Principles of Plant Genetics and Breeding

Common breeding objectives
1 Grain yield. Soybean grain yield is a major breeding
objective. Progress has been made, but not at the rate
achieved in cereal crops such as corn. The major yield
components are numbers of nodes per plant, number
of pods per node, number of seeds per pod, and seed
size. Adapting soybean to new production environments
is followed by improvement in yield potential
for cultivars in those new regions.
2 Agromorphological traits:
(a) Lodging resistance. Lodging resistance is critical
to mechanized production. An attempt to decrease
plant height to improve lodging resistance by
using the dt 1 gene for determinate growth, produced
genotypes that were shorter but lower in
plant yield.
(b) Shattering resistance. Shattering resistance is
important for production to allow the crops to
attain full maturity before harvesting. Sometimes,
inclement weather may force harvesting to be
delayed even after the crop is mature. The mechanical
harvesting process involves physical contact
with moving parts of the combine, which
may promote shattering.
3 Adaptation:
(a) Maturity. Thirteen maturity groups have been
identified for soybean. Late maturity in soybean
has been found to be conditioned by three
dominant genes, E 1 , E 2 , and E 3 . Another gene,
E 4 , has been found to condition sensitivity to
long photoperiods.
(b) Drought and other environmental stresses.
Drought resistance is important in areas where
production is rainfed. Nutrient stress and heat
stress may be important in areas where the production
environment is not ideal.
(c) Herbicide resistance. The modern technology
of genetic engineering has enabled herbicide-
Carlson, J.B., and N.R. Lersten. 1987. Reproductive morphology.
In: Soybeans: Improvement, production, and
uses, 2nd edn (J.R. Wilcox, ed.), pp. 95–134. American
Society of Agronomy, Madison, WI.
Chen, Y., and R.L. Nelson. 2004. Genetic variation and relationship
among cultivated, wild, and semiwild soybean.
Crop Sci. 44:316–325.
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References and suggested reading
tolerant cultivars to be developed for commercial
production (e.g., Roundup Ready®).
4 Disease resistance. Soybean is plagued by numerous
diseases, the major ones including the following:
(a) Bacterial blight. Caused by Pseudomonas
syringae, this pathogen occurs worldwide.
Resistance to the disease has been incorporated
into various breeding programs.
(b) Rots. Phytophthora root rot is the most important
of the rots affecting soybean. Caused by
P. megasperma, this problem affects all stages of
development. Resistance is conditioned by six
dominant genes, Rps 1 –Rps 6 .
(c) Viral disease. The main viral disease of soybean
is the soybean mosaic virus (SMV). It is spread by
aphids. Genes for resistance, Rsv 1 and Rsv 2 , have
been discovered for SMV.
(d) Nematodes. The major species of nematodes of
importance to soybean producers are the cyst
nematode (Heterodera glycines) and root knot
nematode (Meloidogyne spp.). Three recessive
genes, rhg 1 , rhg 2 , and rhg 3 , and one dominant
gene, Rhg 4 , have been identified and incorporated
into breeding programs to develop resistant
cultivars.
5 Insect resistance. Major insect pests of soybean
include the southern green stink bug and beanflies,
which are common in Asia and Africa.
6 Seed compositional traits and quality:
(a) Oil quality. Soybean is the world’s leading
source of vegetable oil, accounting for more than
75% of the market share. Breeding objectives
include an increase in oil content as well as
improvement in oleic acid and reduced linolenic
acid for high oil quality.
(b) Seed protein. Soybean is also the leading source
of protein meal. Because seed oil and seed
protein are negatively correlated, breeding has
tended to focus on developing cultivars with a
high protein and low oil content.
Kisha, T., C.H. Sneller, and B.W. Diers. 1997. Relationship
between genetic distance among parents and genetic variation
in populations of soybean. Crop Sci. 33:1313–1325.
Specht, J.E., D.J. Hume, and S.V. Kumudini. 1999. Soybean
yield potential – A genetic and physiological perspectives.
Crop Sci. 39:1560–1570.