Page 509 - Biosystems Engineering

Agricultur e Management 481

Rapid Tests for Seed Viability
Rapid seed viability tests are required when large quantities of seed
have to be procured in the shortest possible time (within 24 h).
Although a germination test is still the most reliable and satisfactory,
it is a time consuming and slow process. Rapid tests are also useful
when a seed is dormant or a seed has been injured due to heat, frost,
or unsuitable storage. Rapid seed tests are based on enzyme activity as
measured and have proved to be the most useful because the enzyme
activity is closely bound up with the life of the seed. Enzymes are
involved in oxidation/reduction reactions with certain organic com-
pounds, some of which give a different color in their oxidized state
from that in the reduced state. Thus, a simple color change can be used
to demonstrate the presence or absence of dehydrogenase enzymes.
The chemical used for the test is a cream or light yellow soluble
powder 2,3,5-triphenyl tetrazolium chloride (or bromide). The color-
less solution is reduced in the presence of certain enzymes to an insol-
uble formazan. When this occurs in living tissues, the tissue is stained
red. The test reaction should be performed in the dark.
The tetrazolium solution must come in contact with the seed
embryo. Since the coats of many seeds are relatively impermeable to
the chemical, it is often necessary to prepare the seed. A period of
preconditioning in which the seed is soaked in water at 30°C is rec-
ommended. This allows the seed coat to soften and swell and ensure
a bright strain. The preconditioned seed may then

1. Be placed directly in the tetrazolium solution (peas and beans).
2. Be bisected longitudinally (cereal and grasses).
3. Be pricked below the embryo or bisected laterally (small grass
seed).
4. Have the seed coat removed (melon, cotton).
5. Be scratched above the embryo (lettuce).
6. Have a piece cut off the embryo (Pinus).

Some seeds such as small clovers and Brassica sp. may be placed
directly in the tetrazolium solution without preconditioning. After a
period in the solution, the length of which varies with the species and
with the temperature of the test conditions, the seed is removed from
the tetrazolium solution, washed in water, and classified according to
the staining pattern. The time that it takes to complete the test varies
with the species, with the number of species (preferably between 6
and 8), with the strength of the solution (0. 1, 0.5, and 1.0 percent are
used), and with the temperature. It may be difficult to interpret the
results, and the analyst must have a proper understanding of the
importance of the parts of the embryo in order to interpret the stain-
ing pattern correctly.