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 Technical Aspects of Drought Tolerance
By
Bert Visser
 
 
Keywords:  Abiotic stress.
Correct citation: 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
Different mechanisms may render a plant drought tolerant: The use of biotechnology
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 also important.

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 varieties.
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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