Principles of Plant Genetics and Breeding

354 CHAPTER 19
accumulation. Of course, this will have to be done
within reasonable agricultural limits, as dictated by
weather and cropping sequence. Genotypes can be
adapted to new growing conditions (e.g., cold tolerance
to allow the farmer to plant earlier than normal).
2 Tolerance of adverse environmental factors.
Because of the vagaries of the weather and the
presence of other inconsistencies or variation in the
production environment (climate, product management,
etc.), biomass can be enhanced by breeding for
tolerance to these factors. Such breeding efforts may
be directed at developing tolerance to abiotic stresses
(e.g., drought, heat, cold). This would allow the cultivar
to produce acceptable yields in the face of moderate
to severe adverse environmental conditions.
3 Pest and disease resistance. Diseases and pests can
reduce biomass by killing plant tissue (or even an
entire plant in extreme cases), and stunting or reducing
the photosynthetic surface of the plant. Diseaseand
pest-resistance breeding will enhance the biomass
potential of the crop. Breeding to control pests is one
of the major undertakings in plant breeding.
Ideotype concept
Plant breeders may be likened to plant structural and
chemical engineers who manipulate the genetics of
plants to create genotypes with new physical and
biochemical attributes for high general worth. They
manipulate plant morphology (shape, size, number of
organs) to optimize the process of photosynthesis,
which is responsible for creating the dry matter on
which yield depends. Once created, dry matter is partitioned
throughout the plant according to the capacity
of meristems (growing points of the plant) to grow.
Partitioning (pattern of carbon use) is influenced by
both intrinsic (hormonal) and extrinsic (environmental)
factors. Certain plant organs have the capacity to act as
sinks (importers of substrates) while others are sources
(exporters of substrates). However, an organ may be a
source for one substrate at one point in time and then a
sink at another time. For example, leaves are sinks for
nutrients (e.g., nitrates) absorbed from the soil while
they serve as sources for newly formed amino acids.
Plant genotypes differ in patterns for partitioning of
dry matter. This means plant breeders can influence dry
matter partitioning. Pole (indeterminate) cultivars of
legumes differ in patterns of the partitioning of dry
matter from bush (determinate) cultivars. Similarly, in
cereal crops, tall cultivars differ from dwarf cultivars in
the pattern of dry matter partitioning. The concept of
the plant ideotype was first proposed by C. M. Donald
to describe a model of an ideal phenotype that represents
optimum partitioning of dry matter according to
the purpose for which the cultivars will be used. For
example, dwarf (short statured) cultivars are designed
to channel more dry matter into grain development
whereas tall cultivars produce a lot of straw. Tall cultivars
are preferred in cultures where straw is of economic
value (e.g., for crafts or firewood). Consequently, ideotype
development should target specific cultural conditions
(e.g., monoculture, high density mechanized
production, or production under high agronomic
inputs). All breeders, consciously or unconsciously, have
an ideotype in mind when they conduct selection within
a segregating population.
Plant morphological and anatomical traits (e.g., plant
height, leaf size) are relatively easy for the breeder
to identify and quantify. They do not vary in the short
term and also tend to be highly heritable. Consequently,
these traits are most widely targeted for selection by
breeders in these programs. The wheat ideotype defined
by Donald comprised the following:
1 A relatively short and strong stem.
2 A single culm.
3 Erect leaves (near-vertical).
4 A large ear.
5 An erect ear.
6 Simple awns.
It is not practical to specify every trait in modeling an
ideotype. The degree to which a plant model is specified
is left to the discretion of the breeder. A more accurate
ideotype can be modeled if the breeder has adequate
information about the physiological basis of these traits.
The group of traits used to define the ideotype presumably
are those that would contribute the most to crop
economic yield under the range of environmental and
management conditions that the crop would encounter
during its life. As previously indicated, physiological
processes are common to all plants, but there is no
universal ideotype in plant breeding. This is because
of the vast morphological and physiological diversity
of crops, and the wide range in their economic end
products as well as the cultural environments.
The role of partitioning in determining crop yield
depends on the species and the products of interest. In
forages, the total above-ground vegetative material is
harvested as the end product. The importance of partition
in the economic yield in this instance is small. In
other crops, the desired product is enhanced at the