In 1991, the Brazilian national and state research institutions in coordination with CIAT, had initiated a 5-year project for breeding cassava in drier tropics and subtropics with funds from the International Fund For Agriculture Development . Cassava germplasm from northeastern Brazil and the north coast of Colombia was initially screened at four semiarid sites in northeastern Brazil, characterized by extended drought for several months, hot weather, and sandy infertile soils with presence of pests and diseases . Some accessions were selected for their broad adap- tation across sites to contribute in a recombination and selection program . Yields of selected clones at 12 months and 18 months after planting demonstrate the high potential of cassava in these drier areas.
Also, some progenies, via hybridization, have been developed and sent to ITTA, Nigeria, for adaptation and selection under drier areas in sub-Saharan Africa . Farmers had participated in evaluating and selecting adapted materials that resulted in rapid acceptance and, consequently, in the release of several improved clones . The research reviewed here on cassava productivity, ecophysiology, breeding, and responses to environmental stresses was conducted in collaboration with a multidisciplinary team at CIAT. Under favorable environments in lowland and mid-altitude tropical zones with near-optimal climatic and edaphic conditions for the crop to realize its inherent potential, cassava is highly productive in terms of root yield and total biological biomass, conferring a competitive advantage over other tropical energy crops. Under stressful environments with extended drought of several months and low-fertility soils, where major cereal and legume crops might fail, cassava produces reasonably well.
This inherent capacity to tolerate complex stresses is supported by several morphological, physiological and biochemical mechanisms and traits, such as long leaf life, tight stomatal control over gas exchange, high photosynthetic potential and extensive fine root systems. Core germplasm was characterized and several clones tolerant to water stress and low-fertility soils were identified to breed for drier areas in Africa and Latin America. Selection for nutrient use efficient shortto-medium cassava was found to be advantageous for soil fertility conservation while retaining yield potential. Modeling predicts the suitability of cassava in globally warming climate versus other food crops, confirming its high level of tolerance . Cassava has high optimum temperature for photosynthesis and growth, and responds positively to elevated CO2 that point to its potential as food, feed and energy crop in tropical and subtropical zones adversely affected by climate changes. Because of its high costs using sophisticated methodology, currentclimate research is still confined within developed temperate zones .
Yet, there is an urgent need to conduct climate research in representative tropical ecosystems where GCMss cenarios predict the worst consequences for agricultural productivity and food security . The use of the Free Air CO2 Enrichment technology in combination with rain shelters which facilitates evaluating interactions of CO2 with soil water status , may further enhance developing improved cassava cultivars adapted to increasing atmospheric CO2. Moreover, temperature influences may be studied under field conditions using the “Temperature Free-Air Controlled Enhancement ” technique, as was recently described and used at U.S. Arid-Land Agricultural Research Center, USDA, Maricopa, Arizona . Developed countries, who are the main polluters of the atmosphere, via excessive carbon emissions, must shoulder the costs of climate research in tropical and subtropical regions. Oil-rich Arabian/Persian Gulf States should take the lead in supporting developing countries to cope with-and-adapt-to consequences of warming climate. Molecular biology technology is also needed with focus on applications into crop improvement.
For example, this technology is useful for genetically transferring simple qualitative traits controlled by one or two genes, as already had been demonstrated by the successful production and use of insect-resistant transgenic commercial crop cultivars containing the soil bacterium, Bacillus thuringiensis genes that produce the toxic Cry proteins. In contrast, quantitative multigenic traits such as tolerance/resistance to compound abiotic-stresses are unlikely to be easily amenable to genetic engineering via inserting few exotic genes. These traits when expressed at the whole organism level are mostly attributed to a range of morphological, anatomical, physiological and biochemical characteristics and mechanisms. So far, modest advances at the experimental levels were recently reported for the use of genetic molecular markers in cassava selection and breeding efforts for developing drought-tolerant cultivars .