crop yield will play a crucial role in the future. The advances in agricultural biotechnology have revolu-
tionized the genetic analysis and improvement of crop plants and provided not only geneticists but also
physiologists, agronomists, and plant breeders with valuable new tools to identify traits of economic, en-
vironmental, and nutritional importance. The integration of knowledge and biotechniques into the plant
breeder’s set of tools for cultivar development makes plant breeding more precise and shortens the time
needed for cultivar development.
E. Need for Physiologically Superior Genotypes for Sustainable
Cropping SystemsThe plant genetic approach to improving adaptation to major abiotic constraints is ecologically clean, en-
ergy conserving, and much more economical for resource-poor farmers in the tropics than modifying the
soil and crop environment. Hence, it is compatible with national and international goals of economical
food production; conservation of soils, water, and energy; and pollution control.
The shallow rooting ability of less adapted crop and forage cultivars is generally believed not only
to reduce nutrient acquisition from low-fertility acid soils but also to increase susceptibility to seasonal
drought. Developing genotypes that can root more deeply under adverse conditions is an important re-
search objective for improving genetic adaptation to low-fertility soils.
Some of the benefits that can be obtained by integrating stress-resistant cultivars into cropping sys-
tems include fewer input requirements, reduced production costs, and reduced environmental pollution
and soil degradation. Improved genetic adaptation to low-fertility soils will reduce nutrient requirements
of crop and forage cultivars and minimize maintenance fertilizer applications through one of two path-
ways [45]: (1) deeper root growth →more efficient uptake of nutrients from subsoil →less leaching of
nutrients, and (2) more biomass production →less seepage, less leaching →more intensive nutrient cy-
cling→maintenance of higher soil organic matter content →less erosion owing to better soil protection
by vegetation and mulch. Lynch [46] argued that the degree and extent of nutritional limitations to crop
productivity and the economic and ecological liabilities of intensive fertilization are such that, eventually,
nutrient-efficient crops will be an important part of integrated nutrient management of cropping systems.
One important objective of modern agriculture is to maximize crop productivity, preferably within a
sustainable cropping system. As the cropping systems vary from high-input to low-input ones, crop im-
provement strategies have to be modified accordingly. A careful analysis of major factors limiting pro-
duction in each system should dictate whether improvement can be based on breeding strategies, crop
management, or soil management. International agricultural research centers are focusing more on “sus-
tainable” yields rather than maximum yields [47], that is, on achieving high, sustainable crop yields
within a societal framework that imposes significant managerial constraints on the farmer [48]. Some
farmers may well have to abandon the goal of maximum crop yields as a result of new economic and en-
vironmental realities. Physiologically superior genotypes are needed to achieve resource use efficiency
and profitability while minimizing environmental degradation.
III. CASE STUDIES
A. Common Bean
The common bean (Phaseolus vulgarisL.) was originally a crop of the New World, but it is now grown
extensively in all major continental areas. The genus Phaseoluswas domesticated in the upland regions
of Latin America more than 7000 years ago [49–52]. It is the world’s most important food legume, with
an annual production value of over U.S.$10 billion. Latin America produces nearly half (5.1 million
tons) of the world’s supply (11.6 million tons from 14.3 million ha) of dry beans [53]. Beans are grown
in a wide range of environments [54] from sea level to elevations of more than 3000 m [55]. Bean pro-
duction is often relegated to marginal environments, such as those characterized by steep, erosion-prone
slopes or by low soil fertility with seasonal droughts. Nearly 80% of dry bean production occurs on
small-scale farms in the developing countries of tropical Latin America and Africa. Women are the
primary bean growers on small farms in Africa. Widely known as the “poor man’s meat,” the crop pro-
vides an inexpensive source of protein for low-income consumers. Bean consumption is highest in east-
ern and southern Africa, where beans are the second most important protein source after maize and the
ADAPTATION OF BEANS AND FORAGES TO ABIOTIC STRESSES 587