Principles of Plant Genetics and Breeding

348 CHAPTER 18
Further, it is good practice to walk through the field to
rogue out any off-types prior to pollination.
Harvesting and processing
The seed should be harvested at the proper maturity and
moisture content. At physiological maturity in corn, for
example, the kernel moisture is about 30–40%. Safe
harvesting is done at a moisture content of 20% or less.
The timing of this phase in hybrid seed production is
critical because the seed is intended for use as a planting
material and must be of the highest possible germination
capacity. Mechanical damage, physiological immaturity,
and improper seed moisture adversely impact seed
quality, and reduce the germination capacity. Further,
improper seed moisture may predispose the seed to rapid
deterioration in storage.
The processing needed for the seed varies with the
crop. In corn, for example, workers first clean the ears to
remove diseased and discolored ears before shelling. All
seed must be cleaned to remove weed seeds and debris
as much as possible. It is required that the producer of
the seed attach a label providing specific information
including the seed analysis results.
Hybrid seed production of maize
Commercial hybrid seed production in maize is used as
an example because hybrid production in maize is one of
the earliest and most successful exploitations of heterosis.
Both CMS and mechanical detasseling are used in commercial
seed production. In the USA, the single cross is
used in maize hybrid breeding. The female plant is male
sterile (the A-line). It is maintained by crossing with the
B-line in isolation. The A-line is grown in alternating rows
with the pollinator (the R-line) in a ratio of 1 : 2, 2 : 3,
or 2 : 4. Some seed companies mechanically detassel
their maize instead of using a CMS system (Figure 18.2).
Hybrids in horticulture
A review by J. Janick (1996) indicates that hybrid seed is
significantly used in horticultural production. A wide
variety of mating systems are used in hybrid seed production
of these species. These include hand emasculation
(e.g., in sweet pepper, tomato, eggplant), CMS
(e.g., in sugar beet, carrot, onion), self-incompatibility
(e.g., in cauliflower, broccoli), and monoecy (e.g., in
muskmelon, cucumber). The importance of hybrids is
variable among species. The approximate percentages
Figure 18.2 Using a mechanical detasseler to emasculate
corn. (Courtesy of Pioneer Hi-Bred Seed Company.)
of hybrid seed in use in the commercial production of
selected plants are: carrot (90% of fresh market and
40–60% of canning and freezing cultivars), broccoli
(100%), cauliflower (40%), sugar beet (70%), spinach
(90%), muskmelon (80–100%), sweet corn (99%),
tomato (100% of fresh market), and onion (65%).
In the ornamental industry a similar picture prevails.
F 1 hybrid seed is used in begonia (100% by emasculation),
impatiens (100% by CMS), petunia (100% by
CMS), seed geranium (100% by genetic male sterility),
carnation (80% by genetic male sterility), and dianthus
(70% by genetic male sterility).
Exploiting hybrid vigor in asexually
reproducing species
Plants with vegetative propagations
Asexual (vegetative) reproduction is the propagation of
plants using propagules other than seed. Many horticultural
plants are vegetatively propagated. The economic
parts of many important world food crops are non-seed,
such as tubers (e.g., potato), stems (e.g., sugarcane),
and roots (e.g., cassava). Heterosis can be effectively
exploited in species that have the capacity to bear seed
and yet be propagated vegetatively. In such species, the
plant breeder only needs to create one superior genotype.
There is no need for progeny testing. The hybrid
vigor and other traits assembled in the F1 can be maintained
indefinitely, as long as the genotype is propagated
asexually thereafter.
In horticulture, the superior genotype may be propagated
by using techniques such as micropropagation,
grafting, budding, sectioning, and cutting (see Chapter
4). Successful hybrids have been developed in species