three green bean cultivars. Among the three cultivars, the Tender Improved variety was the least and the
Slim Green variety the most severely affected by salinity in all aspects of the stress. Zaiter and Mahfouz
[201] showed that cultivar Badrieh is tolerant of salinity, whereas T No. 1 is susceptible. These authors
indicated that foliar injury symptoms could be used as criteria for salinity tolerance. In the sand culture
system, cultivar Badrieh did not show symptoms at 4 dS m^1 , whereas T No. 1 was found to be sensitive.
B. Tropical Forages
Beef and milk provide 10% of total caloric consumption and 25% of protein consumption in Latin Amer-
ica [202] and are important components in the diet of all economic strata in the tropics [203]. The live-
stock industry in tropical America is far more important relative to other areas of the developing world
(East Asia and Africa). The vast grasslands (240 million ha) that have developed on low-fertility acid soils
(mainly Oxisols and Ultisols) in tropical America offer considerable potential for increasing livestock
productivity and hence lowering the cost of ruminant animal products [204,205]. Undernutrition is the
key constraint to increased livestock productivity on these low-fertility acid soils. Cattle are dependent on
native pastures and, as a result of acid-soil stress, good quality forage is scarce. Using lime to improve
soil chemical properties is not economically feasible because of the low unit value of forage. Therefore,
the selection or breeding of tropical forages (legumes and grasses) adapted to low-fertility acid soils is
considered as the most viable approach to increasing pasture (and cattle) productivity. In Brazil alone, 50
million ha of introduced pastures have been sown in the tropical savannas in the last 30 years [206].
The low-fertility, acid soils of tropical grasslands have low levels of available nutrients and high lev-
els of soluble Al and Mn, and soil pH is lower than 5.0 [205]. The native pastures in low-fertility acid soils
have poor productivity and nutritive value, and animal performance is correspondingly low. Growing im-
proved tropical forage grasses (with the C 4 photosynthetic pathway) and legumes (with C 3 ) in associa-
tions can make introduced pastures productive [207].
The main role of the legume in a grass-legume association is to improve forage quality [207]. Pas-
ture legumes are rich in N and provide an extra source of protein for grazing animals, particularly in the
dry season when grasses supply little nutrition. The legume directly contributes to animal production by
providing protein-rich forage. It can also improve productivity of low-fertility acid soils by increasing the
amount of N available in the soil for associated grasses [208,209]. But legumes in tropical pastures do not
readily persist under grazing and are more difficult for farmers to manage. To give sound management
advice and help farmers benefit from forage legumes, researchers must understand how nutrient supply
influences the physiology of interactions between grasses and legumes grown in association [210].
With good management, grass-legume pastures can increase nutrient cycling, greatly improve ani-
mal production, markedly increase soil biological activity, and also sequester significant amounts of or-
ganic carbon deep in the soil [211–213]. Pasture productivity should then increase with time as long as
the grass-legume balance is maintained. Furthermore, in the Colombian savannas, improved grass-
legume pastures have clearly boosted yields of subsequent crops of upland rice [209,214]. The success of
crop-pasture rotation has established the need for a different group of tropical forages to take advantage
of the higher soil fertility resulting from crop fertilization.
- Identifying Grasses and Legumes Adapted to Low-Fertility Acid Soils
Exploiting the natural variability of forage germplasm has been an important research strategy for CIAT
and its collaborators to identify tropical grass and legume species adapted to the various ecosystems in
acid-soil regions [205,215]. Germplasm is gathered from a wide range of conditions throughout the low-
fertility acid-soil regions of tropical America, Southeast Asia, and Africa.
The germplasm bank held at CIAT contains over 20,000 accessions from more than 700 species in
150 genera. In cooperation with national research institutions, CIAT screens this germplasm for tolerance
of high soil Al and acidity, low P availability, and tolerance of diseases and insects. Ecotypes that pass
this first screening are characterized in terms of tolerance of grazing, minimum nutrient requirements, nu-
tritive value, dry-season performance, and compatibility in grass-legume mixtures. Subsequently, highly
promising ecotypes are assembled into pastures, relevant establishment technology is developed, and cat-
tle liveweight gains measured. The most promising pasture combinations undergo long-term productiv-
ity and economic evaluation, and the respective technological packages are further adapted to the re-
quirements of the predominant farm system in the area. The result is a new generation of forage grasses
and legumes for these regions [205].
ADAPTATION OF BEANS AND FORAGES TO ABIOTIC STRESSES 597