Principles of Plant Genetics and Breeding

interruption in 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 their tolerance to low temperatures. Frost-tender
plants are intolerant of ice in their tissues and are hence
sensitive to chilling injury. The plant (e.g., beans, corn,
tomato) 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. They can survive temperatures of up
to −196°C.
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. Grain
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.
Genetic basis of low-temperature stress tolerance
The capacity of a genotype to tolerate low temperatures
has been extensively studied. Whereas it is agreed that
low-temperature tolerance is a complex trait (quantitative),
researchers are not unanimous on the mode of
gene action governing the expression of the trait.
Reports indicate recessive, additive, partial dominance,
and overdominance as the modes of action that occur in
nature for cold stress. The inconsistency in the results is
partly blamed on the way research is often conducted.
Some workers use controlled freeze tests while others
use field tests. Further, various reports indicate the role
of cytoplasmic factors and non-additive 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.
BREEDING FOR RESISTANCE TO ABIOTIC STRESSES 397
A large amount of low-temperature tolerance research
has been conducted in wheat. Low-temperature
tolerance in cereals depends on a highly integrated system
of structural, regulatory, and developmental genes.
Several vernalization genes have been identified (e.g.,
vrn 1 , vrn 4 ). The vrn 1 is homeoallelic to the locus Sh 2 in
barley and Sp 1 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 (dh 5 , Wcs120).
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 (concept of cold hardiness)
when they have the capacity to withstand freezing
temperatures. On the other hand winter-hardy (winter
hardiness) species are able to avoid or tolerate a variety
of weather-related effects associated with winter (e.g.,
freezing, heaving, desiccation, frost resistance, etc.).
The mechanism 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 the following:
1 Escape. Like drought, cultural practices may be
adapted by producers to prevent the vulnerable stage
of growth coinciding with the presence of the stress
factor.
2 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
supercooled 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