Principles of Plant Genetics and Breeding

516 CHAPTER 30
improvement methods discussed in Part I of this book
are applicable to sorghum breeding. To develop a random
mating population the breeder starts by selecting
20–40 parents. The next step is to incorporate a malesterility
gene by crossing each parent individually to a
male-sterile stock. The F 2 segregates for male sterility.
The selected plants are backcrossed one or two times if
the cytoplasmic male-sterile (CMS) stock lacks good
agronomic qualities. An equal amount of F 2 seed from
all crosses are bulked and grown in isolation for random
mating.
Sorghum is also bred using population improvement
methods for enhancing quantitative traits. The pure-line
method of breeding has been used in sorghum improvement.
With the discovery of CMS and fertility-restoring
genes in the 1950s, hybrid sorghum breeding became
practical and profitable using the A-, B-, R-line breeding
systems as in corn.
The sorghum conversion program (adapting tropical
sorghum to temperate climates) has been a significant
part of the success of sorghum breeding in the USA.
A conversion program starts with a cross of tropical ×
temperate lines at a tropical station. The resulting F 1 is
grown at a temperate experimental station to obtain F 2
seed. The F 2 is planted at a tropical station. Selections
are made and backcrossed (five cycles of backcrosses) to
adapt the tropical line to temperate conditions.
Establishing a breeding nursery
The specific layout of a nursery depends on the task to
be performed and materials to be handled. A section
of the nursery may be allocated to procedures such as
selfing, crossing, population breeding, observations,
forage breeding, and tropical conversion. To reduce
walking, parents to be crossed are planted near each
other. Plant density should be similar to that used by
farmers in production. Spacing varies from 10 to 100 cm
between rows and 5 to 30 cm within rows.
Materials and equipment
Artificial pollination
The equipment used includes pollinating bags (6 ×
12–40 cm), stapler, knife, marking pencils, scissors, hot
water container, clips, string, and apron.
Emasculation
Sorghum breeders control pollen using one of four
general methods – male sterility, hot water emasculation,
hand emasculation, and control of anther dehiscence.
The panicle of the male-sterile plant is bagged
just before anthesis. Using male sterility enables the
breeder to undertake large-scale hybrid seed production.
Hot water emasculation is also used, but usually
when male sterility is not available in the parents, and
when complete pollen control (i.e., the presence of a few
selfs) is not critical. To emasculate in the field, a suitable
panicle (just beginning to flower) is selected and all
opened spikelets removed. The panicle is enclosed in a
rubber or plastic sleeve, tied tightly around the peduncle
but open at the top. Water at 47–48°C is poured into
the sleeve and held for 10 minutes. In the greenhouse,
the particle may be emasculated by directly immersing
the panicle in hot water by inverting the potted
plant. The panicle is left to dry before covering it with a
pollination bag.
Breeders may use hand emasculation when only a
small quantity of seed is needed and complete pollen
control is desired. This method of emasculation requires
skill to succeed. Hand pollination in the field is undertaken
in the afternoon when contamination from other
plants is least. Anthers may be removed by using forceps,
scissors, or other pointed instruments. The bag is placed
over the emasculated panicle.
Anther dehiscence can be delayed by high humidity.
In regions of high humidity, placing polythene or paper
over a particle can delay anther dehiscence by about
30 minutes the next morning. However, under hot
conditions, heat might build up under the bag and
injure the flowers.
Pollination
Pollen collection is best done in the morning between
7 and 12 a.m. The center of the particle yields the
most pollen. It may be necessary to cover the male
plant, the day before pollination to keep out contaminants.
When it is time to pollinate, the panicle is
tilted so that the pollen can be shaken into the bag.
Male-sterile plants and those emasculated by hot water
are ready for pollination after about 5–8 days following
bagging, when flowering is complete and the stigmas
are extruded. The bag containing the pollen is inverted
over the female panicle and shaken to pollinate. The
operator may also remove a branch of the panicle and
brush pollen on the stigma. The pollinated panicle is