stable and productive farming systems via their contributions to soil enhancement [208,209,212,262,263]
and nutrient cycling [264,265]. This is mainly attributed to their generally efficient use of external nutri-
ents and inherently higher and more stable production potential under low fertilizer inputs [205,231]. In-
troduced tropical grass and legume cultivars develop deep and abundant rooting systems that penetrate
well into A1-toxic subsoils, giving them greater potential to survive seasonal droughts and reduce nutri-
ent leaching [211,213,266].
The rooting ability of acid soil–adapted grass and legume cultivars has several consequences. Soil
physical conditions are improved, as shown by higher rates of water infiltration and increased stability
and size of soil aggregates [267,268]. Both roots and aboveground litter contribute to the quantity and
quality of soil organic matter, which, in turn, improves soil biological activity. These improvements in
soil quality attributes lead to significant increases in grain yield of acid soil–adapted upland rice
[205,209].
The ability of acid soil–adapted grasses and legumes to root profusely would also help these species
to become established in degraded and compacted soils with minimum tillage. This ability to reclaim de-
graded lands may be particularly valuable for small farmers in the humid tropics, who often do not have
farm machinery. The vigorous rooting ability of grasses and legumes help prevent soil runoff in regions
where rains are intense and abundant. Some tropical forage legume species can also be used as improved
and accelerated fallow to complement or substitute the fallow of native species, which grow more slowly
[269].
IV. FUTURE PERSPECTIVES
This chapter showed that considerable progress is being made in improving genetic adaptation of com-
mon bean and tropical forages to major abiotic constraints in the tropics. It also highlighted the role of
physiological studies for improving genetic adaptation to major abiotic constraints. Crop physiology has
been called the “retrospective science” by one plant breeder because physiologists elucidate what the
breeders have already achieved [270]. This is because the links between physiology and genetics have not
been established. This situation is likely to change in the future [2], when knowledge of plant physiolog-
ical processes will become extremely important in screening for and measuring phenotypic traits.
Advances in agricultural biotechnology open a new and exciting perspective for dissecting and un-
derstanding the complex regulation of physiological traits and mechanisms controlling crop adaptation
to abiotic stresses. As high-density molecular maps become more readily available for a range of food
and feed crops, physiologists need only to screen the parents of the available mapping populations for
variation in the expression of the trait(s) of interest and then to score the appropriate mapping popula-
tion for the trait. As Prioul et al. [271] pointed out, it is time for plant and crop physiologists to study
marker-characterized segregating populations and marker-specific-near-isogenic lines instead of im-
proved cultivars.
To develop the new technologies, the disciplines of physiology, genetics, molecular biology, and
breeding will need to be brought together. The integrated team efforts would contribute toward develop-
ing food and feed crops that would overcome major abiotic stresses, particularly in the tropics (Figure 1).
We are likely to see continued significant progress in our understanding and ability to modify stress tol-
erance by molecular engineering, using both model and crop plants, based on understanding how stress
affects plant biochemistry and physiology through gene expression [272]. The onset of genomics will pro-
vide massive amounts of information, but success will depend on using that information efficiently to im-
prove crop phenotypes [273]. Screening for and measuring important phenotypic traits are crucial to the
full exploitation of the opportunities offered by molecular marker technology.
A novel area that holds promise, once a physiological mechanism is identified, is the candidate gene
approach [274,275]. This detects expression of a tolerance response to the stress in question. The key re-
sponses may be those that occur days or weeks after exposure to a “realistic” stress, not minutes after the
imposition of a treatment, which is ecologically irrelevant [275]. The candidate gene approach would per-
mit much more precision, both in mapping of important genes and in defining the physiological basis of
yield improvement. One can envisage situations in which the improved adaptation to major abiotic con-
straints combined with adaptation to biotic constraints and improved nutritional quality could have a
tremendous impact on food security and human nutrition in the tropics [2,273,276].
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