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Literature review
the embryo (COPELAND, 1976). Stratification has also been known to decrease the
time to germination and increase growth rate in other species (COPELAND, 1976).
Stratification may also decrease the sensitivity to external conditions, so that a seed
might, for example, germinate at a less suitable temperature (COPELAND, 1976).
The germination rate may also be improved by exposing seeds to different day/night
temperatures (COPELAND, 1976). In some cases embryonic dormancy may be
broken by exposure to a certain light intensity, wavelength and/or photoperiod
(COPELAND, 1976). In some cases dormancy caused by inhibitory chemicals within
the embryo can be broken by gibberellic acid (COPELAND, 1976). Other chemicals
which are known to break non-deep physiological dormancy include potassium
nitrate, kinetin and ethylene. The effects of endogenous physical dormancy also
diminishes as seed age increases (COPELAND, 1976).
In the case of morphological dormancy, germination is prevented at the time of
maturity due to the morphological characteristics of the embryo. The embryo is
underdeveloped or even undifferentiated at the time of dispersal and a period of
growth, known as after-ripening, is required before the seed can successfully
germinate (COPELAND, 1976; BASKIN & BASKIN, 1998). Morphological dormancy
occurs in seeds with rudimentary and linear embryos. Most of the interior of these
seeds is occupied by endosperm and the embryo may only be 1% of the seed
volume or less (BASKIN & BASKIN, 1998).
Morphophysiological dormancy is essentially a combination of the two dormancy
types. In this case the embryo must grow to a species-specific critical size and the
physiological dormancy of the seed must occur before germination can take place
(BASKIN & BASKIN, 1998).
The primary reason for dormancy of seeds with exogenous physical dormancy is the
impermeability of their seed or fruit coats to water (BASKIN & BASKIN, 1998). Seeds
with physical dormancy frequently have a palisade layer of lignified cells in the testa
or pericarp (BASKIN & BASKIN, 1998). The breakdown of such hard seed coats in
the natural environment occurs through the gradual processes of hydration and
dehydration, exposure to hot and cold temperatures, scorching by fire and
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