Principles of Plant Genetics and Breeding

ultimately its productivity and economic value. The
common stresses that plants may be exposed to in
agroecological systems include the following:
1 Drought. This is the environmental condition caused
by lack of rainfall.
2 Heat. Heat stress occurs when temperatures are high
enough to cause irreversible damage to plant function.
3 Cold. Cold stress manifests when plants are exposed
to low temperatures that cause physiological disruptions
that may be irreversible.
4 Salinity. Stress from salinity occurs when the dissolved
salts accumulate in the soil solution to an extent that
plant growth is inhibited.
5 Mineral toxicity. Mineral toxicity occurs when an
element in the soil solution is present at a concentration
such that plants are physiologically impaired.
6 Oxidation. Oxygen-free radicals (or activated oxygen)
are known to cause degenerative conditions in
plant cells.
7 Water-logging. Excessive soil moisture as a result of
prolonged rainfall can cause anoxic soil conditions,
leading to roots suffering from lack of oxygen.
8 Mineral deficiency. Inadequate amounts of essential
soil minerals for plant growth cause crop injury.
Over the years, plant breeders have devoted attention
and resources to addressing these environmental stresses
to varying degrees and with varying success. The most
widely studied stresses will be discussed in more detail in
the next sections.
Tolerance to stress or resistance
to stress?
The terms tolerance and resistance are used in the
literature to describe the mechanisms by which a plant
responds to stress. Often, they are used as though they
were interchangeable. According to J. Lewitt, from a
physiological standpoint, a plant’s response to stress
may be characterized as “avoidance” (i.e., the environmental
factor is excluded from the plant tissue) or
“tolerance” (i.e., the factor penetrates the tissue but the
tissue survives). The term resistance, from a physiological
standpoint, is mechanism-neutral (implying neither
tolerance nor exclusion). When the term is applied to
bacteria, the development of resistance to an antibiotic
has evolutionary stages. Full antibiotic resistance is not
necessarily conferred by an immediate change in the
bacterial genome. It is preceded by tolerance. Because
bacteria can survive in the presence of an antibiotic, they
have the opportunity to develop resistance.
BREEDING FOR RESISTANCE TO ABIOTIC STRESSES 387
Researchers who use resistance to describe plant
response to a stress (cause) appear to view resistance as a
generic term for describing a number of mechanisms
of which tolerance is one. In breeding for response to
a stress, the ultimate goal of the breeder is to transfer
genes to the cultivar that would enable it to perform to a
desirable degree in spite of the stress. In this chapter, the
term that is most widely associated with a particular
stress in the literature is used.
Screening for stress resistance
Because of the complexity of environmental stress as
previously discussed, simple, practical, and effective tests
that can readily be used by breeders as selection aids are
not widely available. A. Acevedo and E. Fereres (1993)
summarized the criteria for the development and use of
screening tests as follows:
1 Genetic variation should occur in the germplasm
pool for the trait under consideration.
2 The heritability for the trait should be greater than
the heritability for yield per se.
3 The trait should be correlated with a yield-based
stress-resistance index.
4 It is desirable for the trait to be causally related to yield.
5 The screening test should be easy, rapid, and economical
to apply.
Some of the traits associated with resistance to economic
stress for which genetic variation has been found
in wheat include osmotic adjustment, proline accumulation,
leaf area per plant, epicuticular wax content, organ
pubescence, tolerance in translocation, root growth,
and many more. However, few of these traits can be
readily applied as selection aids by plant breeders.
Drought stress
Water is the most limiting factor in crop production.
In tropical regions of the world, moisture extremes are
prevalent. There is either too much of it when the rain
falls, or there is little or lack of rainfall. Drought is
responsible for severe food shortages and famine in
developing countries.
What is drought stress?
Drought occurs both above ground (atmospheric
drought) and below ground (soil drought). The