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Literature review
oligosaccharides (RAGHAVAN, 1976; BEWLEY & BLACK, 1994). Most seeds store
their major food reserves within the embryo; usually the cotyledons (BEWLEY &
BLACK, 1994). Some plants also have their seed storage reserves within extra-
embryonic tissues. These extra-embryonic tissues used for storage include the
endosperm (Gymnosperms) or the perisperm (Coffea arabica). Both embryonic and
extra-embryonic tissues can also be used for storage; such as in maize (BEWLEY &
BLACK, 1994).
2.8.2 Seed germination
Germination starts with the uptake of water by a seed and ends with the onset of
elongation of the embryonic axis, usually the radicle (BEWLEY & BLACK, 1994). It
also includes the steps of protein hydration, sub cellular changes, respiration,
macromolecular syntheses and cell elongation (RAGHAVAN, 1976; BEWLEY &
BLACK, 1994). The combined effect of these steps is to transform a dehydrated,
dormant embryo into an embryo which grows actively and accumulates water
(MAYER, 1977; BEWLEY & BLACK, 1994). A seed in which none of these processes
have taken place is said to be quiescent. They characteristically have low moisture
content (5-15%) and an extremely slow metabolic rate (BEWLEY & BLACK, 1994).
Seeds are able to survive in this state for a number of years. Quiescent seeds
require an environment of suitable temperature, hydration and the presence of
oxygen in order to germinate (BEWLEY & BLACK, 1994). The major cellular
processes involved in the initiating and facilitating of radicle emergence include
respiration, RNA and protein synthesis and enzyme and organelle activity
(RAGHAVAN, 1976; BEWLEY & BLACK, 1994).
During imbibition various structural and physical changes occur (BEWLEY & BLACK,
1994). The completion of imbibition requires a small amount of water (not more than
three times the seed’s dry weight). For successful subsequent root and shoot growth
a larger and more constant supply of water is essential (BEWLEY & BLACK, 1994).
The uptake of water by the seed can be seen as triphasic (Figure 2.31) (BEWLEY &
BLACK, 1994). Phase 1 involves imbibition, where a strong osmotic gradient results
in the uptake of water from the soil (BEWLEY & BLACK, 1994). Different areas of the
testa and different organs within the seed often absorb variable amounts of water.
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