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Literature review
degradation due to extreme soil acidity, microbial breakdown and digestion by
animals (COPELAND, 1976). Another reason for exogenous physical dormancy is
sometimes the impermeability of seed coats to gasses (KHAN, 1977). Physical
dormancy is often found in combination with chemical dormancy.
Chemically dormant seeds do not germinate due to the presence of inhibitors in the
pericarp. These inhibitors are usually removed by leaching (BASKIN & BASKIN,
1998). These inhibitors include chemicals such as ABA (Abscisic acid) and phenolic
compounds (KHAN, 1977). Many studies have shown that there is a interaction
between phytochromes activated by red light and inhibitors (KHAN, 1977).
Mechanical dormancy is usually the result of a hard, woody fruit wall or seed coat.
This woody structure is usually the endocarp or the mesocarp (BASKIN & BASKIN,
1998). Upon germination the endocarp often splits into two halves (BASKIN &
BASKIN, 1998). In such cases dormancy can sometimes be broken with a period of
cold stratification (KHAN, 1977).
2.8.8 Seed longevity and viability
Some seeds are viable after several years, decades or even after a few hundred
years (BEWLEY & BLACK, 1982). Longevity is largely dependent on storage
conditions. Factors that influence the longevity of seeds in storage include
temperature, moisture and oxygen pressure (BEWLEY & BLACK, 1982). A low
temperature and moisture content usually equates to a longer period of viability
(BEWLEY & BLACK, 1982). Higher oxygen pressure results in a shorter period of
sustained viability (BEWLEY & BLACK, 1982). Unorthodox or recalcitrant seeds
cannot withstand drying (BEWLEY & BLACK, 1982). These seeds must retain a
relatively high moisture content to remain viable (BEWLEY & BLACK, 1982). Even in
relatively moist storage conditions they are rarely viable for more than a few months
(BEWLEY & BLACK, 1982). The majority of seed plants are however orthodox and
can remain viable for a prolonged period under suitable storage conditions (BEWLEY
& BLACK, 1982). Various mathematical equations have been derived to relate the
viability of seeds with their storage environment (BEWLEY & BLACK, 1982). To test
the longevity of seeds the relative number of normal seedlings produced by seed
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