Principles of Plant Genetics and Breeding

362 CHAPTER 19
kind of adaptation has its value. A cultivar that is responsive
to the environment may be released for a narrowly
defined area of adaptation, whereas another that has a
low G × E interaction may be suitable for release for use
over a wider region of production.
Lodging resistance
Lodging resistance may be defined as the resistance
to leaning, bending, or breaking of the plant prior to
harvesting.
Nature, types, and economic importance
There are two basic types of lodging that may be caused
by biotic or abiotic factors. Lodging may originate at the
root level (root lodging) or at the stem or stalk level
(stalk lodging). Soil-borne disease and insect pests may
destroy plant roots, causing the plant to lean over starting
at the root level. Disease and insect attack can also cause
the stem or stalk to lodge. For example, the European
corn borer weakens the stalk and predisposes it to bending.
Strong winds and other weather factors such as hail,
ice, or snow, are common causes of lodging in plants.
Certain plant characteristics make plants susceptible
to lodging. These include tall plant height, thin stems,
excessive vegetative growth, and succulence. In addition,
the crop cultural environment may encourage
lodging. For example, high nitrogen fertilization promotes
luxuriant vegetative growth, and hence excessive
amounts produce top-heavy and succulent plants that
are also prone to disease and insect attack. Stems that
have been attacked by pests are weak and lodge easily.
Breeding for lodging resistance is important because
lodging results in yield reduction. Lodging prior to
pod filling results in partial fruit or seed development.
Lodging at maturity may also make pods or cobs inaccessible
to mechanized harvesters (combines) and hence
left unharvested. Lodged plants are exposed to disease
infestation. For example, in cereal crops such as wheat,
contamination by mycotoxins produced by Fusarium
species may increase.
Genetics and breeding
Breeding for lodging resistance is challenging, partly
because it is a quantitative trait and hence conditioned by
many genes. Furthermore, its expression is significantly
impacted by the environment. It is difficult to score for
lodging resistance on a phenotypic basis in the field
because the factors that cause lodging may occur at different
stages in plant development, or never at all. Over
the years, researchers have tried to find above-ground
morphological traits that strongly correlate with lodging
resistance in various species. Even though culm
parameters (e.g., weight, diameter) have been reported
to correlate significantly with lodging resistance in wheat,
such an observation is not universal. Consequently,
developing a reliable morphological index for field
selection in a breeding program is difficult.
Nonetheless, resistance to lodging may be improved
by targeting a combination of the following traits
(depending on species) – short stature, stronger stalk,
sturdy stem, thick rind, stiff culm, strong root system,
resistance to stalk or stem diseases and insect pests, and
other traits. It should be pointed out that no single trait
or group of traits has proven to be universally reliable as
an index of lodging resistance.
To improve selection, QTLs for lodging resistance
have been identified and used as molecular markers in
some breeding programs using marker-assisted selection.
Furthermore, dwarfing genes have been used to
breed short-statured cultivars in crops such as rice
and wheat, resulting in increased lodging resistance. In
corn, for example, recurrent selection has been used to
improve lodging resistance. The success of the Green
Revolution is attributed to the development of semidwarf
cultivars that were high yielding and responsive to
the environment without lodging.
Shattering resistance
Dry fruits that split open upon maturity to discharge
their seeds are called dehiscent fruits. Whereas shattering
is advantageous in the wild, it is undesirable in
modern crop production.
Nature, types, and economic importance
Some fruits split along only one side (called a follicle),
while others split along two sides (called a legume), or
multiple sides (called a capsule). This natural mechanism
of seed dispersal has adaptive value to plants in the
wild. In crop production, the splitting of dry fruits to
release their seeds prior to harvesting is called grain
shattering. In serious cases, some cultivars can loose
over 90% of their seed to shattering if harvesting is
delayed by just a few days. Whereas shattering is often
identified with pod-bearing species (e.g., soybean, pea),
it also occurs in cereal crops such as wheat and rice.