Types of Seed Dormancy

This review will focus on the process of primary seed dormancy. As noted above, primary dormancy is characterised by a transient inability of mature dispersed seeds to germinate under conditions that are normally conducive to germination (Grappin et al., 2000); the inception of primary dormancy occurs during seed development (reviewed in Bewley, 1997) . Dormancy can also be induced in mature, already dispersed, nondormant seeds (known as induced or secondary dormancy) by environmental conditions that are unfavourable for germination, e.g. anoxia, unsuitable temperatures or illumination (Bewley, 1997). In induced dormancy, the seed loses its sensitivity to dormancy-breaking factors (e.g. light, nitrate, etc., depending on the species) (Bewley and Black, 1994).

Primary dormancy is generally classed into two major types: embryo dormancy and coat-imposed dormancy (more accurately termed coat-enhanced dormancy) (Bewley and Black, 1994). In embryo dormancy, it is the embryo that is dormant and the embryonic axis will not elongate even if the embryo is excised from its enclosing seed tissues and placed on water. In coat-enhanced dormancy, the embryo, when isolated is capable of germination, but the intact seed is dormant; thus, it is the surrounding seed tissues that impose the block to germination. The inhibitory nature of the enclosing seed tissues may be attributed to one or a combination of the following effects: (1) interference with water uptake; (2) mechanical restraint; (3) interference with gas exchange; (4) supply of inhibitors to the embryo or promotion of the synthesis of inhibitors within the embryo and (5) prevention of the exit of inhibitors from the embryo (Bewley and Black, 1994). For many seeds, more than one of these factors operates to maintain coat-enhanced dormancy.

A variety of germination inhibitors have been identified within the seed tissues that enclose the embryo. While their presence does not necessarily imply a causal role in preventing germination, in many cases, where repeated washing (leaching) of seeds relieves dormancy, inhibitors are known to be removed (Bewley and Black, 1994). The inhibitor that has received the most attention with respect to dormancy imposition and maintenance is ABA .

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