Principles of Plant Genetics and Breeding

346 CHAPTER 18
Storage of seed
It is critical that germplasm be stored such that its
viability is retained over the duration of storage. Seed
germplasm should be stored at low seed moisture content
in an environment in which the humidity, temperature,
and oxygen content are low. To this end, seed to
be stored is usually dried to about 10.0–12.5% moisture
content, and stored at a temperature of less than 21°C.
The specific requirements differ among species. The
Harrington rule of thumb suggests that seed viability
is retained for a longer time if the sum of the storage
temperature (°F) and relative humidity (%) is less than
100°F. Relative humidity is more important in the
storage of soybean. In corn, a sum of 60 is desired for
long-term storage of corn. The rate of decline in seed
viability in storage also varies among species. Storage in
a household freezer may suffice for certain species, especially
small-seeded legumes (e.g., alfalfa, clover). The
oxygen level in the storage environment may be reduced
by introducing gasses such as carbon dioxide, nitrogen,
or argon. The seed may also be stored in a vacuum.
Selection of parents (inbred lines)
The choice of parents to be used in a cross is the most
critical step in a plant breeding program for the development
of hybrids. The choice of parents depends on the
specific objectives of the breeding program and what
germplasm is available. Once the inbred lines have been
developed, the breeder has the task of identifying a few
lines with potential for use as parents in hybrid production.
The number of inbred lines that would emerge
from a random mating population in which a number of
loci are segregating is given by 2n. Hence, for n = 10,
there will be 1,024 inbreds. First, the large pool of
inbreds needs to be significantly reduced by phenotypic
selection to identify a small number of high performing
inbreds. This is effective for traits of high heritability.
The next step is to subject the promising lines to a more
rigorous test of their performance in crosses (combining
ability test, see Chapter 8). Combining ability tests, as
previously described, entail crossing each inbred with all
other inbreds to be evaluated. Suppose 50 inbreds were
selected, the cross combinations required in a combining
ability test is given by n(n − 1) = 50(50 − 1) = 2,450
crosses! To handle this large number, the practice is
to use a common tester. As previously indicated, the
breeder should select parents from different heterotic
groups (interheterotic cross) rather than within the
same group. A general combining ability (GCA) test
should be conducted first, to be followed by a specific
combining ability (SCA) test to identify specific pairs of
inbreds with exceptional performance in crosses. This
sequence of activities is of practical and strategic importance
in quickly reducing the large number of inbreds to
a manageable size by the time of the more involved evaluations.
Certain inbreds have high GCA, being able to
produce high performing hybrids with a series of other
inbreds. On the other hand, certain inbreds are able to
“nick” with only a few in that set of inbreds tested. The
key decision in combining ability testing is the type of
tester to use. A tester can have a broad genetic base
(e.g., open-pollinated cultivars) or a narrow genetic
base (e.g., elite inbreds, related inbred lines).
Where a hybrid breeding program already exists,
breeders may want to develop one or two new inbreds
to replace those in the program that have been shown to
have weaknesses. To replace an inbred in an established
single cross, for example, the opposite inbred should be
used as a tester. Substitute inbred lines may be developed
by backcross procedures (so the inbred is least
genetically reorganized), or by isolating new inbreds
from the same genetic source. New inbreds may also be
developed from completely new sources.
Field establishment
Once a breeder has identified superior inbreds, these
lines are used as parents for producing hybrid seed.
Considerations for maximizing hybrid seed production
in the field include the following.
1 Field preparation. The field should be properly
prepared to obtain a seedbed suitable for the seed
size. The field should be free from weeds (use preand
postemergence weed control as appropriate).
Competition from weeds will adversely affect crop
establishment.
2 Planting time. It is important that the planting be
timed such that the seed will germinate promptly for
good establishment. Also, the time of pollination
should coincide with good weather. In fact, the
whole operation, from planting to harvesting, should
occur within the growing season, making maximum
use of the growing condition for optimum seed yield.
The breeder may use heat units to calculate the best
time for planting the parents (Table 18.1).
3 Synchronization of flowering. Because a hybrid
depends on two different genotypes, the breeder
should synchronize the flowering of these inbreds so
that both male and female plants would be ready at
that same time for effective pollination. A technique