FUTURE WORK - Two areas of research and development need to be addressed on the potential use of cassava as a mainstay crop tolerant to extreme water deficits and to marginal lands in the tropics and subtropics where severe drought prevails and shortages of food and feed are chronic, particularly in Sub-Saharan Africa: (1) Through the use of modern molecular biology tools, more basic research on cassava biochemical photosynthetic characterization should be done to search for genetic diversity within cultivated cassava and wild Manihot species in order to identify genetic sources with high photosynthetic rates and enhanced activities of key photosynthetic enzymes, focusing on PEPC and C3-C4 traits (El-Sharkawy and de Tafur, 2007; El-Sharkawy et al., 2008).
The use of molecular markers and marker assisted selection should facilitate and enhance the conventional breeding process for drought-tolerance in cassava (Setter and Fregene, 2007). Also, the molecular basis for tolerance to water stress and identification of possible controlling genes should be investigated using contrasting germplasm (e.g., Lokko et al., 2007). This line of research should be internationally supported within an interinstitutional/interdisciplinary collaborative network, including research institutes in developed countries. (2) National programs, perhaps supported with the Geographical Information Systems (GIS), should make the necessary mapping of additional new lands and suitable climatic conditions for potential expansion in cassava production where other staple food crops will probably fail to produce. This effort should be backed up by developing the necessary adapted cultivars endowed with early storage root filling characteristics (i.e., early bulking) and better leaf retention (Lenis et al., 2006) and duration coupled with resistance to pests and diseases. Agronomic/physiological research in the area of plant-soil-water relations and plant nutrition on the newly bred cultivars would provide the technical knowledge needed for cropping systems and natural resources managements. Crop modeling, based on sound field research, should play a role in this case (El-Sharkawy, 2005).
CONCLUSIONS
The research on cassava summarized here illustrates the effectiveness and utility of ecophysiological research in improving the genetic base and in developing genotypes more adaptable to drought in the marginal environments in the tropics. Linking and integrating physiological research with breeding efforts within a commodity-oriented multidisciplinary research team at CIAT, and in collaboration with national programs, were pivotal in delivering the needed information and improved technology. Plant traits related to productivity and to tolerance to water stress, such as high leaf photosynthetic capacity and longer leaf life and duration, extensive fine rooting systems and stomatal control of water losses, were identified and selected for parental materials used in breeding for improved and more adaptable genotypes. Furthermore, the research has revealed important information on the physiological mechanisms underlying cassava productivity and tolerance to prolonged drought that should help to develop better crop management in both favorable and stressful environments. More research is needed to elucidate further the biochemical and molecular characteristics of cassava photosynthesis in relation to productivity, particularly under stress. This would require essential changes in the current counterproductive policy of the international research system, which is based on short-term research projects that do not ensure needed solutions to farmers problems. This policy should be reversed and replaced by the previously adopted and more effective long-term core funding of interdisciplinary and integrated research approach (El-Sharkawy, 2006a,b). National programs should be responsible for conducting the applied and adaptive research aspects that are required for fulfillment of their national needs.
Across institutions and countries, collaborative research, as that illustrated by the project conducted by CIAT and EMBRAPA/CNPMF, Brazil, on breeding improved cassava cultivars for the seasonally dry and semiarid environments, should not only be encouraged but also emulated in other crops. In view of the adverse effects on agricultural productivity in tropical countries that might result from observed global climate changes, more research on drought tolerance is warranted. Cassava, with its inherent capacity to tolerate and produce reasonably well under prolonged drought, should be expected to provide more essential food and feed than any other crop in marginal environments.
Acknowledgement: I am grateful for the sincere collaboration I received during the course of this research from the many workers, colleagues, associates, visiting scientists, students, secretaries at CIAT, and the Colombian farmers who generously offered logistic support and hospitality at their private farms and homes. The financial grants from The International Fund for Agricultural Development (IFAD), Rome, helped in initiating the collaborative breeding project between CIAT and the federal research organizations in Brazil (EMBRAPA/CNPMF) in seasonally dry and semiarid areas of northeastern Brazil. Without this support, the achievements highlighted here would have never been realized. I am also thankful to my daughter, Farah El-Sharkawy Navarro, for her support in organizing the data in tables and figures and in searching the internet for needed information and references. The invaluable inputs provided by Dr. Mary Beth Kirkham, Kansas State University, Manhattan, Kansas, were also appreciated. The support from the editorial staff of BJPP in editing the original manuscript and in redrawing the many figures is also appreciated.
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Braz. J. Plant Physiol. vol.19 no.4 Londrina Oct./Dec. 2007
Mabrouk A. El-Sharkawy*Centro Internacional de Agricultura Tropical (CIAT), Cali, Colombia. Present address: A. A. 26360, Cali, Colombia
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