tified, such as BAT 477, A 195, and BAT 1289 [112]. The superior adaptation of BAT 477 to water
deficits was attributed to dehydration postponement through greater root length density and deeper soil
moisture extraction [100]. Castonguay and Markhart [113], on measuring saturated rates of photosynthe-
sis in water-stressed leaves of common and tepary beans, found that genotypic variability in drought tol-
erance between common and tepary beans is not related to differences in mesophyll tolerance of dehy-
dration. Tepary bean relies more on dehydration postponement than on drought tolerance. Severe drought
impaired N mobilization, harvest index, and water use efficiency in common bean [110].
Grafting diverse shoot genotypes on selected root genotypes of common bean and evaluating yield
under soil water deficits showed variation in shoot genotype. However, the effect of shoot genotype on
growth and yield under water deficits was found to be small compared with that of root genotype [114].
Field research under rain-fed conditions indicated that water use efficiency (based on carbon isotope
discrimination) is not a promising indicator of adaptation to water deficit in the common bean [96].
Other physiological traits such as shoot dry weight and leaf N concentration appeared the most promis-
ing, being based on heritability, strong general combining ability effects, and correlations with seed
yield across trials [108]. Phenotypic plasticity is considered another mechanism contributing to in-
creased performance under drought [109]. This particular attribute allows genotypes to shorten their
growing cycle dramatically at later planting dates to avoid drought conditions or low temperatures later
in the growing season.
Identification of a shoot trait or traits that reflect rooting ability and adaptation to drought will min-
imize labor-intensive root measurements in a breeding program. Studies of other grain legumes such as
peanut and soybean have indicated that water use efficiency is negatively associated with certain shoot
traits such as specific leaf area (leaf area per unit leaf dry weight) and leaf ash content [12,115]. The de-
crease in specific leaf area in drought-adapted genotypes may also be related to the accumulation of non-
structural carbohydrates in leaves. Understanding the relationships between grain yield and shoot traits
such as specific leaf area, leaf ash, and leaf nonstructural carbohydrates, using contrasting genotypes, may
help identify the specific shoot traits related to adaptation to drought in common bean.
- Breeding for Improved Adaptation to Drought
Progress in breeding for adaptation to drought in common bean has been slow, although several selection
criteria for resistance to drought have been identified [94,116,117]. Because seed yield is the most im-
portant economic trait, the most practical method for improving performance is through the direct mea-
surement of yield-related characteristics [118]. Studies on inheritance of seed yield of the common bean
under rain-fed conditions in contrasting environments by White et al. [108] indicated that an efficient sys-
tem for breeding for increased seed yield under drought can be developed by using early generation yield
testing of population bulks. They suggested that potential parents adapted to drought should first be tested
for combining ability in environments of the region before using them extensively in hybridization and
selection programs. Singh [119] reported an increase in yield under drought through hybridization be-
tween races and gene pools, involving high-yielding and water stress–tolerant progenitors derived from
different origins, such as those found in the Mexican highlands. New sources for drought tolerance were
found in cultivars grown in Jalisco and Durango, Mexico (S. Singh, personal communication).
Schneider et al. [111] evaluated the performance of two common bean populations consisting of 78
and 95 recombinant inbred lines (RILs) under conditions with and without drought. They examined seed
yield under drought, yield potential, drought susceptibility index, harvest index, and geometric mean as
potential indicators of drought-resistant genotypes. Among the plant traits measured, they found that the
100-seed weight was the most highly heritable trait in both populations. They also found that the geo-
metric mean of the two drought treatments (with and without) was the single strongest indicator of per-
formance under both drought and no-drought treatments. On the basis of this study, they suggested that
the most effective breeding strategy would involve selection based, first, on the geometric mean, followed
by selection based on yield under drought stress. Using the RILs of the same two populations, Schneider
et al. [120] studied the possibility of molecular marker–assisted selection (MAS) to improve drought re-
sistance in common bean. Using one-way analysis of variance and multiple regression, they identified
four RAPD (random amplified polymorphic DNA) markers in one population and five in another that
were consistently and significantly associated with yield under drought, yield without drought, and/or ge-
ometric mean yield across a broad range of environments. From this study, they concluded that the rela-
tive value of MAS is inversely proportional to the heritability of the trait under examination.
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