Brazil) have identified several heat-tolerant lines (e.g., HF 465-63-2). Their leading line, IPA 7, is not
only heat tolerant but also resistant to a major fungal disease (Macrophominaspp.) that affects bean per-
formance under drought [90].
Various studies [54,79,91] of the effects of photoperiod on common bean indicate that adaptation of
common bean is strongly affected by photoperiod, and the species shows considerable genetic variation
in photoperiod response. White and Laing [92] characterized this variation among 4000 bean genotypes
and found that about 60% of genotypes were photoperiod sensitive and that small-seeded and bush-inde-
terminate materials had the highest proportion of day-neutral genotypes. The authors also constructed a
frequency distribution of photoperiod response in 3060 genotypes, which showed three distinct peaks,
suggesting simple genetic control.
Interaction between air temperature and photoperiod plays a great role in bean production [79].
The small-seeded day-neutral genotypes are physiologically the most efficient, especially at warmer
sites and higher latitudes [74]. Many highland cultivars are poorly adapted to lowland areas at the same
latitude because of temperature differences between altitudes. Acosta-Gallegos et al. [93] determined
the effect of sowing date on the growth and seed yield in highland environments. They found that vari-
ation in maturity had a more consistent effect on growth and seed yield than on days to flowering, in-
dicating that growth and seed yield are more affected by the duration of the reproductive phase than by
the duration of the vegetative phase. The authors suggested that part of the variation in growth and seed
yield may be due to genotypic differences in photoperiod or temperature response. Highland bean cul-
tivars can produce flowers and seeds when exposed to a 6-hr daylight regime [73]. Temperatures of the
rooting zone and subsoil may be key factors in determining days to flowering, seed germination, and
tap or lateral root formation.
- Physiological Response to Water Deficits
About 60% of common beans produced worldwide are grown in regions subjected to water stress, mak-
ing drought the second largest contributor after disease to yield reduction [94,95]. Increased adaptation
of common bean genotypes to soil water deficits would contribute to both stability and expansion of pro-
duction in drought-prone environments such as northeast Brazil and the central highlands of Mexico.
Bean cultivars adapted to drought would require less water for irrigation and would therefore contribute
to the conservation of an important natural resource. The short growing season reduces the common
bean’s water requirements to levels below those of other species generally considered as more drought
adapted [96].
“Drought resistance” is a general term encompassing a diversity of mechanisms that enable plants to
survive and produce in periods of dry weather. “Drought tolerance” involves the maintenance of a posi-
tive turgor pressure at low tissue water potential. Drought tolerance mechanisms include osmotic adjust-
ment and dehydration tolerance achieved via protoplasm resistance. “Drought avoidance” is the mainte-
nance of a high tissue water potential (i.e., maintenance of green, turgid tissue) during a period of a high
evaporative demand or a period of increasing soil water deficit.
Under rain-fed conditions, water deficit can occur more than once during a crop’s growth cycle,
caused by erratic patterns of rainfall distribution, or may kill the crop [95]. The intensity and duration of
stress determine the degree of yield reduction relative to its yield potential. Research approaches that have
most successfully improved drought performance (1) used realistic soil conditions, (2) tested with ade-
quate water and with limited water, (3) understood the sources of crop failure in the proposed growing
area, and (4) targeted a limited number of traits for genetic improvement [14].
Although common bean is not a drought-tolerant species [54], it is grown over a wide range of habi-
tats where it is exposed to seasonal droughts and wide fluctuations in soil moisture availability between
years. Research efforts on common bean adaptation to drought involve studying the effects of water stress
on plant growth, development, and seed yield [73,97,98]; developing field screening methods [99–101];
evaluating and identifying sources of drought tolerance in germplasm [94,102–104]; and evaluating phys-
iological traits related to underlying mechanisms of adaptation to drought [105–110].
Performance under drought can be evaluated in terms of three discrete groups of characteristics: mor-
phological, physiological, and phenological [111]. Loss of leaf area is the most important morphological
adaptation and results from a reduced number of leaves, reduced size of younger leaves, inhibited expan-
sion of developing foliage, or leaf loss accentuated by senescence, all of which result in decreased seed
yield [105]. Through field screening, some relatively drought-tolerant lines of bean germplasm were iden-
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