Approaches to crop improvement generally fall into one or two broadly defined systems:
• Formal systems, in which modern science is brought to bear on crop improvement by institutions such as government plant-breeding stations, university departments, and private breeding companies, with the aim of producing cultivars for wide, often commercial, distribution to farmers.
• Informal systems, in which farmers and local communities, mainly in developing countries, breed and select cultivars primarily to meet their own needs and circumstances.
Both systems coexist in many regions and each depends, to a greater or lesser extent, on the other.
Formal systems of crop improvement normally aim to produce high-yielding cultivars that are broadly adapted across a wide range of agroecosystems. Special attention is given to breeding cultivars with specific resistances to pests and diseases and tolerances to abiotic stresses. Quality characteristics are generally determined by the preferences of large consumer groups, often in importing countries, or by the demands of processors. New cultivars must meet legal requirements for distinctness, uniformity, and stability to be officially registered. Plant breeding is an expensive process, and there is an ever-growing need to show financial returns on investment in crop improvement from the public as well as private sectors. Under these circumstances, it is perhaps not surprising that the majority of modern cultivars are widely grown or high-value crops and tend to have a relatively narrow genetic base.
In the formal sector, breeding for adaptation will continue to be concerned with improving adaptability in existing environments, extending the areas in which individual crops are grown, or seeking improved stability across a range of environments. New variation will be sought and traditional cultivars and wild relatives will continue to provide the necessary variation so long as their conservation is secured. Improved knowledge of the distribution of diversity and of the effect of specific environmental variables on that distribution will improve our capacity to locate desired characters. Improved understanding of the significance and nature of coadapted gene complexes will enable breeders to use adapted germplasm with much greater efficiency.
In formal systems, breeders are generally concerned with adaptation in one of three ways:
• To develop cultivars that are better adapted to the agroecosystems in which the target crop is currently produced;
• To develop cultivars that are adapted to new agroecosystems (geographic areas or farming systems); or
• To develop cultivars that are competitive over large areas in the hope that they will exhibit greater stability across seasons (by broadening adaptability).
The first of these is a more or less continuing part of most breeding programs and includes breeding objectives such as the incorporation of improved frost resistance in northern areas, better drought tolerance in arid lands, or disease and insect resistance. The second objective has resulted in dramatic increases of area for a number of crops such as sorghum in the U.S. (Maunder, 1992) or canola in Canada.3 The third has been a major objective of international breeding programs in the 1960s and 1970s and resulted in such cultivars as the IR36 rice with its 75% coverage of the Southeast Asian rice acreage. To achieve such objectives, use may be made of genes having a large effect on specific, identified adaptive features, such as pheno-logical characteristics; photoperiod response (e.g., day length in sorghum); tolerance of extremes of temperature, soil moisture, or soil chemical factors; or resistance to pests and diseases. However, a significant part is played by intensive directional selection for characters under largely additive genetic control, such as flowering time, maturation period, or by selection for broad adaptability as evidenced by low genotype x environment interaction, as in the case of IR36 (Evans, 1993).
In contrast to formal plant-breeding systems, farmer- and community-level crop improvement efforts are more concerned with adapting cultivars to local conditions and systems. In regions of high ecological diversity, a large number of microenvironments may have to be catered to, and farmer circumstances change over time. In continuing to adapt local cultivars to meet their needs, farmer selection pressure may have the effect of producing a series of small adaptive shifts, primarily within the context of complex gene systems. Their high levels of heterogeneity compared with modern commercial cultivars, comparative stability across seasons, location specificity, and generalized, rather than highly specific, tolerances and resistances often generally characterize farmer varieties or landraces.
Brush (1995) has shown that farmers in widely differing environments continue to grow and use traditional cultivars on at least part of their land, even when modern cultivars are available. Traditional cultivars (landraces) may meet their specific requirements and may be better adapted to the environments in which they are grown. There is growing evidence (Riley, 1996) that throughout the world farmers, particularly in traditional agricultural systems, actively seek to improve their crops through exchange with other farmers and through selection within and among their landraces. There have been reports of farmers making crosses, and gene flow between crops
and their wild relatives can provide additional sources of variation especially in centers of origin, although the extent to which this occurs has been questioned (Wood and Lenné, 1993). Thus, landraces, or farmer cultivars, are not static, but represent diverse and dynamic gene pools that, under the pressures of both farmer and natural selection, evolve over time.
Smallholder farmers, particularly those in marginal environments, have to cope with great environmental variability. They generally lack the economic and institutional resources to transform their environments to meet the requirements of their crops (Eyzaguirre and Iwanaga, 1995). As a result, they rely on within-landrace variation or the inherent plasticity of their crops to ensure at least a minimum level of production over the seasons. For many, meeting immediate family needs and short- or medium-term survival are primary objectives. Strategies for achieving them frequently depend on the maintenance and management of diversity within and among crops and landraces in their farming systems. They must balance, for example, straw vs. grain yields, high yield vs. the ability to yield under stress conditions, and ease of hand-harvesting vs. optimum plant architecture for mechanical harvesting. Under these circumstances, farmers' strategies for managing their genetic resources have resulted in an enormous diversity among and within their crops and cultivars.
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