As mentioned before, bean cultivars show great diversity in phenology, and this is a major factor de-
termining yield potential and adaptation to rain-fed environments [73]. Many landraces and cultivars of
the Mexican highlands showed a plastic response to planting date such that, with increasingly later plant-
ings, they became earlier flowering [109]. This phenological plasticity appears to be an adaptive strategy
by common bean to rain-fed environments. In drought-stricken northeast Brazil, BAT 477 can tolerate
drought very well. This line combined drought tolerance with resistance to Macrophominadisease that
prevails in drought areas. A more recent bred line, SEA 5, was found to be better adapted to drought than
BAT 477. This was mainly attributed to its ability to partition a greater proportion of assimilates to grain
production [121].
- Physiological Responses to Low Phosphorus Supply
Phosphorus (P) deficiency is widespread, covering an area estimated at over 2 billion ha [122]. Cochrane
et al. [123] estimated that 86% of the tropical soils of Latin America have levels of P less than 7 ppm
(Bray II) available in the topsoil. These soils have a high capacity [124] to fix P in forms that are mostly
unavailable to plants, thus imposing agronomic and economic constraints. Application of P fertilizer is
common practice and is necessary if agricultural productivity is not to be seriously limited. Improved cul-
tivars with genetic adaptation to low-P soils may be a viable alternative or complement to P fertilization,
particularly for crop-livestock systems in the tropics [18,125].
The two major components of P efficiency are P acquisition efficiency and P use efficiency [18].
Phosphorus acquisition efficiency refers to the plant’s ability to acquire greater amounts of P per unit root
length, whereas P use efficiency refers to the plant’s ability to produce yield per unit of acquired P from
soil. With a given P supply in soil, P acquisition per plant might be improved in at least three ways: (1)
with a root system that provides greater contact with P, (2) with greater uptake per unit of root due to en-
hanced uptake mechanisms, and (3) with an ability to use insoluble organic or inorganic P forms that are
relatively unavailable or poorly available to plants [7,18,126,127]. Association with arbuscular mycor-
rhizae (AM) significantly affects each of these attributes [128]. Uptake of P by upland rice, pigeonpea,
and groundnut, which all associate with AM, was found to be higher than that by buckwheat, castor, cot-
ton, maize, sorghum, and soybean in soils with low P availability [129].
Among the edaphic stresses, P deficiency is the primary constraint to common bean production in
the tropics and subtropics, limiting seed yield on at least 60% of the bean-producing areas of Latin Amer-
ica and Africa [130,131]. The symbiotic nitrogen fixation (SNF) of common bean is more affected by P
deficiency [132] than that of other crops such as soybean [133]. High SNF in common bean was reported
to be related to nodule number, nodule mass, late nodule senescence, early nodulation, and secondary
nodulation [134,135]. Screening 220 lines (Andean and Mesoamerican origin) under greenhouse condi-
tions resulted in the identification of contrasting lines that may be useful for further improvement of SNF
potential and adaptation to P-deficient soils [132]. Beans with SNF and tolerance of P deficiency were
mostly found among late-flowering, type IV lines but included three early-flowering, type III lines. High
P concentration in seeds produces seedlings less dependent on soil P supply and therefore could enhance
nodulation and SNF of common bean [136].
Substantial genetic variation in P efficiency in common bean has been demonstrated under both field
and greenhouse conditions [58,121,137–156] (Table 1). Beebe et al. [154] studied the relationship be-
tween geographic origin and response to low P supply in soil in a selection of 364 genotypes drawn from
the gene bank held at CIAT. They found highly significant variation in P efficiency among genotypes in
all growth habits. Wild beans usually performed relatively poorly, indicating that P efficiency traits in
common bean have been acquired during or after domestication.
Attempts by Singh et al. [145] to improve P efficiency in common bean were not successful because
of the confounding effects of other edaphic and climatic factors rather than because of P deficiency per
se. Thus, the interactions of genotype season, genotype P levels, and genotype season P levels
underscore the difficulty of relying on yield performance as a sole criterion for selection in a breeding
program [148]. Identifying specific mechanisms of P efficiency would be far more reliable and prefer-
able. If a multigenic character such as P efficiency could be resolved into physiological mechanisms gov-
erned by discrete traits, these traits could be tagged with molecular markers more reliably than P effi-
ciency could be measured as a quantitative trait by seed yield trials [46].
The genetic control of P efficiency is well known to be complex because of the involvement of this
important nutrient in several aspects of plant metabolism [157,158]. The most pronounced effect of P de-
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