third most important caloric source after cassava and maize. Beans are also nutritionally important in
Central America, Mexico, and Brazil.
The cropping system used ranges from the highly mechanized, irrigated, and intensive production of
monocropped bush beans to complex associations of indeterminate or climbing beans with maize, other
cereals, sugarcane, coffee, or plantain [56]. Soil and crop management inputs in such multiple-cropping
systems are often limited, with the result that seed yield can range from less than 500 kg ha^1 in parts of
Latin America and Africa to as much as 5000 kg ha^1 under experimental conditions.
Research to enhance common bean genetically is complicated by the diversity of edaphic and cli-
matic conditions under which the crop is grown, compounded by highly specific local preferences for par-
ticular grain types or colors. However, great progress has been achieved in developing genotypes resis-
tant to several biotic constraints [56]. Whereas the success in improving genetic adaptation to major
abiotic constraints has been substantial, progress in improving yield potential has been limited indeed.
Different perspectives on the reasons why are given in previous reviews [53–59].
A world collection of beans, comprising more than 40,000 accessions, is held at the Centro Interna-
cional de Agricultura Tropical (CIAT) in Cali, Colombia. This collection includes indigenous wild and
weedy specimens, unimproved landraces, pure lines of Phaseolus vulgaris, and numerous related species.
Phaseolus vulgariswas domesticated several times in the pre-Colombian era in both Mesoamerica and
the Andean region, resulting in at least two major gene pools of cultivated bean, one Mesoamerican and
one Andean [60]. These two gene pools are distinguished by yield potential, morphology [61,62],
isozymes [63], DNA molecular markers [64–66], and physiological traits related to photosynthesis
[67,68].
Singh [69] developed a key for identifying different growth habits in common bean. Bush types
fall into three groups: type I plants form a determinate inflorescence at the end of stems and branches.
Typically, they have a low number of nodes and a short flowering period and are early maturing. Type
II and type III plants have indeterminate growth, the stems and branches ending in a vegetative guide.
Type II plants are erect, have little guide development, and are usually intermediately maturing. Type
III plants have a more prostrate growth, are usually strongly branched, and show moderate ability to
climb if given support. Type IV plants have indeterminate growth and very weak and excessively long
stems and branches that possess strong climbing ability. Type I is considered to have the lowest yield
potential [54]. In general, indeterminate cultivars provide greater yield stability than determinate culti-
vars [70,71].
The process of matching growth habit to changing environment, economy, and technology is a ma-
jor challenge in genetically enhancing the common bean. Vandenberg and Nleya [72] indicated that com-
mon bean germplasm suitable for direct harvest systems could be developed by introducing parents that
can contribute to the genetic enhancement of pod distribution in the overall plant canopy. They have iden-
tified the following plant traits that may optimize canopy structure at harvest: (1) long internodes in the
lower stem, (2) consistent internode elongation under a wide range of environmental conditions, (3) re-
duced stem stunting during early season growth, (4) increased stem length, (5) increased stem strength,
particularly in the more basal internodes, (6) reduced pod length without decreasing seed size, (7) in-
creased pod curvature so that tips do not extend below the cutter bar, (8) long upright peduncles, (9) flow-
ering beginning on upper nodes, (10) high fertility at the upper nodes, and (11) a sufficient number of
main stem nodes to maximize productivity in the available growing season.
White and Izquierdo [73] discussed physiological processes that determine bean yield and applied
that information to analyze limitations to yield potential and stress tolerance. They identified several char-
acteristics that may possibly confer general stress adaptation: an ability for recuperative growth, presum-
ably by remobilizing carbohydrate or nitrogen (N) reserves and having an indeterminate growth habit;
good competitive ability; high tissue concentrations of phenolic compounds with inhibitory effects on a
broad range of pathogens or pest organisms; greater partitioning of photoassimilates to root growth; and
buffer ability for adequate pod retention and seed filling.
Large-seeded bush bean cultivars usually give lower yields than small-seeded ones, especially in
warm, tropical environments. Andean genotypes are predominantly large-seeded, whereas small seeded-
ness is associated with the Mesoamerican region of domestication [54,61]. Large-seeded genotypes tend
to have a lower relative growth rate (RGR) than small-seeded types [54,74]. This poorer performance of
large-seeded lines is not limited to RGR because seed yield is also often negatively associated with seed
size among bush bean cultivars [75].
588 RAO