Principles of Plant Genetics and Breeding

Shatter-sensitive cultivars are susceptible to high loses
during harvesting. The physical contact of the harvesting
equipment with the plant may be enough pressure
to trigger shattering. However, most susceptible cultivars
spontaneously shatter their seeds when the environmental
condition is right (dry, sunny, and windy).
Shattered seeds are not only lost but also become a
nuisance when they germinate as volunteer plants in the
next year’s crop. Being weeds, the volunteer crops are
controlled at additional production cost.
Breeding grain-shattering resistance
Like lodging resistance, grain-shattering resistance is a
complex trait. There is a large variation in the degree of
shattering in existing rice gene pools, ranging from extremely
shattering to extremely hard to thresh. Furthermore,
researchers have identified at least five genes that
condition shattering in rice, including sh1, sh2, and sh4.
Reduced plant height
Modern production of certain cereal crops is dominated
by semidwarfs or dwarf cultivars (e.g., rice, wheat, sorghum).
These cultivars have advantages in mechanized
agriculture and high input production systems.
Nature, types, and economic importance
Reduced plant height is associated with, or promotes,
lodging resistance. Similarly, early maturity also reduces
plant internode length. Producers desire crop cultivars
with reduced plant stature because they are easier to
harvest mechanically. They produce less straw after the
economic product has been harvested. However, in
certain cultures, the straw is used for crafts or firewood,
and hence tall cultivars are preferred.
Reduced stature also increases the harvest index.
Dwarf cultivars can be more closely spaced in the field
for increased crop yield. These cultivars are also environmentally
more responsive, responding to agronomic
inputs, especially fertilizers, for increased productivity.
Genetics and germplasm resources
A variety of height-reducing genes have been discovered
in various species. These genes reduce plant height
when in the recessive form. In castor bean, the dwarf
internode gene, di, reduces plant height by 25–50%.
This reduction in height makes the plant sturdier and
BREEDING FOR PHYSIOLOGICAL AND MORPHOLOGICAL TRAITS 363
more lodging resistant. It reduces the swaying of the
pods in the wind thereby reducing shattering. In wheat,
the reduced stature plants are called semidwarfs. The
dwarfing genes in wheat are designated Rht (reduce
height), of which about 20 have been discovered. These
genes differ in their effects on the plant. For example,
Rht8, discovered in wheat cultivars from Yugoslavia, is
widely used because of its less adverse effect on kernel
density and weight. Furthermore, the wheat dwarfing
genes increase grain yield by increasing plant tillering
and number of seeds per plant. Sometimes, wheat
breeders intensify the height reduction by including
two different Rht genes in a cultivar. Such cultivars,
called double dwarfs, are shorter and tiller more than
single dwarfs. Using monosomic analysis, scientists have
associated Rht8 with chromosome 4A and Rht2 with
chromosome 4D.
Dwarfing genes have been used in rice cultivar development.
One set of genes designated d has been known to
reduce kernel size and grain yield along with reducing
internode length, and hence is not used in commercial
cultivar development. Instead, the sd1 gene, a spontaneous
mutant discovered in a Taiwanese cultivar, “Deegeo-woo-gen”,
has been used to develop successful
cultivars of rice. The gene has been induced by mutagenesis
and used in cultivar development. There are several
major dwarfing genes in oats, of which, so far, the
dominant Dw6 and semidwarf Dw7 are readily available
for use in breeding. However, the use of these genes in
improving lodging resistance has been limited because
their use results in reduced grain quality in many
environments.
Sorghum breeding has also benefitted significantly
from the discovery and use of dwarfing genes. Four
dwarfing genes – dw1, dw2, dw3, and dw4, have been
discovered. These genes produce a type of dwarfism
described as brachytic dwarfism, which reduces plant
stature without significantly affecting leaf number, leaf
size, plant maturity, or yield. The gene dw3 is observed
to be mutable (to Dw3), resulting in one tall mutant
occurring out of about every 600–1,200 plants in a
field of dwarf plants. These rogues must be removed
before harvesting to keep the seed as pure as possible.
Furthermore, the reduction in plant stature is dependent
upon the genotype with respect to the dwarfing
loci. Cultivars may have one dwarfing gene (e.g., Dw1,
Dw2, Dw3, dw4), two dwarfing genes (e.g., Dw1, Dw2,
dw3, dw4), three dwarfing genes (e.g., dw1, dw2, Dw3,
dw4), or four dwarfing genes (dw1, dw2, dw3, dw4).
Most sorghum cultivars in the USA are three-dwarfs
(have three dwarfing genes). They are superior in yield