Principles of Plant Genetics and Breeding

Breeding for drought resistance
Drought resistance is highly cropping region specific.
Drought resistance in crops is a major breeding objective
in dryland farming systems. In more advanced
agricultural production systems, the goal of combating
drought is to obtain economic plant production in spite
of the stress; plant survival is not the goal. However, in
less advanced agriculture economies, plant survival is
critical to producing some yield. The yield, albeit small,
may mean the difference between famine and livelihood.
Underlying principles
In order to formulate proper and effective breeding
objectives in a drought breeding program, the breeder
should understand the nature of the trait to be manipulated.
Earlier plant breeding efforts were directed at
developing a genotype that had water-saving capacity.
However, this concept has proven to be inadequate
in addressing modern crop development. Researchers
(J. B. Passioura and C. T. de Wit) have produced a
model to describe the relationship between crop yield
and water use:
Yield = T × WUE × HI
where T = total seasonal crop transpiration, WUE = crop
water use efficiency, and HI = crop harvest index. This
relationship clearly indicates that the focus in breeding
for drought resistance should be on water use (efficient
water use), rather than water saving. In order for a
crop to sustain yield, it should be able to use water
under stress. Plants cannot live without water. Drought
resistance is a finite trait. What is desirable in modern
crop production is a plant with an ability to use water
efficiently when this resource is limited by drought.
A successful drought breeding program depends on
the formulation of an appropriate and relevant ideotype
for a drought target environment. This task is very
challenging, requiring the breeder to put together a
conceptual description of detailed morphological, physiological,
and developmental attributes of the ideal
genotype.
Characterization of the drought environment
Generally, constraints to crop production manifest as a
combination of physiological stress factors (rather than
one), even though one may predominate. For example,
drought spells may be associated with low or high
BREEDING FOR RESISTANCE TO ABIOTIC STRESSES 389
temperature, depending on the region. In irrigated production,
drought is often associated with salinity stress.
Further, the soil may introduce additional factors that
limit crop production (e.g., unfavorable pH, toxic levels
of elements such as aluminum, and deficiencies of other
such micronutrients). To complicate the stress system,
the predominant factors may be unpredictable in occurrence,
intensity, and duration. Two drought conditions
may have significantly different composite properties.
For example a drought event may occur with either a
low vapor pressure deficit or high vapor pressure deficit.
Characterization of the drought environment is
needed for several reasons, including the following.
1 Drought resistance is highly location-specific. This is
because various factors (edaphic, biotic, agronomic)
that are often location-specific are involved.
2 The drought-resistant genotypes to be developed
need to be evaluated under a specific set of environmental
conditions. Genotypes that are adapted to a
given set of drought conditions may not be equally
adapted to a different set of drought conditions.
It is clear then that breeders will have to characterize
the target production region to know the most effective
combination of adaptation features in genotypes best
suited for successful production. Environmental characterization
is a challenging undertaking. Consequently,
breeders often limit the activity to only assembling qualitative
meteorological data and soil descriptions. These
data may be obtained at the regional level, or may even
be as specific as farm level, and collected over seasons
and years. Breeders engaged in breeding for resistance
to drought should work closely with experts including
agronomists, soil scientists, and meteorologists, or at
least be familiar with techniques for computing soil
moisture variations and rainfall probabilities. The modern
technology of GIS (geographic information system)
is helpful in detailed characterization of environments.
Plant traits affecting drought response
Researchers have identified numerous stress response
genes. However, these genes are not necessarily stress
adaptive in terms of drought resistance. Nonetheless,
both adaptive and non-adaptive genes are important
in plant performance under drought stress. A good
example of this fact is the role of genes that condition
early flowering. Whereas these genes are expressed
in any environment (i.e., not drought induced), early
flowering may play a role in plant response to, and