Principles of Plant Genetics and Breeding

386 CHAPTER 21
technologies are not readily available or affordable
(which, incidentally, are the regions in most need of
food increases), the alternative is to breed cultivars that
are able to resist these environmental stresses enough to
produce acceptable crop yields.
Each species has natural limits of adaptability. There
are tropical plants and temperate plants. Breeders are
able, within limits, to adapt certain tropical plants to
temperate production, and vice versa. To achieve this,
plants are genetically modified to resist environmental
stresses in their new environment. Sometimes, in modern
crop production, breeders develop cultivars to resist
certain environmental stresses to make better use of the
production environment. For example, cold resistance
enables producers to plant early in the season while the
soil is still too cold for normal planting. This may be
done to extend the growing season for higher productivity
or for some other reasons.
Resistance to abiotic stress and
yield potential
Plant growth and development is the product of the
interaction between the genotype (genetic potential)
and the environment in which the plant grows. Any
stress in the environment will adversely impact growth
and development. Plants perform well in environments
to which they are well adapted. Yield potential was
previously defined as the highest yield attainable by a
genotype growing in an environment to which it is
adapted and in which there is no environmental stress
(i.e., optimum growing conditions). Except, perhaps,
under controlled environments, it is very difficult to
find a production environment (especially on a large
scale) in which some environmental stress of some sort
does not occur. Stress resistance is an inherent part of
all cultivar development programs. Prior to releasing a
cultivar, genotypes of high potential are evaluated at
different locations and over several years to determine
adaptedness.
High yield potential has been achieved in cereal crops
through enhanced harvest indexes, whereby dry matter
is redistributed into the grain (by reducing its distribution
to vegetative parts – roots, stems, leaves). However,
this pattern of dry matter distribution also reduces the
ability of these genotypes with high yield potential to
cope with environmental stresses. Studies on genotype
× environment (G × E) interactions have shown crossover
effects whereby, under conditions of severe stress,
genotypes of high yield potential perform poorly. Fur-
ther, as the abiotic constraints intensify (e.g., drought,
low temperature, high temperature), a report by the
Centro Internationale de Mejoramiento de Maiz y Trigo
(CIMMYT) indicates that it becomes more difficult to
improve genetic and agronomic yield of crops.
R. A. Fisher and R. Maurer partitioned stress effects
on yield into parameters that measure sensitivity to
stress (S) and the extent of the stress (D) and yield
potential (Y p ):
Y = Y p (1 − S × D)
where D = (1 − X¯ / X¯ p ), and X¯ and X¯ p are the mean
yield of all cultivars under stressed and optimal conditions,
respectively. This relationship may be manipulated
algebraically to obtain:
S = (1 − Y/Y p )/D
= (Y p − Y)(Y p × D)
where D = a constant for a particular trial. Hence, S is a
measure of the yield decrease due to the stress relative to
the potential yield, with a low value of S being desirable.
The problem with using S as a measure of adaptation
to stress is that there are cases where S has been positively
correlated with Y p (i.e., cultivars whose yield was
affected little by the stress also had very low yield potential).
In other words, cultivars with low S also may have
low stress resistance (Y ) and would not have been useful
to the producer in the first place. The correlation
between S and Y p indicates that it may not be possible
(or it would be challenging) to combine the desirable
traits associated with a low S and high yield potential.
Types of abiotic environmental stresses
J. Lewitt (1972) observed that plants respond to stress
by strain reactions, which take the form that may be
classified as either plastic or elastic, and manifest as G ×
E interactions. Plastic responses produce a permanent
change in the phenotype, whereas elastic responses
are flexible and permit the normal state of the plant
to return. Plants respond physiologically to stress by
changing this reaction norm. Plant growth and development
depend on biochemical processes (e.g., photosynthesis,
respiration) that in turn depend on factors in
the environment in order to proceed optimally. As previously
indicated, when conditions in the environment
are less than optimum, the plant experiences stress,
which adversely affects its growth and development, and