tems, allowing plants to adapt to and withstand wide environmental variations. Research on physiolog-
ical aspects of crop adaptation to abiotic environments should aid plant breeders, geneticists, and
molecular biologists to manipulate crop genotypes for improved yield potential, stress resistance, and
nutritional quality.
Improved adaptation of a crop to its environment can be achieved by two general approaches: the
growth environment may be altered, or the plant genotype may be improved. Often a combined approach
is the most effective.
Plant growth is closely related to the assimilation of carbon, the element’s partitioning into different
plant structures, and its loss through respiration, all of which must be accompanied by water and nutrient
uptake. Assimilated carbon enters a pool of carbohydrates, and from there it is used either in respiration
or in the growth of assimilatory and supportive structures. Partitioning of dry matter into leaves has a pos-
itive feedback on plant productivity because of its effects on total leaf area, but it inevitably increases de-
mand for nutrients and water under conditions in which too few carbohydrates are available for root
growth. These simultaneous parallel requirements need to be balanced by the plants.
Several authors have reviewed research efforts on improving plant adaptation to different climatic
and edaphic stress factors [5–18]. In general, these reviews discuss physiological processes in detail rel-
ative to whole plant stress tolerance. The most successful approaches to improving crop and forage adap-
tation to abiotic stresses have historically used field-based evaluations to identify tolerant cultivars, fol-
lowed by breeding and selection of genotypes that combine performance in stressful environments with
other desirable plant attributes.
In this chapter, I have attempted to evaluate the role of physiological research in improving crop
adaptation to abiotic stresses in the tropics, using case studies of common bean (Phaseolus vulgarisL.)
and tropical forages.
II. ROLE OF PHYSIOLOGICAL RESEARCH IN CROP IMPROVEMENT
Plant physiology explores the full range of plant behavior, whereas crop physiology concentrates on how
cultivars and related genotypes differ and how one may excel others under particular environmental or
stress conditions [3]. A plant’s genetic characteristics determine its potential maximum size, rate of pho-
tosynthesis, rate of dry matter production, and the form and nature of its storage organs, including those
that are usually harvested for food or feed. Environmental factors such as water availability, temperature,
photoperiod, light intensity, and availability of nutrients determine to what extent this potential can be
reached. The main challenge is to recognize improved genotypes and to determine where energy-depen-
dent inputs (fertilizer, irrigation, pesticides, etc.) can be used with greatest effect and efficiency. Attempts
to answer such problems form the basis of crop physiology.
An effective crop improvement program for genetically enhancing crop adaptation to abiotic stress
factors would involve (1) identifying germplasm tolerant of the abiotic stress factors of interest, (2) char-
acterizing plant traits and mechanisms responsible for superior genetic adaptation, (3) determining mech-
anisms of inheritance for key plant traits, (4) identifying quantitative trait loci (QTLs) associated with key
traits involved in stress tolerance for which marker-assisted selection in populations is feasible, and (5)
developing an integrated genetic enhancement scheme.
Physiological research can make substantial contributions to crop improvement through character-
izing germplasm for yield potential, making physiological analyses of yield potential, identifying key
physiological traits, integrating physiological tools, and developing resource use–efficient genotypes
for sustainable cropping systems.
A. Characterizing Germplasm for Yield Potential
In the 20th century, the plant breeder has at hand several new techniques that both speed up breeding
and increase the range of genetic variation [19]. These include “classical” genetic modification meth-
ods such as inducing mutations by treatment with chemicals or x-rays; anther and ovule culture, which
allows the production of completely homozygous plants, thereby cutting out some of the requirements
for selfing; and embryo rescue techniques, which permit previously incompatible species to produce vi-
able offspring. The second type includes “cellular” modification, which generates somatic variation
through tissue culture by producing novel hybrids through cell fusion. These increase the gene pool
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