Principles of Plant Genetics and Breeding

510 CHAPTER 30
short, day-neutral genotypes for use in temperate regions,
starting with 1,500 accessions. A large part of this material
is also maintained in long-term storage at the National
Seed Storage Laboratory, Fort Collins, Colorado.
Germplasm resources
In the USA, sorghum germplasm is maintained at
the Regional Plant Introduction Station, Experiment,
Georgia, with duplicate samples stored at the National
Seed Storage Laboratory, Fort Collins. A more comprehensive
collection with accessions in excess of 25,000 is
maintained at the International Crops Research Institute
for the Semi-Arid Tropics (ICRISAT) in India.
Cytogenetics
Sorghum comprises three known ploidy levels: x = 5,
x = 10, and x = 20. Important sorghum species are:
S. bicolor (L.) Moench (2n = 2x = 20) S. propinquun
(Kunth) Hitche (2n = 2x = 20), and S. halepense (L.)
Pers. (2n = 4x = 40)
Of these three, the most important to crop production
is S. bicolor. Grassy species such as S. arundinuceuin,
S. verticillijlorum, and S. aethiopicum have the
same chromosome number (2n = 2x = 20), and can be
crossed with S. bicolor. Sorghum has also been successfully
crossed with sugarcane and corn.
Cultivated races are readily crossable with each other
to produce fertile hybrids. In sorghum production,
natural interspecific crossing between S. bicolor and
S. halepense is a common source of off-types in the field.
Two types of products arise from this type of cross –
sterile plants with 30 chromosomes and fertile plants
with 40 chromosomes, the former often persisting in
the field as perennial weeds.
Cytogenetic male sterility (CMS) and fertilityrestoration
systems have been discovered in sorghum
and are used in hybrid sorghum production. The CMS
in sorghum genotypes was developed by backcrossing
chromosomes of kafir into the cytoplasm of milo.
Similarly, genetic male sterility (ms) has been discovered
in sorghum male-sterile plants (Msms).
Genetics
Midrib color is controlled by a single dominant gene
(D), whereas resistance of hydrogen cyanide (HCN)
appears to be controlled by more than one factor.
Researchers have determined that grain color in sorghum
is influenced by pericarp color, pericarp thickness,
presence of testa, color of testa, endosperm color,
glume color, and plant color. Each of these characteristics
is determined by one or a few genes. For example,
two genes, R and Y, determine whether the pericarp
will be red (RY ), colorless or white (Ry, ry), or lemon
yellow (rY ). The kernel starch is conditioned by a dominant
allele (Wx); wxwx conditions a waxy endosperm.
Similarly, sugary endosperm is controlled by a single
locus, susu.
Genetic male-sterility genes occur in sorghum, the
most commonly used being the ms 3 . It has stable expression
over different environments. CMS also occurs in
sorghum, conditioned by an interaction between two
major genes, msc 1 and msc 2 .
Important genes that have impacted sorghum breeding
are those affecting maturity and plant height.
Sorghum is a short-day plant. Maturity is influenced
by photoperiod and temperature. Genes that influence
sorghum maturity are designated Ma 1 , Ma 2 , Ma 3 , and
Ma 4 ; tropical cultivars being dominant at the Ma 1 locus.
A genotype of Ma 1 Ma 2 Ma 3 Ma 4 takes about 90.5 days
to anthesis, while ma 1 ma 2 ma 3 Ma 4 takes 55.3 days to
anthesis. Similarly, four independent recessive genes,
dw 1 , dw 2 , dw 3 , and dw 4 , reduce internode length of the
sorghum stalk without affecting time of blooming and
leaf size. A genotype Dw 1 Dw 2 Dw 3 Dw 4 has a flag leaf
at 127 cm, while dw 1 dw 2 dw 3 dw 4 has a flag leaf located
43 cm above ground on the stalk. Sorghum cultivars
containing dwarf genes are identified by the number of
specific dwarf genes they contain as 1-dwarf (contains
one dwarf gene, e.g., dw 2 ), 2-dwarf (contains two
dwarf genes, e.g., dw 2 dw 3 ), and so on. US commercial
sorghum hybrids are 3-dwarf.
General botany
Sorghum (Sorghum bicolor (L.) Moench) is known by
common names such as milo, kafir, and guinea corn.
The annual sorghums have n = 10 and include grain
sorghum, sorgo, broomcorn, and sudangrass. Sorghum
halepense (johnsongrass) is a perennial sorghum with
2n = 20. The sorghum plant has culms that may stand
0.6–4.5 m tall, depending on the type and variety. It
may produce two or more tillers. The stalk is solid. The
center of the stem can be dry or juicy, insipid or sweet
to taste. A dry stalked variety has leaves with a white or
yellow midrib, while a juicy stalked variety has a dull