Research was conducted to examine the physiological basis for the lower productivity of the large-
seeded Andean genotypes [76,77]. Andean lines were found to have less yield and slower seed growth
rate (land area basis) than lines of Mesoamerican background [76]. Studies showed that values for large-
seeded Andean lines were smaller than for small-seeded Mesoamerican lines on such attributes as vege-
tative growth, single-leaf carbon exchange rate (CER), internal leaf anatomy, RGR, net assimilation rate
(NAR), specific leaf weight (SLW), specific leaf N (SLN), leaf thickness, and mesophyll-cell surface area
per unit leaf area exposed to air. These studies also found a positive correlation between RGR and CER
and postulated that the smaller RGR of the large-seeded Andean lines is a function of their slower CER,
which results from thinner leaves with less photosynthetic apparatus per unit leaf area.
White et al. [78] tested the effect of growth habit on the yield of large-seeded bush cultivars and con-
cluded that the mere change in stem type from determinate to indeterminate growth habit did not increase
the yield potential or stability of the large-seeded indeterminate near-isogenic lines. The single gene
change affecting growth habit was therefore in itself not sufficient to improve yield when the rest of the
plant’s genetic composition remained unaltered. White et al. [78] suggested the following yield-increas-
ing traits, which could be genetically manipulated in the indeterminate growth habits: increased number
of nodes and branches, delayed and extended flowering periods, and the ability to recover from stress at
flowering through regrowth. They also pointed out the need to develop an optimal ideotype for the large-
seeded indeterminate beans, based on architectural components and physiological mechanisms that con-
tribute to greater seed yields and stress adaptation.
- Adaptation to Temperature and Photoperiod
Both temperature and photoperiod have strong effects on growth and development in the common bean
[79,80]. In the tropics, high air temperature is normally accompanied by high soil temperature in the root-
ing zone (top 20 cm of soil). Poor root formation due to high temperatures can lead to drought stress. In
semiarid regions, high temperatures and drought often act together to reduce bean yields significantly.
The effects of high temperatures include flower fall, abortion, reduced pollen grain viability, impaired
pollen tube formation in the styles, and reduced seed size [81,82].
Two components contribute to plant adaptation to high temperature [8]: (1) heat avoidance, in which
plant tissues subjected to high solar radiation or hot air have lower temperatures than control plants, and
(2) heat tolerance, whereby essential plant functions are maintained when tissues become hot.
Beans are grown in a very wide range of latitudes and the mean air temperature varies between 14
and 35°C. Temperatures of air and rooting zone can determine seed germination, root growth, taproot for-
mation, and flowering. High temperatures negatively affect pollen-stigma interaction, pollen germina-
tion, pollen tube growth, and fertilization. Consequently, if plants are exposed to high temperatures for 1
to 6 days before flowering, pod set is very low [83].
Extreme temperatures, that is, lower than 10°C and higher than 40°C, can result in a poor germina-
tion rate [84]. White and Montes-R. [85] characterized the germination response of 20 genotypes of com-
mon bean by fitting cumulative counts, using a maximum-likelihood analysis. They found that the ger-
mination rate increased from a base temperature typically near 8°C to an optimal development
temperature (TO) of 29 to 34°C. Base temperature did not differ among common bean genotypes.
Mesoamerican germplasm showed slightly higher TOthan Andean germplasm, but TOvaried widely
within each of the two gene pools. The only accession of tepary bean (P. acutifolius) evaluated, ‘Sonora
32’, was the most tolerant of high temperatures at germination.
Bean cultivars tolerant of cold conditions are needed for mountainous regions and bean-growing re-
gions in the higher latitudes. Beans suffer cold stress either during seed germination or, later, at the pod-
filling stage. Beans possess significant genetic variability for cold tolerance [86]. Cold-tolerant cultivars
can be selected, using a laboratory test, at 12°C in the F 2 generation when appropriate parents and/or
sources are identified [87]. Selecting bean lines for cold tolerance at seedling stage is possible [88]. The
Universidade Federal de Lavras, Brazil, is attempting to breed cultivars that are cold tolerant during early
growth stages and at maturity.
Selecting for improved adaptation to high temperatures in beans is possible [89]. Masaya and White
[79] suggest that the most difficult part of developing bean cultivars adapted to extreme temperatures is
not so much the search for adequate physiological response to temperatures as for resistance to associated
biotic constraints such as fungal diseases (e.g., web blight) under hot, humid conditions and various root
rots under cool conditions. Researchers at the Instituto Pernambucano de Pesquisa Agropecuária (IPA,
ADAPTATION OF BEANS AND FORAGES TO ABIOTIC STRESSES 589