Another population of RILs was developed from the cross BAT 881 G 21212 to study yield po-
tential under low-P conditions [184] (S. Beebe, personal communication). Linkage groups were estab-
lished and QTL analysis carried out. One linkage group was particularly important for yield at low P, and
a long segment of more than 80 cM appeared to carry several QTLs for yield. The entire segment ac-
counted for more than 300 kg/ha at low P—a remarkable effect under very difficult production conditions.
Field studies indicated that G 21212 is particularly efficient in mobilizing photosynthates to grain when
grown in P-deficient soil [121]. Thus, it is possible that the QTLs identified are unlike those that were
identified for P acquisition.
The AFLP (amplified fragment length polymorphism) technique, combined with selective genotyp-
ing, was used to map QTLs associated with tolerance of low P in rice [185]. Molecular markers for QTLs
may serve to select the desired traits, but whether a breeder bases selection on a molecular marker or on
a trait will depend on the relative ease and cost of each approach. In any case, physiological analysis and
QTL analysis are highly complementary approaches. The use of QTL analysis strengthens physiological
analysis of traits. On the other hand, QTLs per se often express variably across environments, and phys-
iological analysis can offer the breeder more understanding of the biological significance of a given QTL
and its potential value. The possibilities of using other species as sources for improving P efficiency in
common bean, by wide crossing or genetic transformation, are still to be exploited.
- Selecting for Improved Adaptation to Other Soil Constraints
Knowledge of specific nutritional requirements of common bean can help determine more precisely the
amount of fertilizer needed to overcome soil constraints and maintain productivity over time. Substantial
progress was made in defining the nutritional requirements of common bean [58,143]. Table 2 shows crit-
ical nutrient values for soil and plant analysis compared with normal range in plant tissue to detect edaphic
constraints to bean production in the tropics.
Studies of genotypic variation of common bean for tolerance of various edaphic constraints have
been reviewed [95,148,149,155]. Results demonstrate the feasibility of selecting and breeding for toler-
ance of certain edaphic constraints [95,186] (Table 1). In most regions, N deficiency limits common bean
and associated crop production. Tolerance of low N supply in soil has several components [187]. These
include rate and duration of N acquisition, efficiency of N use in vegetative growth, timing of transition
to reproductive growth, rate and duration of N accumulation in seeds, and efficiency of N use in seed for-
mation. Studies on photosynthetic N use efficiency in relation to leaf longevity in common bean indicate
ADAPTATION OF BEANS AND FORAGES TO ABIOTIC STRESSES 595
TABLE 2 Critical Nutrient Values for Soil and Plant Analyses to Detect Edaphic Constraints to Common
Bean (Phaseolus vulgarisL.) Production in the Tropics
CriticalElement Soil Plant Normal range for plants
N 25 (g kg^1 ) 52–54 (g kg^1 )
P 11–15 (mg kg^1 ) 2.0 (g kg^1 ) 4.0–6.0 (g kg^1 )
K 0.15 (cmol kg^1 ) 15 (g kg^1 ) 15–35 (g kg^1 )
Ca 4.5 (cmol kg^1 ) 5.0 (g kg^1 ) 15–25 (g kg^1 )
Mg 2.0 (cmol kg^1 ) 2.0 (g kg^1 ) 3.5–13 (g kg^1 )
S 4–15 (mg kg^1 ) 1.6–6.4 (g kg^1 )
B (mg kg^1 ) 0.4–0.6 20 10–50
Zn (mg kg^1 ) 0.8 15 35–100
Mn (mg kg^1 ) 5.0 20 50–400
Cu (mg kg^1 ) 0.6 5.0 5–15
Fe (mg kg^1 ) 2.0 5.0 100–800
Exch. Al (cmol kg^1 ) 1.0 50 100–800
Al saturation (%) 50 (organic soil)
10 (mineral soil)
pH 5.0–7.8
5.5–6.5 (optimal)
Mn toxicity (mg kg^1 ) 20–80
Source: Adapted from Refs. 58 and 143.