Principles of Plant Genetics and Breeding

446 CHAPTER 24
paper in dishes). The rolled towel is kept at 20°C for
16 hours, followed by 30°C for 8 hours. The germinated
seedlings are counted. Seed viability may also be
determined by a biochemical test, the tetrazolium
test. Seeds are soaked in 2,3,5-triphenyltetrazolium
chloride solution. Living tissue changes color to red
while non-living tissue remains uncolored.
2 Purity test. Seed purity is determined at two levels –
genetic and physical. The sample seed should reflect
the physical features of the cultivar and be distinguished
from other seeds and weed seeds. A genetic
purity test requires a chemical test that may be as relatively
simple as isozyme analysis, or as sophisticated as
DNA profiling or fingerprinting (see Chapter 14).
3 Vigor testing. Seed vigor determines the capacity of
seed to emerge rapidly and uniformly and develop
into normal seedlings under a range of conditions.
Common tests of vigor include accelerated aging
(seeds are subjected to high temperature of between
40 and 45°C, and high humidity, before conducting
a germination test), cold test (seeds in a rolled towel
or containers are held at 10°C for 7 days before
moving to a 25°C environment), and electrical conductivity
of seed leakage.
4 Seed health. The seed sample is examined for the
presence of pathogens by visual inspection, seed incubation,
or biochemical testing.
5 Noxious weed seed. This is seed from a species that
sooner or later becomes aggressive and difficult to control.
The weed is officially prohibited from being reintroduced
into a production region. A noxious seed in
one state may not be classified as such in another state.
Tagging commercial seed
Once tested, the seed is ready to be made available to
consumers, but not until it is properly identified with
a tag or label. A tag on the seed bag identifies each class
of seed. Customarily, a white tag identifies breeder or
foundation seed, while a purple tag is used to identify
registered seed. A certified seed receives a blue tag.
On some occasions, a green tag is used to identify a
cultivar that the developer opts not to have certified
but nonetheless has achieved the standards of certified
seed. Such a cultivar may be simultaneously released
by different companies under different brand names
(called branded cultivars) for marketing purposes. An
example of a tag and the information it displays is presented
in Figure 24.2 and Table 24.1.
Figure 24.2 A sample seed tag. (Courtesy of Pioneer Seed
Company.)