ing for yield. Furthermore, stress tolerance at one developmental stage does not always confer tolerance
at another stage. In addition, many methods proposed by physiologists to monitor stress tolerance are
based on the performance of individual cells, tissues, organs, or individual plants and do not provide a
good indication of the whole plant response to stress when grown in a spaced-plant nursery or in a com-
petitive environment in the field. Ceccarelli et al. [30] argued that selection for a single trait is often un-
successful, particularly in unpredictable environments where the frequency, timing, and severity of
stresses are unknown.
Simulation modeling can make an important contribution to improving plant adaptation to stressful
environments. Our ability to assess accurately the interaction of numerous processes over a crop’s life cy-
cle is limited, and the development of models can remove much of the “hunch taking” in selecting relevant
physiological traits for genetic manipulation [31,32]. Seed yield can be described as the rate of photosyn-
thate accumulation, the intensity or fraction of current assimilate allocated to seed, the duration of pho-
toassimilate partitioning to seed, and the extent of remobilization of previously assimilated materials to the
seed. Boote and Tollenaar [33] used crop growth simulation to evaluate hypothetical yield response to
many genetic traits. Using a modeling approach, they made a systematic evaluation of the importance of
plant traits as they affect the five “P’s” of yield potential: prior events (vegetative canopy with sufficient
tillering and fruiting sites), photosynthesis, partitioning, pod- or grain-filling period, and prior accumula-
tion and remobilization of photosynthates and minerals. They found that of the five P’s listed, duration of
the pod-filling period is the most likely to account for past, present, and future yield increase. They sug-
gested that yield improvement could also come from increased stress tolerance to the extent that photo-
synthesis is maintained, seed fill is longer, and mobilization is slower.
D. Integrating Physiological Tools and Molecular Genetics for Crop
ImprovementThe use of genetics in plant biology aims at the physiological and molecular genetic characterization of
the phenotypic variation for the trait under study [34]. Testing possible associations between physiologi-
cal and biochemical traits by comparing plant phenotypes and looking for correlations between them is
not highly reliable [35]. Advances in molecular marker technologies offer powerful alternative methods
to examine the relationships between traits. Using these techniques, it became clear that even for highly
complex traits such as crop yield, a small number of QTLs explained a large part of the genetic variabil-
ity [36,37]. Information from various genetic linkage maps will have to be integrated to facilitate com-
parison between detected QTLs and known major genes on the conventional genetic map [38].
The combination of genetics and plant physiology allows genetic markers to be associated with spe-
cific responses to stress [39]. Abiotic stress work on gene pools of small-grain cereals such as barley fre-
quently shows that adaptive and developmental genes are strongly associated with the stress response [40].
Using barley as a model plant for application of molecular markers, Forster et al. [40] expressed concern
that much of the genetic variation for improving abiotic stress tolerance has been lost during domestica-
tion, selection, and modern breeding, leaving pleiotropic effects of the selected genes for crop develop-
ment and adaptation. Their work indicated that transfer of such genes from primitive landraces and related
wild species is critical in matching improved cultivars to their targeted agronomic environments. The ap-
plication of marker technologies to the redomestication of crops by exploiting the potential gold mine of
favorable alleles existing in the crop’s wild relatives provides the best relatively short-term opportunity for
achieving the necessary advances in crop performance [2,41].
The ability to map DNA sequences physically to specific locations on a chromosome has advantages
over more widely used genetic mapping procedures [42]. Stuber et al. [43] indicated that new investiga-
tions, using DNA-based marker technology as a tool for plant geneticists and plant breeders, will continue
to add evidence on the projected role of markers, not only for identifying useful genes (or chromosomal
segments) in various germplasm sources but also for transferring these genes into desired cultivars or
lines. They also pointed out that the synergy of empirical breeding, marker-assisted selection, and ge-
nomics will “produce a greater effect than the sum of the various individual actions.”
The need for integrating the knowledge available for different crops has never been greater. Im-
provements in crop simulation techniques and in the understanding of crop genetics suggest the possibil-
ity of integrating genetic information on physiological traits into crop simulation models [44]. In view of
the increasing demand for food by the world’s growing population, the development and improvement of
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