Principles of Plant Genetics and Breeding

Greenhouse nursery
Breeders may use a greenhouse to help synchronize
flowering. Greenhouses enable plant breeders to manipulate
temperature, photoperiod, light, and other plant
growth environmental conditions to synchronize flowering.
Increasing the photoperiod tends to reduce days to
heading. A greenhouse nursery also enables the plant
breeder to move pots around to facilitate crossing.
Artificial pollination
Materials and equipment
Emasculation is needed in wheat crossing. The equipment
and materials used include scissors, curved and
straight-type forceps, soda straws, squeeze-type bottles,
paper clips, tags, and glassine bags (5 × 17 cm).
Emasculation
Preparation of the female for hybridization by emasculation
is done 1–3 days before normal anthesis. The anthers
at this stage should be light green but not yellow or
cream colored. The spikes to be emasculated should be
carefully selected. Further, the stigmas should be clearly
visible extending to about one-quarter the length of
the floret. One to three of the upper and basal spikelets
are usually non-functional and are removed with the
scissors. In awned varieties, the awns are clipped. Only
the primary and secondary florets are retained emasculated.
To emasculate, gently but firmly grasp the spike
between the thumb and index finger of one hand. Then,
lightly press the thumb below the tertiary floret and
pull the center florets downward and outward with the
forceps. The three anthers in each floret are removed by
carefully inserting a pair of forceps between the lemma
and palea and spreading them. Care should be exercised
to avoid damaging the stigma. Cover the emasculated
spike with a bag.
Florets may also be emasculated by cutting off about
one-third of the florets with the scissors to expose the
anthers, which are then removed with the forceps. This
procedure predisposes the florets to rapid desiccation
and is favored under cool and humid environments.
Another procedure entails cutting the primary and secondary
florets at about 5–7 days before anthesis. The
remaining parts of the anthers eventually degenerate
without forming viable pollen. This procedure is quick
but requires greater skill and experience for high success.
BREEDING WHEAT 481
Pollination
Emasculated flowers should be pollinated within 2–4
days for best results. Also, a mature and receptive stigma
is one with fully developed feathery features, whereas
recently extruded anthers provide good pollen. Forceps
are used to remove anthers from the florets. The glassine
bag is removed from the emasculated spike so the pollen
can be gently brushed on one or several stigmas. A soft
brush may also be used to transfer pollen to stigmas.
CIMMYT in Mexico developed a pollination technique
that is suitable when pollen is present in copious
amounts. First, the upper third of each primary and secondary
floret of the male spike is removed to allow the
mature anthers to freely extrude to release the pollen.
The top of the bag covering the female spike is cut to
allow the male spike to be inverted into the bag, parallel
to the female spike. The male spike is then vigorously
rotated by twirling the peduncle between the thumb
and forefinger. The bag is resealed, with the male spike
in place, for an additional day for enhanced pollination.
In another method, the approach method, the female
spike is positioned lower than the male spike and the
two covered with a glassine bag. Such maneuvering is
easier in the greenhouse. It can be used in the field if the
female and male plants are planted in close proximity or
the male spike is detached. To prolong the supply of
pollen, culms of the detached spikes can be placed in
water to keep them alive.
Natural pollination
As previously indicated, about 1–4% outcrossing occurs
in wheat, depending on the variety. To facilitate largescale
hybridization and eliminate the tedium of emasculation,
repeated backcrossing is used to convert breeding
lines into female CMS or A-lines. This scheme enables
breeders to undertake composite cross-breeding, production
of synthetics, and the hybridizations needed
for recurrent selection. However, the stability of the
male fertility-restoration gene is difficult to maintain,
being conditioned by several genes and affected by the
genetic background in which the genes occurs.
Seed development
A pollinated spike is properly targeted for identification.
Successful pollination and fertilization can be verified
after 3–5 days of pollination. Kernel development is