Principles of Plant Genetics and Breeding

476 CHAPTER 27
Reproductive biology
Floral biology
Wheat has a determinate, composite spike inflorescence.
Each spike bears 10–30 spikelets, which are borne singly
at nodes on alternate sides of a zig-zag rachis. A spike
may be awnless, awnleted, or awned. A spikelet consists
of 1–5 flowers (or florets) attached alternatively to
opposite sides of the rachilla (central axis). Except in
some club wheats, only two or three kernels mature,
because one or more of the upper florets are usually
sterile. A spikelet is subtended by a pair of empty bracts
and glumes.
A floret consists of a lemma and palea, which enclose
these stamens and a pistil, plus two lodicules that regulate
the opening of the flowers and anthers. Wheat
flowers bloom under temperatures of 13–25°C. The
flowering is usually diurnal, the highest peak occurring
in the morning, and a lower peak in the afternoon.
Blooming begins in the spikelets located above the
middle of the spike and proceeds both upward and
downward. It takes about 2–3 days for a wheat spike
to complete blooming, after the appearance of the
first anthers. The flowering period may last from 14 to
21 days.
Pollination
Wheat is predominantly self-pollinated with about
1–4% natural cross-pollination. Pollen shed usually
starts inside the floret, but about 80% of anther dehiscence
occurs outside the floret. The primary and secondary
florets produce larger and more viable pollen
grains than other florets. Wheat pollen remains viable
for up to about 30 minutes after shedding. Once pollinated,
the pollen tube growth starts within 15–60
minutes. Even though the stigma remains receptive
for up to 13 days, it is most receptive within 3 days of
anthesis. Xenia may occur when plants with the blue
aleurone trait are used as males in a cross.
Common breeding methods
A sample of some of the steps used at CIMMYT are
summarized below as an example.
F 1 Make simple crosses. Evaluate on the basis of disease
resistance, agronomic traits, and hybrid vigor. Bulk
and harvest seed for F 2 .
F 2 Space plant 2,000–3,000 F 2 under optimal conditions
(high fertility, moisture). Select plants based
on disease resistance, lodging, tillering, maturity, etc.
F 3 Grow progeny rows in 2 m long three-row plots at
dense spacing. Select desirable plots and then select
and bulk the best heads in each plot. Selection environment
is variable (irrigated, rainfed, acid soil, etc.)
F 4 Grow selected plants in dense-planted rows and
treat as in F 3 .
F 5 Space plant 100 plants per plot of selected F 4 families
under favorable conditions. Evaluate on the basis of
disease resistance, desirable agronomic traits, and
spike fertility.
F 6 Grow selected plants individually as F 6 plots of three
rows, 2 m long. Select and bulk agronomically superior
lines for yield testing under various conditions
(irrigated, rainfed, hot climate, acid soil, etc.).
Various approaches are adopted in wheat improvement.
Some cultivars are developed though the introduction
of genotypes and adapting them to new
production environments. Evaluation of germplasm is
also a way of identifying genotypes for use as parents
in future breeding.
Modern wheat breeding depends primarily on
hybridization to create variability for selection. Being a
self-pollinating species, pure-line selection is often used
in wheat improvement. As needed, backcrossing may
be used to introgress desirable genes into existing commercial
cultivars. Many traits in wheat are influenced by
several genes rather than one or two, because wheat is a
polyploid species. Consequently, it is uncommon for
breeders to observe unexpected phenotypes in the F 1 .
Hybridization may also bring together the three independent
complementary genetic systems that condition
lethals, partial lethals, or reduced productivity in the F 1 .
Specific undesirable traits resulting from hybridization
include hybrid chlorosis and grass-clump dwarfness.
Hybrid necrosis is conditioned by a complementary
two-gene, multiallelic system, whereas hybrid chlorosis
is controlled by a complementary two-gene system.
Genotypes with these undesirable genes are known.
Breeders can reduce the incidence of these F 1 defects by
carefully selecting parents for hybridization. The incidence
of grass-clump dwarfness may be reduced by growing
plants under high temperature and also using GA.
Hybrid wheat for commercial production has not
been a practical breeding approach because of lack of
sufficient heterosis upon crossing and low seed set.
Other practical problems include the complexity of fertility
restoration in wheat hybridization. Several major
and minor genes are involved.