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Literature review
germinated under controlled conditions each year serves as a comparative measure
of seed viability (HARTMANN & KESTER, 1965).
The tetrazolium test for seed viability is used by soaking seeds in a solution of 2,3,5-
triphenyltetrazolium chloride (TTC) (HARTMANN & KESTER, 1965). This chemical is
absorbed by living tissue and changed into an insoluble red compound, formazan, by
NADPH dehydrogenases (HARTMANN & KESTER, 1965; COPELAND, 1976;
LEADEM, 1984; BAND & HENDRY, 1993). Non-living tissue remains uncoloured
(HARTMANN & KESTER, 1965). The reaction takes place equally well in dormant
and non-dormant seeds and results are obtained in less than 24 hours. The test is
used as a rapid assessment of viability or as a viability test of dormant seeds that do
not respond to other methods (HARTMANN & KESTER, 1965; INTERNATIONAL
SEED TESTING ASSOCIATION, 1999a). A 1% solution is commonly used, although
a 0.05% solution may sometimes be satisfactory (HARTMANN & KESTER, 1965;
BAND & HENDRY, 1993). It should be used at a pH of 6.5 to 7.5 (INTERNATIONAL
SEED TESTING ASSOCIATION, 1999a).
The intact seeds of some species may be soaked in a TTC solution whereas some
seeds require to be soaked in water first so that tissues are hydrated (COPELAND,
1976). Some seeds should be soaked in a solution with a respiration stimulant such
as hydrogen peroxide (COPELAND, 1976). Other seeds require procedures to be
followed before staining that include the removal of any hard covering and sectioning
of the seeds so that the embryo may be exposed to the TTC solution (HARTMANN &
KESTER, 1965). The embryo is then incubated in 1% TTC for 2 hours in the dark
after which the excess tetrazolium is washed off with water (COPELAND, 1976;
BAND & HENDRY, 1993).
The amount of staining is then observed. The location and intensity of the formazan
stain is important to accurately define the viability of the embryo (COPELAND, 1976;
LEADEM, 1984). If the areas of cell division are unstained or abnormally stained the
potential for germination to occur is lowered (COPELAND, 1976). A number of broad
classes, defining the germinability of the embryo are given in Table 2.4.
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