Principles of Plant Genetics and Breeding

and brown being dominant over gray. Stem termination
is controlled by two genes, Dt 1 and Dt 2 , with Dt 1 conferring
intermediate stem while Dt 2 conditions semideterminacy,
with dt 1 conditioning determinancy. Dt 1
and Dt 2 are dominant to dt 1 and dt 2 , respectively; dt 1
is epistatic to Dt 2 and dt 2 . Pod color at maturity is
controlled by two genes: L 1 L 2 and L 1 l 2 produce black
pods, while l 1 L 2 produce brown color. Tan pods are
conditioned by l 1 l 2 . Six independent recessive genes df 1
to df 6 are known to condition dwarfism in soybean,
while five major and independent genes, E 1 to E 5 , condition
flowering and maturity, with E 1 and E 2 delaying
flowering and maturity.
General botany
Soybean (Glycine max (L.) Merrill) is an annual summer
legume. Cultivated soybean is usually erect with a
well-defined main stem and branches, and numerous
leaves. Both determinate and indeterminate cultivars
are used in production. The leaves and stems are
usually pubescent. The flowers are either purple or
white, and are borne in axillary racemes on peduncles
at the nodes. The plant produces a large number of
flowers, but only about two-thirds to three-quarters
of them produce pods. The pods are also pubescent.
They range in color from light straw to black, containing
1–4 seeds (occasionally five). The seeds are
usually unicolored and may be straw-yellow, greenishyellow,
green, brown, or black. Bicolored seeds exist,
such as yellow with a saddle of black or brown. The
hilum is also colored in various patterns – yellow, buff,
brown, and black. Soybean is primarily self-pollinated.
In the proper soil environment, soybean is infected
by the bacterium Rhizobium, resulting in roundish
nodules on the roots in which the nitrogen-fixing bacteria
live.
Reproductive biology
Floral biology
Soybean flowers are borne in the axil of a branch in
clusters of 3–15. Soybean has the floral features of the
family Papillionadae, comprising a large standard petal,
two wing petals, and two keel petals. The five petals
enclose a pistil surrounded by 10 stamens, nine of which
are fused into a tube, the one nearest the standard being
BREEDING SOYBEAN 521
free. The style curves towards the standard petal bearing
a club-shaped stigma.
Pollination
The soybean flower is cleistogamous, with self-pollination
occurring shortly before or after the flower opens.
Pollen shed depends mainly on temperature. Under
warmer conditions (30°C or higher) pollen shed may
occur before 7–9 a.m. Anther dehiscence under moderate
temperature starts later in the morning and continues
for much of the day. Cool temperature reduces
pollen shed and often causes flowers to prematurely selfpollinate
before the breeder can emasculate the stem.
Common breeding methods
As previously indicated, plant introductions played a
significant role in early soybean breeding programs.
Selections were made from introductions to develop
commercial cultivars. Modern soybean breeders use a
wide variety of methods in their programs, including
backcrossing to transfer qualitative traits and single-seed
descent to accelerate breeding programs. Pedigree
selection is also used because the plants can be well
spaced for individual observations.
Hybridization is most commonly used in soybean
breeding programs for gene transfer. Both qualitative
and quantitative traits have been improved in breeding
programs using hybridization to create new variability
for selection. Recurrent selection is possible if male
sterility is incorporated into the breeding program.
Establishing a breeding nursery
Field nursery
To facilitate crossing in the field, the two parents to be
crossed are planted in opposite rows with wide spacing
(65–100 cm) between them for the operator to move
freely. Having the two parents to be crossed planted side
by side facilitates that crossing process and reduces the
chance of crossing the wrong parents.
Greenhouse nursery
Soybeans can be readily hybridized in the greenhouse.
Potted plants can be moved around and positioned at
levels that are convenient for the crossing operation.