Principles of Plant Genetics and Breeding

usually results from one component with extreme value.
Furthermore, yield components are determined sequentially.
As such, they tend to exhibit yield compensation,
the phenomenon whereby deficiency or low value for
the first component in the sequence of developmental
events is made up by high values for the subsequent
components. The next effect is that yield is maintained
at a certain level. However, yield compensation is not a
perfect phenomenon. For example, it may occur over a
wide range of plant densities in certain species. In beans,
a reduction in pod number can be compensated for by
an increase in seed number per pod and weight per seed.
Concept of yield potential
A given crop has an inherent optimum capacity to perform
under a given environment. This capacity is called
its yield potential. It can be measured through yield
trials. It is the maximum attainable crop yield from a
specific soil–water regime under ideal production conditions
(experimental conditions whereby there is no limit
Introduction
BREEDING FOR PHYSIOLOGICAL AND MORPHOLOGICAL TRAITS 357
on access to any needed production input). It is suggested
that only about 20–40% of this yield potential
can be attained economically in actual production on
farmers’ fields. When a farmer has reached an economic
yield potential for the crop, attempts can be made to
increase field yields in a variety of ways. The farmer can
use production resources more efficiently; agronomic
innovations that are more responsive to local needs and
conditions can be introduced to the farmer by extension
agents; and the government may institute incentive
policies (e.g., credit) for farmers. However, improved
cultivars constitute perhaps the most effective approach.
To do this, a breeder would have to assemble appropriate
variability (genotypes with complementary genes
contributing to yield potential) and hybridize them to
generate transgressive segregates with superior yield.
Biotechnology can be used to develop new cultivars to
cope with the constraints to the rise in field yields (abiotic
and biotic stresses). Molecular markers may be used
to assist the breeding of especially complex traits while
recombinant DNA technology may be used to incorporate
desirable unique genes from unrelated sources.
Once a technology is proven successful in a crop (e.g. Roundup Ready® soybean; Padgette et al. 1996), researchers can theoretically
transfer the technology to other crops. Industry refers to this as product extension. Given the complexity of the crop and
trait, researchers determine how much additional optimization is required to achieve commercial success in subsequent crops.
This development process can take 5–8 years and involves many different aspects of science. Costs can approach US$40 million
(Context Network 2004) over this period of time, requiring researchers to be strategic, focused, and precise. The development
timeline and process described below occurred between 1997 and 2004 (Figure 1).
Product
concept
Gene
discovery
Sally Metz
Transformation
Greenhouse
and field
evaluations
Testing
Industry highlights
Bringing Roundup Ready® technology to wheat
Monsanto Corporation, 800 North Lindbergh Blvd, St Louis, MO 63167, USA
Line
selection
Regulatory
submissions
Variety
development
Management
practices
Grain
handling
Product
introduction
plan
Figure 1 The steps involved in the development and commercialization of Roundup Ready® wheat.
Commercial
launch