Principles of Plant Genetics and Breeding

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Artificial pollination
Equipment and materials
The equipment and materials used in soybean
hybridization include fine-tip forceps, tags, a bottle with
alcohol, Petri dish, pencil, and headband-mounted
magnifiers.
Emasculation
Soybean is emasculated in the bud stage. The suitable
buds are those ready to open the following day. At this
stage the corolla is visible at the tip of the bud. To emasculate,
the flower is grasped between the thumb and
index finger, keeping the hand steady to avoid snapping
the pedicel. The calyx is removed with a pair of forceps
by grasping the sepal and pulling with the forceps, and
carefully pulling up while gently wiggling the forceps. It
is important to remove all unemasculated flowers from
the leaf axil. The leaf axil is tagged. If more that two cultivars
are involved in the crossing program, the forceps
should be sterilized between emasculations by dipping
the tip in a bottle of alcohol.
Pollination
Pollination immediately follows emasculation. Recently
opened flowers have fresh pollen, and suitable flowers
from the pollen source are picked. The stamen column
and the style are together removed with a forceps, and
used as a brush to dust pollen onto the stigma of the
female flower. When a flower yields copious amounts of
pollen, more than one stigma may be pollinated with
one male stamenal column.
Under humid conditions, as often occurs in the
southern US, anther dehiscence occurs in the morning
when the female parts are less mature and more difficult
to emasculate and the flowers may also be damp from
the morning dew. The male flowers may be picked into
Petri dishes and stored in desiccators until the afternoon.
Pollen remains viable for about 2 days when
stored in a cool dry place.
Pollinated flowers are tagged. The success rate of pollination
depends on the skill of the operator and can
range from zero to about 40%. The failure of a cross may
be due to factors including immature female, injury to
the stigma, injury to the pedicel, high temperature, and
inadequate pollen.
Natural pollination
Natural pollination is facilitated by the use of genetic
male sterility. Breeders using recurrent selection method
may benefit from this pollination method.
Seed development and harvesting
Signs of successful cross are visible within 7 days of
pollination. Sometimes, new pods may develop in the
axis where the artificially pollinated flowers occur. These
should be removed. A successful cross can be identified
by the presence of the calyx scar resulting from the
emasculation process.
Mature soybean should be harvested on time to avoid
field loses. The crop is ready to harvest when the seeds
are at the hard-dough stage. The moisture content of
the seed should be 12–14%. Drier seed (less than 12%)
increases the incidence of seed coat cracking and
splitting and shattering of the seed. The crop may be
harvested at high seed moisture content (17–18%)
provided postharvest drying is available.