Principles of Plant Genetics and Breeding

are less successful, being of poor fertility and often
exhibiting undesirable alien traits. The technique of alien
chromosome additions has been used in an attempt to
reduce the undesirable effects introduced by the wild
species.
Genetics
As previously noted, dwarfing genes occur in wheat
and have been used in breeding to develop cultivars
with short stature (semidwarf wheat) (see Chapter
1). Early work in Japan produced dwarfing genes.
Designated Rht, over 20 dwarfing genes have been
identified, the most commonly used in wheat breeding
including Rht 1 , Rht 2 , and Rht 8 . The first two, called the
Norin 10 dwarfing genes, also belong to a group of
dwarfing genes called gibberellic acid (GA)-insensitive
dwarfing genes. Cultivars with these genes fail to
respond to the application of GA. Rht 3 and Rht 10 genes
confer extreme dwarfism on plants, the latter having
a greater effect. Practical application to commercial
breeding is yet to materialize. Rht 4 and Rht 8 plus others
are called the GA-sensitive dwarfing genes. Monosomic
analysis was used to locate the Rht 1 gene and Rht 2 gene
on chromosomes 4A and 4D, respectively. Chromosome
substitution can be used to transfer these genes in
breeding programs. The dwarfing genes increase grain
yield by increasing tillering and the number of seeds
per plant.
Other genes of interest in wheat breeding include
awnedness, pubescence, grain color, and glume color.
The awnedness trait is inhibited by three dominant
alleles at three independent loci. Hd conditions hooded
awn, while B 1 and B 2 condition awnless or tipped awned
phenotypes. A genotype of hdb 1 b 2 produces a bearded
or fully awned phenotype. Pubescence in the glume
and other parts of the plant is conditioned by a variety
of dominant alleles, e.g., Hg producing hairy glume,
while Hp conditions hairy peduncle. Red grain color
is conditioned by three independent dominant alleles
acting in additive fashion (R 1 R 2 R 3 ), while white grain
occurs when the genotype is r 1 r 2 r 3 . Consequently, when
all three alleles occur in one genotype, the seed color is
very dark red.
Anthocyanin pigmentation occurs in various parts of
the plant. For example, red auricles are conditioned by a
single dominant allele, Ra. The red color of glumes is
controlled by two dominant alleles, Rg 1 and Rg 2 , while
photoinsensitivity is controlled by alleles at three independent
loci, designated ppd 1 , ppd 2 , and ppd 3 .
BREEDING WHEAT 475
General botany
Wheat (Tricticum spp.) is an annual plant. It has a
spikelet inflorescence. A floret is composed of a lemma,
palea, and a caryopsis or grain that has a deep furrow
and a hairy tip or brush. The floret may be awned or
awnless. Awned varieties are common in regions of low
rainfall and warm temperatures. The presence of awns
also tends to influence transpiration rate, accelerating
the drying of ripe grain. Consequently, the tips of
awnless spikes tend to be blasted in hot dry weather.
The grain may also be amber, red, purple, or creamy
white in color.
Under normal high density production conditions, a
wheat plant may produce 2–3 tillers. However, when
amply spaced on fertile soils, a plant may produce
30–100 tillers. The spike (head) of a plant may contain
14–17 spikelets, each spike containing about 25–30
grains. Large spikes may contain between 50 and 75
grains. The grain size varies within the spikelet, the
largest being the second grain from the bottom and
decreasing in size progressively towards the tip of
the spike.
Wheat is predominantly self-pollinated. Anthers
assume a pendant position soon after the flower opens.
Blooming occurs at temperatures between 13 and 25°C
starting with the spikelet around the middle of the
spike and proceeding upwards and downwards. The
wheat kernel or berry is a caryopsis about 3–10 mm
long and 3–5 mm wide. It has a multilayered pericarp
that is removed along with the testa, nucellus, and
aleurone layers during milling. The endosperm makes
up about 85% of a well-developed kernel. Below the
aleurone layer occurs a complex protein called gluten
that has cohesive properties. It is responsible for the
ability of wheat flour to hold together, stretch, and
retain gas as fermented dough rises. This property is
available to the flour of only one other species, rye
flour.
Wheat is classified based on three primary characteristics
– agronomic, kernel color, and endosperm quality.
There are two seed coat colors – red or white. Red
is conditioned by three dominant genes, while the true
whites comprise recessive alleles of all three genes.
Most wheat varieties in the USA are red. Kernel hardness
is classified into two – hard or soft. Upon
milling, hard wheat yields coarse flour. White wheats,
lacking this starch–protein complex, produce a higher
yield of fine flour upon milling. Hard wheat is used
for bread-making because its gluten protein is cohesive
and elastic.