Technical Aspects of Drought Tolerance
||Visser, B. (1994), "Technical Aspects of Drought Tolerance."
Biotechnology and Development Monitor, No. 18, p. 5.
Migration of farmers, for example due to human population growth, increases
the pressure on often arid or semiarid lands. In particular these
migrant farmers attempt to maintain their familiar farming systems, which
are often illsuited to the possibilities of these marginal lands.
Many have stressed that changes in the prevalent farming systems and
their adaptation to the specific environments is a highly important strategy
to improve yields and food security. For example, the replacement of maize
by sorghum and millets, as well as the acceptance of cassava as a crop
for human consumption, have been propagated in Eastern and Southern Africa.
As a complementary activity, the breeding of more drought tolerant varieties
of preferred staple crops, such as maize, wheat and rice, should be undertaken.
Drought tolerance is generally defined as the property of a given cultivar
to show a relatively small yield reduction upon exposure to drought. This
implies that breeding for stress adaptation is at the expense of yield
potential. Drought tolerance has appeared to be a difficult research aim
in breeding programmes due to several technical limitations:
Adaptive plant mechanisms
Definition of drought. Drought stress is highly heterogeneous in
time (over the seasons and years) and space (between and within sites),
and is extremely unpredictable. This makes it very difficult to identify
a representative drought stress condition.
Environmental influences. Since the phenotype is the product of
genotype and environment, assessment of the desired genotype is highly
dependent on the proper environmental conditions. The unpredictable and
variable forms in which drought stress will manifests itself, makes selection
of promising individual plants and breeding for drought tolerance extremely
Multiple genes involved. Drought tolerance has been shown to be
a highly complex trait, influenced by many different genes. In fact, drought
tolerance should not be regarded as a unique heritable trait, but as a
complex of often fully unrelated plant properties.
Interrelation with other stresses. Drought can hardly be separated
from other important abiotic stresses such as temperature and salinity.
Due to these interrelations, no single mechanism exists by which multiple
stresses are alleviated.
Different mechanisms may render a plant drought tolerant:
The use of biotechnology
- The ability of
a plant to escape periods of drought, in particular during the most
sensitive periods of its development. One breeding strategy is to shorten
the life cycle of a crop to enable it to mature safely during a rainfall
period. For example, in the Sahel very short season cowpeas now avoid drought
by maturing before any substantial stress develops, in less than 65 days.
The ability of a plant to endure or withstand a dry period by maintaining
a favourable internal water balance under drought condition. Osmotic adjustment,
in which the plant increases the concentration of organic molecules in
the cell water solution to 'bind' water
is one example. A thicker layer of waxy material at the plant surface and
a more extensive and deeper rooting are others.
The ability of a plant to recover from a dry period by producing
new leaves from buds that were able to survive the dry spell, is commonly
considered less interesting from the breeder's point of view. For example,
the alternately branched forms of groundnuts bear large numbers of terminal
vegetative buds, which may help them to produce new leaves more rapidly
after damaging dry periods.
The common approach in breeding for drought tolerance is to select
for drought tolerance components. Since several of these components are
difficult to measure, indirect selection might be applied. It is here that
novel biotechnological tools can be of use. Breeders can select for socalled
molecular markers which have exhibited a strong genetic linkage with the
component that is to be selected. This linkage may be purely physical:
The molecular marker is a stretch of DNA which is located close to a gene
influencing the drought tolerance component.
The advantage of indirect selection for associated molecular markers
instead of for the genetic trait itself is that the presence of the marker
in the offspring of a crossing can be demonstrated, independently from
interfering environmental influences, at the plantlet level in the laboratory.
A bottleneck that remains in this approach is that it must be possible
to measure the component with reasonable accuracy to first establish the
linkage with specific molecular markers. The more complex the trait, the
more difficult it will be to identify a limited number of major molecular
markers. Restriction fragment length polymorphic DNAs (RFLPs) as
well as random amplified polymorphic DNAs (RAPDs) can be used as
molecular markers. Both tools form the basis of different technical approaches
to measure the desired linkage.
Markerassisted breeding is a highinput longterm activity
which heavily depends on the availability of a large number of molecular
markers. The availability of a genomic map in which the genetic traits
and the molecular markers of a species have been physically arranged is
Current research at international institutes
For several crops markerassisted breeding for drought tolerance
is now expanding. The efforts of the international research institutes
funded by the Consultative Group on International Agricultural Research
(CGIAR) are explicitly directed at improved genetic material for agriculture
in developing countries and rely in their breeding programmes on germplasm
from these countries. The status and quality of the breeding programmes
will, to a large extent, determine whether and over which time period the
use of molecular markers will advance the selection of more droughttolerant
In general, worldwide research on drought tolerance breeding has focused
on cereals. In developing countries, the crops that receive attention for
drought tolerance include maize, upland rice, wheat, sorghum, cowpea, pigeonpea
and Phaseolus bean.
For maize, rice and soya bean, and to some extent for wheat and Phaseolus,
genomic maps have been developed and molecular markers are available. Whereas
breeding research in maize and soya bean has been largely an affair of
private US companies, the International Rice Research Institute
(IRRI, Philippines), with the support of the Rockefeller Foundation, has
played a central role in rice research. The International Maize and
Wheat Improvement Center (CIMMYT, Mexico) has developed markerassisted
breeding programmes in maize, on drought tolerance (anthesissilk interval)
and on insect resistance. In wheat, CIMMYT carries out research on disease
resistance and drought tolerance.
At the Centre for Tropical Agriculture (CIAT, Colombia) transfer
of drought tolerance from tepary to Phaseolus bean (genetic exchange
between different species!) assisted by molecular markers is part of the
bean improvement programme. The International Crops Research Institute
for the SemiArid Tropics (ICRISAT, India), in collaboration with
several international partners, has developed a genomic map for sorghum
using maize markers, and mapping for drought tolerance in sorghum and pearl
millet has been identified as a priority. Similar research in chickpea,
pigeon pea and groundnut is in a preliminary stage. In addition, considerable
public research efforts into conventional and/or markerassisted breeding
for droughttolerance have been undertaken in droughtprone countries,
such as Australia, USA, Israel and India.
Bert Visser (DGIS, Ministry of Foreign Affairs, The Netherlands)
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