|Keywords:||Abiotic stress; Latin America/Carribean; Small-scale farming.|
|Correct citation:||Jaffé, W. and Rojas, M. (1994), "Abiotic Stress and Biotechnology in Latin America." Biotechnology and Development Monitor, No. 18, p. 6-7.|
Production increases in Latin America could more easily be realized by raising the productivity on existing agricultural lands, than by advancing the agricultural frontier. Overcoming abiotic stress would play a minor role in this general strategy. However, for the social and economic development of large groups of small farmers, who are usually cultivating marginal areas, attention on abiotic stress might be more important.
Abiotic stress constitutes, as elsewhere, a major constraint for agricultural production in Latin America and the Caribbean (LAC). Most of the good soils in favourable locations are already occupied, creating pressures to expand agriculture to more vulnerable, lower quality soils in generally fragile environments. Some of these soils can be exploited if various constraints, among them principally abiotic stress, could be overcome. Those fragile environments on which these constraints can not be overcome should preferably be left unused for conservation or other purposes.
To relieve the pressure on fragile environments and even to permit the setting aside of large areas for conservation, production increase should be realized on existing agricultural lands. In many cases this would require the development of alternative production technologies, in order to avoid the highinput, highmechanization model of many highproductive agricultural systems. This clearly is the current first priority for agricultural technology development in LAC and overcoming abiotic stress has perhaps a minor role to play in this general strategy.
In most traditional agricultural areas in LAC abiotic stress has never been the most important factor limiting yields and productivity, with some exceptions in specific production situations, like irrigation and highaltitude zones. Abiotic stress is principally a constraint for the incorporation of non, or only lightly, utilized areas into agricultural production. Examples of these newly exploited areas are the tropical savannas.
It is important to notice the social dimension of this issue. Current agricultural production in hostile and difficult environments is principally done by poor peasants. The lack of suitable technologies for these production situations is one of the most important limitations for these peasants to attain economically significant production. Overcoming abiotic stress is therefore one element in the economic and social development of large groups of people.
Geographic dispersion of abiotic stress
Determined by geographic, climatic and ecological factors, abiotic stresses vary in their relative importance in different subregions and countries. The high Andes and the Southern Cone countries face freezing temperatures. Nearly 70 per cent of land devoted to potato production in the Andean countries is under cold stress. When temperature descends below 0o C., potato is damaged considerably. In some cases, temperature can drop below minus 10o C., resulting in the total loss of the crop.
Drought is a problem in the northeast of Brazil, the Andean foot hills of Argentina and Bolivia, the Pacific coast of Chile and Peru, northern Mexico and smaller regions in Central America. Sixty per cent of Latin America's bean production regions are believed to suffer moderate to severe drought.
The irrigated areas in the droughtvulnerable regions present various degrees of salinization. Worldwide, salinization is causing as much land to be abandoned as is currently being brought under irrigation. Irrigated land constitutes a sizeable proportion in LAC, albeit lower than in other developing and developed countries. For example, this proportion is 35 per cent in Chile and Peru and 30 per cent in Mexico.
Perhaps the most important abiotic stressrelated problem, in terms of the size of affected area and its agricultural and conservation potential, is the soil quality in tropical, humid regions. The Amazon basin is the largest example, but other important zones are the coastal regions of the Andean countries and parts of Central America. Many of the soils in humid regions are very acid (<pH 5) and crop growth is hindered by high content of aluminum and/or manganese. Crop production is drastically reduced when aluminum saturation of the active cation exchange sites is greater than 60 per cent and tends to be optimal when aluminum saturation is zero.
Acid soils comprise 40 per cent of the world's arable lands and are believed to cover more than 800 million hectares of the forest and tropical savanna ecosystems of tropical America. Nearly 75 per cent of the Amazon Basin contains acid and infertile soils classified as oxisols and ultisols. About 383 million hectares, i.e. 79 per cent of the Amazon area, suffers from aluminum toxicity.
Intensified agriculture and ranching in the less fragile savannas surrounding the Amazonian rain forest would be perhaps the best strategy to protect it from encroachment and cultivation. The savannas would act as a buffer zone, helping to reduce pressure for further deforestation of the Amazon. Savanna soils, however, have three characteristics that restrict farming: High acidity, low nutrient content and a variable erosion potential, depending mostly on topography. Annual cropping systems, particularly in the Cerrados of Brazil and the Llanos of Venezuela, give good yields but they eventually degrade the soil. As a result of this degradation, the medium to longterm productivity of the savannas' annual cropping systems declines, even with a high level of chemical inputs.
Tropical savannas are found in four of the six Amazon countries, totalling 140 million hectares. Beside the Cerrados of Brazil and the Llanos of Venezuela, they include the Llanos Orientales of Colombia. Brazilian conservationists hold the opinion that erosionprone savanna areas, those that are hilly or have thin soil, should not be farmed. They agree, however, that most other savannas can be farmed. Since 1940, Brazil's Cerrados, the largest of the savannas, have increasingly attracted more ranchers.
Stategies to overcome abiotic stresses
Tolerance to cold environments for potato production in the Andes, acid soils in tropical savannas, occurrence of drought and floods have traditionally been targets for plant breeders in international and national research centres in the region. Conventional plant breeding techniques have been used to develop new varieties able to produce greater yields in spite of such abiotic stresses. Among the examples of research involving many regional centres is the United Nations Development Programme (UNDP) financed project titled "Increasing Food Production in Warmer and Stressed Environments", which started in 1982. It aims at the development of wheat varieties tolerant to abiotic stress produced by heat, drought and acid soils with associated aluminum toxicity, which are some of the most important constraints for wheat cultivation in the tropics. The programme consists of several research projects at institutes in developing and developed countries, coordinated by the International Maize and Wheat Improvement Centre (CIMMYT), Mexico. Some success has been achieved in developing varieties for acid soils with aluminum toxicity and a wider base of rust resistance. However, the expression of genetic resistance to drought, heat and spot blotch has not been clearly demonstrated.
Several research programmes aim at developing the tropical savannas on a lowinput, sustainable basis. One such programme is being conducted by International Centre for Tropical Agriculture (CIAT), located in Cali, Colombia. Their work focuses on combining crop and pasture systems by alternating production of forage grasses with annual crops specially bred for adaptation to lownutrient, acidic savanna soil. CIAT developed acidtolerant rice grass and legume cultivars. Varieties of rice, cassava, soya beans, and sorghum have been selected for their ability to grow under savanna conditions with limited application of lime, used to decrease soil acidity, and fertilizer. CIAT's savanna agricultural system is characterized by a low level of needed inputs. Land preparation does not require deep ploughing or other intensive practices. It assumes a sixyear rotation, with the fields allowed to lie fallow the last year.
CIAT estimates that the system will obtain rice yields of about 1.8 metric tonnes per hectare, 44 percent better than current yields. Pastures used for beef production are projected to yield 200 killogrammes per hectare, a marked advance from current average yields of 12 killogrammes per hectare. Adoption of crop/pasture rotation practices could provide a sustainable agricultural production model for currently underutilized tropical savannas. The savanna area in South America suitable for this croppasture technology is estimated at 76 million hectares, or 54 per cent of LAC's total savanna area.
These examples illustrate the potential of conventional approaches to develop stress resistance. But creating stresstolerant plant varieties and animal races through conventional breeding is a long and difficult process, constrained by the lack of knowledge of its genetic basis. Since its birth in the seventies, modern biotechnology has been envisaged as giving new possibilities complementary to traditional research strategies.
The potential of biotechnology
In spite of the explosive development of biotechnology in the last 20 years, its use for obtaining new abiotic stresstolerant varieties has been very limited. The lack of basic scientific information of the molecular biology of crops and animals is certainly the greatest constraint to the use of biotechnology for stress tolerance. Abiotic stress tolerance is the result of many genes expressed simultaneously or in a certain order. Often this expression concerns plant architecture (roots, length of the plant, leaf configuration) and complex physiological responses controlled by groups of genes. In contrast to resistance to pests and insects, which can be obtained through modification of quite a few genes, these traits will be much more difficult to modify. Modification of a complex of genes is still beyond the possibilities of the current genetic engineering and transformation technologies. But most important, very little is known about the molecular basis of important physiological and biochemical responses and characteristics of plants and animals.
This limitation has not prevented some innovative biotechnology approaches to developing stress tolerance. One example is the introduction of an antifreezing protein of an antarctic fish into plants to achieve cold tolerance. Research groups in LAC are currently participating in this effort, and have used this project to develop technological capabilities in genetic engineering .
One of the biotechniques considered most promising for abiotic stress plant breeding is the use of restriction fragment length polymorphism (RFLP) markers. RFLP markers can be used for the identification and selection of singlegene traits associated with stress tolerance (such as osmotic adjustment, photoperiodic response, and water use efficiency). High expectations are put on the use of RFLP markers to increase the efficiency of breeding.
RFLP technology is being applied to plant breeding by some advanced plant breeding groups in LAC. CIAT is one of them, and some of its proposed new projects in the area of biotechnology will apply RFLP markers to overcome abiotic stress for its mandate crops (cassava, pastures and rice).
Even if it is still early to expect biotechnology to contribute to practical production problems in the area of stress tolerance, it is definitely making a great contribution to the understanding of the plant and animal physiology and biochemistry on the molecular and genetic level. Genetic engineering and other biotechniques are being widely used to study the mechanisms responsible for stress tolerance. This basic scientific research concentrates worldwide on drought, salinity and temperature responses of plants on the molecular level. In the last years, impressive advances have been achieved by the relatively few groups involved in this research. Many important genes have been identified and some of the physiological responses to these stresses are beginning to be understood. But little work is being done on soil toxicity (acid soils, aluminum toxicity) which is one of the most important problems of the region. The reason for this limited attention is that acid soils and aluminium toxicity are only found in developing countries. Drought, on the contrary, is also a problem in the temperate climates of developed countries where most of the research groups active in stress tolerance are located.
Lac's options in stress tolerance
Biotechnology is currently not being applied to resolve abiotic stress problems in LAC. But even if stress tolerance is currently not a priority in agricultural technology development in LAC in general, the importance of some of the abiotic constraints in specific areas and production systems justifies a certain level of scientific and technological effort, especially concerning themes that are not addressed elsewhere. The most important of these surely is the problem of acid and aluminum toxicity of the tropical savannas and other soils in the region, were local scientific groups could develop true competitive advantages visavis groups in countries without these types of soils.
The problem of stress tolerance in LAC has to be approached with a medium
to longterm perspective, with special emphasis on enhancing the research
capacity in the molecular biology of stress tolerance in the region. Universities
and basic research institutes, supported by national biotechnology programmes,
could play an important role in this. Conventional plant breeding efforts
have to be strengthened in parallel to the development of basic science
Walter Jaffé/Miguel Rojas
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