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Literature review
Phase 2 is seen as a lag phase. Here the matrix forces of the seed cells that caused
the strong osmotic gradient in phase 1 are no longer active (BEWLEY & BLACK,
1994). During this phase major metabolic events take place in preparation for radicle
emergence (BEWLEY & BLACK, 1994). Only germinating seeds, and not dormant
seeds, enter phase 3. This stage is associated with changes in the cells of the radicle
and radicle elongation (JANN & AMEN, 1977; BEWLEY & BLACK, 1994). These
changes are facilitated by the uptake of water (JANN & AMEN, 1977). This uptake of
water is a result of the production of low-molecular-weight osmotically active
substances (BEWLEY & BLACK, 1994). These substances are produced as a result
of hydrolysis of stored reserves (BEWLEY & BLACK, 1994). The kinetics of water
uptake is however more complex than this, as many seeds distribute water to
different seed parts at different rates.
Figure 2.31: The triphasic pattern of water uptake by germinating seeds, with arrow showing
the time of radicle protrusion (BEWLEY & BLACK, 1994).
For germination to be completed, the radicle must expand and penetrate the
surrounding structures (BEWLEY & BLACK, 1994). This does not require cell
division. Instead, the radicle cells elongate as the radicle penetrates through the
surrounding tissues and cell division starts some time after the testa is eventually
ruptured. There are a number of possible requirements for radicle elongation
(BEWLEY & BLACK, 1994). One such requirement is the lowering of the osmotic
potential as a result of the accumulation of solutes within the radicle; this increases
water uptake and raises the turgor pressure, which facilitates cell elongation. The
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