available to breeders and can produce results much more rapidly than hybridization in plants with long
generation times. The third type (embryo rescue) includes “molecular” techniques that involve the in-
sertion and integration of a short segment of alien DNA into the plant genome. The process of insert-
ing and integrating DNA is known as genetic engineering, genetic manipulation, genetic modification,
transformation, or transgenesis.
Yield potential is defined as the yield of a cultivar when grown in an environment to which it is
adapted, with unlimited nutrients and water and with pests, diseases, weeds, lodging, and other stresses
effectively controlled [20]. Evans and Fischer [20] distinguished yield potential from potential yield,
which they defined as the maximum yield that can be reached by a crop in a given environment, as de-
termined, for example, by simulation models with plausible physiological and agronomic assumptions.
Evans [1] assessed the progress in yield potential for many crops in many environments by growing his-
torical series of leading cultivars side by side. Several studies indicated that both the rate of progress and
the extent of increase in yield potential have differed greatly among crops [20–25]. A major outcome of
these efforts was the realization that improvement in yield potential was greater for cereals and cotton
than for grain legumes and root and tuber crops.
Increases in yield in recent years, across all crops, owe as much to innovation and improvements in
agronomy as to plant breeding, more with some crops and less with others, more at some stages and less
at others [26]. Crop yields could continue rising because of agronomic innovation and improvement on
the one hand and breeding for improved stress resistance on the other, especially as the global environ-
ment changes. Breeders of a range of crops in most agricultural environments have devised technologies
for crossing and testing that have successfully improved yields. Miflin [2] pointed out that, in the excite-
ment of the tremendous advances in genetics across all organisms, it is important not to forget the role of
the environment in crop performance and that food comes from successful phenotypes.
B. Physiology of Yield Potential
Conceptually, high yield can be achieved by (1) maximizing the extent and duration of solar radiation in-
terception, (2) using the captured energy efficiently in photosynthesis, (3) partitioning assimilates in ways
that provide optimal proportions of economic product to other plant structures, and (4) maintaining those
plant organs at a minimum cost of energy [27]. A key requirement for achieving high and stable yields is
flexibility in morphogenesis and acclimation of physiological systems to overcome biotic and abiotic
constraints. A retrospective analysis of the physiological basis of genetic yield improvement in temper-
ate-climate maize indicated that a large proportion of yield improvement may be attributable to the ca-
pacity of newer hybrids to better tolerate stress conditions [25]. Increased stress tolerance in maize was
associated with lower plant-to-plant variability. Studies of the physiological basis of yield improvement
in soybean suggested that recently released cultivars not only supply more photoassimilates during the
seed-filling period than old cultivars but also display improved N 2 fixation and better tolerance of the
stress of high plant populations [28].
C. Identifying Important Physiological Traits
The effectiveness of selection for physiological traits depends on factors such as heritability, genetic cor-
relation between traits, inputs required for measuring a trait, intensity of selection, and the manner in
which the selection is integrated into the breeding program [29]. Studies of plant response to different cli-
matic and edaphic stress factors indicate that genetic variation is available for a number of important
physiological traits [6,11,16–18]. Plant breeders have tried to incorporate this genetic variation into cul-
tivars that exhibit whole plant stress tolerance. Most breeders are not convinced that selection based on
physiological traits will give better results, believing that improvements in field experimentation and
computerization will ensure continued success of empirical selection for stress tolerance.
Among the several reasons why breeders have seldom adopted physiological traits as selection cri-
teria [11,16] are that (1) the genetic control of stress tolerance is poorly understood; (2) if understood,
stress tolerance is often controlled by multiple genes; and (3) variation for stress tolerance usually exhibits
a large environmental component or large genotype-by-environment interaction, making direct selection
for a physiological trait in a single environment difficult. Yield increases associated with a particular trait
are small, and breeders have not been convinced that selecting for the trait is more efficient than select-
ADAPTATION OF BEANS AND FORAGES TO ABIOTIC STRESSES 585