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Literature review
2.8.7 Seed dormancy and the influence of temperature and stratification
The imposition of dormancy is normally controlled endogenously and germination is
initiated in response to a certain combination of environmental variables. These
environmental variables include temperature, availability of minerals, light and
weathering (KOLLER, 1972). With the termination of dormancy, the metabolic
processes of synthesis and growth are resumed (KOLLER, 1972). Some seeds
exhibit no dormancy, such as mangrove seeds which germinate on the tree itself,
while other seeds such as lotus or lupin seeds may remain dormant for centuries or
millennia (THIMANN, 1977).
There are, generally speaking, two types of organic seed dormancy; endogenous
and exogenous (BASKIN & BASKIN, 1998). In endogenous dormancy there is some
characteristic of the embryo that prevents germination, while in exogenous dormancy
it is some characteristic of the structures that cover the embryo that prevents
germination. These structures include the endosperm, perisperm, testa and fruit walls
(BASKIN & BASKIN, 1998). Seeds may, for example, be unable to germinate
because of seed or fruit coats that are impermeable to water (BASKIN & BASKIN,
1998). Before water uptake and subsequent germination can take place these blocks
to germination must be removed. There are a number of endogenous and
exogenous dormancy types (Table 2.3).
Endogenous physiological dormancy is generally caused by a physiologically
inhibiting mechanism of the embryo that prevents germination. The structures that
cover the embryo may however also play a substantial role (BASKIN & BASKIN,
1998). Physiological dormancy can be differentiated into non-deep, intermediate and
deep physiological dormancy (BASKIN & BASKIN, 1998). Embryos of seeds in non-
deep and intermediate dormancy tend to germinate when isolated from the
surrounding tissues while those of seeds in deep physiological dormancy do not
(BASKIN & BASKIN, 1998).
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