Agriculture Reference
In-Depth Information
(a)
(b)
Figure 9.5
Artificial hybridization in canola breeding (a) and wheat breeding (b). Note that pollination bags cover racemes
and ears that have been pollinated so as to avoid unwanted crosses.
Artificial pollination therefore demands that nat-
urally inbreeding lines (or lines which are self-
compatible) be
emasculated
to avoid self-pollination.
Emasculation in most crop species can be achieved by
manually removing the male plant parts (i.e. anthers)
before they are mature and pollen is dehisced. In some
cases it is possible to use chemical emasculation where
specific chemicals applied at the critical growth stage
will render the plants male sterile. Chemical emascula-
tion is, however, not widely used in routine breeding
and mechanical emasculation is used almost exclusively
employed as a means of avoiding selfing in crossing
designs.
After the chosen female plants have been emascu-
lated, within a few days pollen from the male parents can
usually be applied to the receptive female stigma. Pollen
can be transferred manually, often using a small paint
brush or by removing dehisced male parent anthers
and brushing pollen onto the female stigma. Cross-
pollination can also be achieved simply by having
emasculated females grown in close proximity to male
flowers and allowing pollen to naturally pass from male
to female. When this is to be done it is common to place
emasculated female flowers and pollen fertile male flow-
ers together within a pollination bag to ensure that the
designed hybridization occurs and to avoid the female
from being pollinated by stray pollen which may, for
example, be blown in the air. If necessary, within these
bags, suitable pollinating insects can also be placed to
help pollination efficiency
In several crop species there are self-incompatibility
systems, which have developed naturally to maximize
heterozygosity of plants within the species (e.g. as
exists in many
Brassica
species). Similarly, many crop
species have male sterility systems (either nuclear or
cytoplasmic in inheritance), which can be utilized in
cross-pollination systems. In both these cases it is not
necessary for the female parents to be emasculated to
guarantee cross pollination.
Irrespective of the breeding system, it is common
to place pollination bags over flowers either prior to
pollination (to avoid unwanted crosses) or after polli-
nation (to ensure that no further pollination takes place)
(Figure 9.5). It should be remembered that bagging
crosses will be time consuming and that if not done
carefully can have an adverse effect on the potential
success of the artificial hybridization. Damage can be
caused during the bagging operation or the bag may
create an environment unsuitable for seed production.
It also is important to label carefully at each stage,
otherwise the origin of any seed produced may be in
doubt.
With many crop species, particularly crops which are
clonally propagated and where the end product does
not involve the botanical seed (e.g. potato, banana,
sugarcane) it is not always easy to have parental lines
develop sexually reproductive parts. In a number of
instances flower induction can be achieved by manip-
ulation of environmental conditions by adding or
reducing nutrient levels manipulation of day length
