that leaf longevity is an important factor in leaf N use efficiency but that genetic variation for shoot ar-
chitecture is not important in determining the N use efficiency of individual leaves [188].
Common bean is widely regarded as weak in nodulation and N 2 fixation [55,134]. This is partly the
result of the marginal soil conditions under which it is commonly grown, partly a result of competition
from indigenous but often ineffective soil rhizobia, and partly a result of selection for early flowering and
short growth season in many areas. Bush types fix less N 2 than indeterminate and climbing types
[189,190]. There is significant genetic variability within growth types for N 2 -fixing ability [191]. Several
research centers have breeding programs under way to improve N 2 fixation in common bean [192]. Be-
cause seed yield gains from N 2 fixation have proved limited, even monocropped beans are often fertilized
with N fertilizers [193].
Genetic analyses of F 1 , F 2 , and backcross progenies from diverse germplasm of snap bean grown in
nutrient solutions with a low K supply indicated a single gene control of K efficiency (dry matter yield
per unit K absorbed) [194]. Reciprocal F 1 progenies from crosses between efficient and inefficient strains
showed no maternal effects. The gene for K efficiency was homozygous recessive in the efficient geno-
types. Differential responses among strains grown at low K appeared to be associated with K use rather
than K uptake or high accumulation of K and did not appear to be associated with Na substitution for K
in the plant [194,195].
In a pan-African effort, Wortmann et al. [149] evaluated 280 entries from African bean-breeding pro-
grams for tolerance of low availabilities of soil N, P, and K, and toxicities of Al and Mn. Several entries
were identified as tolerant of each of the stresses (Table 1). Especially promising were RWR 382, RAO
55, ACC 433, XAN 76, and MMS 224 for low-P tolerance; ICA Pijao and EMP 84 for low-K tolerance;
Muhinga, Ntekerabasilumu, and 7/4 ACC for tolerance of Al toxicity; and MCM 5001 and XAN 76 for
tolerance of Mn toxicity. Several varieties, including XAN 76, RAO 55, and OBA 1, performed well un-
der several edaphic stresses.
In Latin America, Thung et al. [142] proposed a field screening method to evaluate Al tolerance in
beans, using seed yield as a selection parameter. Attempts were made to screen bean germplasm for Al
resistance, using nutrient solutions [196,197]. The response of seven cultivars of beans exposed to toxic
levels of Al was assessed, using root elongation rate and callose accumulation in 5-mm root tips as early
markers of Al injury [197]. Based on root elongation rate, which is very sensitive to Al toxicity, Massot
et al. [197] identified ‘F-15’ and ‘Superba’ as the most Al-tolerant cultivars. Callose synthesis correlated
positively with internal Al concentration and negatively with root elongation rate. Results indicated that
while both callose accumulation and root elongation rate could be useful in classifying the bean cultivars
for Al tolerance, root elongation rate is the more sensitive parameter.
Field screening of 5000 accessions of germplasm collection and bred lines over the past few years in
an Al-toxic soil at Quilichao, Colombia, has resulted in the identification of 77 genotypes for further test-
ing and analysis [121]. Among the 77 genotypes tested, four bred lines, A 774, VAX 1 (interspecific),
FEB 190, and FEB 192, were found to be outstanding in their adaptation to Al-toxic soil conditions. Grain
yield of a bred line, A 774, was 60% greater than that of a widely adapted cultivar, Carioca, with no lime
treatment. A 774, VAX 1, and FEB 190 were also responsive to lime and P application. To date, the ge-
netic control of Al resistance has not been elucidated, much less the association of mechanisms studied,
as has been possible with P.
Sources of genetic tolerance of Mn toxicity were identified in common bean using three growing
conditions: nutrient solution culture, silica sand culture, and Mn-amended soil [198]. Six genotypes (Ar-
gentino, BAT 271, Calima, EMP 84, H6 Mulatinho, and Pintado) out of 25 screened were tolerant of a
toxic level of Mn in solution culture. The tolerance of Mn observed in solution culture correlated with tol-
erance observed in the silica sand system. Some genotypes that performed very well in solution culture
and silica sand suffered severe yield reduction in Mn-amended soil. This study indicated that screening
of genotypes in solution culture is useful to identify sources of tolerance of Mn toxicity, but performance
of those genotypes in soil might be confounded by other edaphic stress factors common to low-fertility
tropical soils. In another study, González and Lynch [199] characterized the mechanisms of Mn tolerance
in common bean using two contrasting (tolerant and sensitive) bean genotypes. They demonstrated that
Mn compartmentation occurs at both the tissue and organelle level and that Mn accumulation in the epi-
dermis-enriched fraction could contribute to Mn tolerance in common bean.
Common bean shows genetic variability for tolerance of soil salinity [95,200]. Pessarakli [200] dis-
cussed the effects of salt stress on dry matter production, total N,^15 N, crude protein, and water uptake by
596 RAO