. Temperate fruits
exhibit chilling injury starting at 0–4 _C, whereas the starting temperature is
8 _C for subtropical fruits and 12 _C for tropical fruits. Grains such as corn
and rice suffer chilling injury at temperatures below 10 _C. When chilling temperatures
occur at the seedling stage, susceptible
crops suffer stand loss. Also, crop maturity is
delayed while yield is reduced.
The capacity of a genotype to tolerate low
temperature has been extensively studied. It is agreed that low-temperature
tolerance is a complex trait that may be influenced by several mechanisms. Reports
indicate recessive, additive, partial dominance,and overdominance as the modes
of action that occur in nature for cold stress. The diversity in the results is
partly blamed on the way research is often conducted. Some workers use controlled-freeze
tests while others use field tests. Furthermore, various reports indicate a
role of cytoplasmic factors and nonadditive
gene effects, even though such effects are generally
believed to be minor. Genes that condition varying levels of low-temperature
tolerance occur within and among species. This genetic variability has been
exploited to a degree in cultivar development within production regions.
A large amount of low-temperature
tolerance research has been conducted in wheat. Lowtemperature tolerance in
cereals depends on a highly integrated system of structural, regulatory, and
developmental genes. Several vernalization genes have
been identified.The vrn1 is homeoallelic to the
locus Sh2 in barley and Sp1 in rye. These two genes have been linked to genetic
differences in low-temperature tolerance. Winter cereals also produce several
proteins in response to low-temperature stress, for example, the dehydrin
families of genes.
Mechanisms of resistance to low temperature
Like drought resistance, certain physiological or
morphological adaptations can make plants either avoid or tolerate stress due to
low temperatures. Plants are described as cold-hardy when they have the
capacity to withstand freezing temperatures. On the other hand, winter-hardy species
are able to avoid or tolerate a variety of weather-related effects associated
with winter.The mechanisms of low-temperature resistance may be grouped into
two.
Chilling resistance. The factors that confer resistance
to chilling are believed to operate at the cell membrane level where they
influence membrane fluidity. Chilling resistant seeds are known to imbibe
moisture slowly. The presence of phenols in the seed coat of legumes is implicated
in conferring chilling resistance.
Freezing resistance. Several mechanisms are used
by plants to resist freezing, including:
_ Escape. Like drought, cultural
practices may be adopted by producers to prevent the vulnerable stage of growth
coinciding with the presence of the stress factor.
_ Avoidance. One of the injuries
of low temperature results from the intracellular formation of ice following
nucleation of ice in the tissue. Water may remain super cooled without forming
ice crystals. Certain compounds that are capable of promoting ice nucleation are
active at low temperatures. Bacteria such
as Pseudomonas syringae are capable of producing ice
nucleating proteins. The first field test of a bioengineered organism was the
testing of “ice minus”, a microbe genetically engineered to be incapable of
producing the bacterial protein that causes ice nucleation. This was intended
to be an approach to help frost-sensitive plants survive frost.
_ Tolerance. Freezing tolerance
occurs when a plant is able to withstand both intracellular and extracellular ice
formation. A group of genes called the cold-response genes is believed to play
a significant role in freezing tolerance. These genes are activated when the
plant is exposed for a period to low but non-freezing temperatures conferring hardening
on the plant. It is the hardening that makes plants survive freezing
temperatures.
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