Principles of Plant Genetics and Breeding

520 CHAPTER 31
the US from China, India, Manchuria, Korea, Taiwan,
and Japan. In 1852, J. J. Jackson is reported to have first
planted soybean as an ornamental plant in Davenport,
Iowa. Most of the production of soybean in the US
prior to the 1920s occurred in the southern states,
mostly for hay, and then spread to the Corn Belt after
about 1924.
Soybean is a subtropical plant. However, it is grown
over a wide range of ecological zones, ranging from the
tropics to 52°N. Its climatic requirements are similar to
those of corn.
History of breeding in the USA
Soybean was introduced into the USA in about the late
1700s. Initially, the crop was grown primarily as a forage
species. The early introductions had weedy characteristics.
They also shattered profusely, making them unsuitable
for mechanized production. Producing soybean
for seed started in the 1920s. Formal soybean research
was initiated as a joint activity between the US Department
of Agriculture (USDA) research wing and the
Agricultural Experiment Stations of land grant universities
in the midwest and southern states. In 1936, a cooperative
state and federal program was established at the
University of Illinois, Champaign/Urbana, to conduct
soybean research. This program is now known as the US
Regional Soybean Laboratory.
Seed companies became involved in soybean breeding
as a result of the United States Plant Variety Act of
1970. With this protection against unauthorized use of
proprietary material, seed companies could make money
developing and selling improved seed. Modern cultivars
were developed for seed, shifting the emphasis away from
soybean as a forage crop to soybean as an oilseed crop.
Cultivars
Soybean may be produced for forage, the varieties for
this purpose generally being small seeded, finer stemmed,
and very leafy. There are also cultivars for edible, dry,
or green-shelled beans. These cultivars usually have
straw-yellow or olive-yellow seed and a yellow, brown,
or black hilum. Soybean grown for grain is grouped into
13 maturity classes, ranging from 000 to X. The 000
group consists of the earliest maturing cultivars while
the X group consists of the latest maturing cultivars.
Groups 000–IV are considered indeterminate, while
groups V–X are determinate cultivars. Further, early
maturing cultivars (000–IV) are adapted to the northern
production regions, while those with high maturity
class designation are adapted to southern production
regions. The 000 cultivars are adapted to the short
summer growing seasons of northwest USA and Canada.
Cultivars in groups II and III are best adapted to the
midwest growing area. Maturity groups VIII and higher
are grown in the southern or coastal plain counties.
Germplasm resources
The USDA maintains about 15,000 accessions of G. max
and a smaller number of other Glycine species at Urbana,
Illinois, and Stoneville, Mississippi. Soybean accessions
are also held by the Applied Genetics of Korea Atomic
Energy Research Institute in Seoul, Korea, the National
Institute of Agricultural Sciences at Hiratsuka, Japan,
and the Asian Vegetable Research and Development
Center, Taiwan.
Cytogenetics
The genus Glycine has two subgenera – Soja and Gycine.
The subgenus Soja consists of two species: G. max (2n =
2x = 40), the cultivated species, and G. soja (L.) Sieb
or G. ussuriensis (2n = 2x = 40), a wild species. These
two species are cross-fertile. There are 15 wild species of
soybean of which G. tabacinia and G. tomentella have
polyploidy forms (including 2n = 4x = 80).
Genetics
Several hundreds of genes for qualitative traits have
been identified for soybean. Among them are four recessive
genes for genetic male sterility, designated ms 1 , ms 2 ,
ms 3 , and ms 4 . Linkage groups have been identified for
13 of the 20 soybean chromosomes. Soybean has pigmentation
on various parts of the plant. The black and
brown seed coat and hilum color are conditioned by two
gene pairs, Tr and Rr. The expression of these genes is
modified in some conditions by brow seed coat (TrO),
which is dominant over reddish-brown color (Tro), and
green seed coat (G), which is dominant over yellow seed
coat (g).
Soybean may have either brown (tawny) or gray
pubescence, the trait being conditioned by a single gene