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Table 2.4: Topographic stain evaluation classes for the TTC test (LEADEM, 1984).
Class Description Viability
1 Embryo completely stained Germinable
2 Very pale staining Possibly germinable
Literature review
3 Cotyledons unstained Non-germinable or possibly germinable
4 Radicle unstained Non-germinable or probably not germinable
5 No staining Non-germinable
The meristems should be well stained in order to insure healthy germination and
growth of the embryo (LEADEM, 1984). A typical viable embryo should be at least
75% stained and its tissue should be firm with a smooth surface (LEADEM, 1984).
The TTC stain test leaves a number of uncertainties. A sample may, for example, not
be stained because the stain never penetrates the tissue and the non-enzymatic
reduction of TTC is also possible in dead and living tissue (BAND & HENDRY, 1993).
A second viability test should be used to confirm the tetrazolium result (BAND &
HENDRY, 1993).
2.8.9 After-ripening
In the period of after-ripening individual or collective changes take place so that a
seed that was once dormant can germinate (COPELAND, 1976). This period may be
determined by physical and chemical stimuli in the environment such as a
phytochrome response and or the balance of inhibiting and promoting substances in
the environment (COPELAND, 1976). It may also be determined by the effect of the
environment on the balance of inhibiting and promoting substances within the seed
and embryo itself and morphological growth and development of the embryo
(COPELAND, 1976).
After-ripening can sometimes be hastened by growth promoting substances, low-
temperature stratification, alternating temperature and light exposure treatments
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