During seed maturation
, the embryo enters a quiescent phase in response to desiccation.
Seed germination can be defined as the resumption of growth of the embryo of the mature seed; it depends on the same environmental conditions as vegetative growth does.
Water and oxygen must be available, the temperature must be suitable, and there must be no inhibitory substances present.
In many cases a viable (living) seed will not germinate even if all the necessary environmental conditions for growth are satisfied. This phenomenon is termed seed dormancy.
Seed dormancy introduces a temporal delay in the germination process that provides additional time for seed dispersal over greater geographic distances. It also maximizes seedling survival by preventing germination under unfavorable conditions.
Types of dormancy
Two types of seed dormancy have been recognized:
coat-imposed dormancy and embryo dormancy.
(i) Coat-imposed dormancy.
Dormancy imposed on the embryo by the seed coat and other enclosing tissues, such as endosperm, pericarp, or extrafloral organs, is known as coat-imposed dormancy. The embryos of such seeds will germinate readily in the presence of water and oxygen once the seed coat and other surrounding tissues have been either removed or damaged. There are five basic mechanisms of coat-imposed dormancy
1. Prevention of water uptake.
2. Mechanical constraint. The first visible sign of germination is typically the radicle breaking through the seed coat. In some cases, however, the seed coat may be too rigid for the radicle to penetrate. For the seeds to germinate, the endosperm cell walls must be weakened by the production of cell wall–degrading enzymes.
3. Interference with gas exchange. Lowered permeability of seed coats to oxygen suggests that the seed coat inhibits germination by limiting the oxygen supply to the embryo.
4. Retention of inhibitors. The seed coat may prevent the escape of inhibitors from the seed.
5. Inhibitor production. Seed coats and pericarps may contain relatively high concentrations of growth inhibitors, including ABA, that can suppress germination of the embryo.
(ii) Embryo dormancy.
The second type of seed dormancy is embryo dormancy, a dormancy that is intrinsic to the embryo and is not due to any influence of the seed coat or other surrounding tissues. In some cases, embryo dormancy can be relieved by amputation of the cotyledons. Species in which the cotyledons exert an inhibitory effect include European hazel (Corylus avellana) and European ash (Fraxinus excelsior).
A fascinating demonstration of the cotyledon’s ability to inhibit growth is found in species (e.g., peach) in which the isolated dormant embryos germinate but grow extremely slowly to form a dwarf plant. If the cotyledons are removed at an early stage of development, however, the plant abruptly shifts to normal growth. Embryo dormancy is thought to be due to the presence of inhibitors, especially ABA, as well as the absence of growth promoters, such as GA (gibberellic acid). The loss of embryo dormancy is often associated with a sharp drop in the ratio of ABAto GA.
Primary versus secondary seed dormancy.
Different types of seed dormancy also can be distinguished on the basis of the timing of dormancy onset rather than the cause of dormancy:
• Seeds that are released from the plant in a dormant state are said to exhibit primary dormancy.
• Seeds that are released from the plant in a nondormant state, but that become dormant if the conditions for germination are unfavorable, exhibit secondary dormancy.
For example, seeds of Avena sativa (oat) can become dormant in the presence of temperatures higher than the maximum for germination, whereas seeds of Phacelia dubia (small-flower scorpion weed) become dormant at temperatures below the minimum for germination. The mechanisms of secondary dormancy are poorly understood.
Environmental Factors Control the Release from Seed Dormancy
Various external factors release the seed from embryo dormancy, and dormant seeds typically respond to more than one of three factors:
Many seeds lose their dormancy when their moisture content is reduced to a certain level by drying—a phenomenon known as afterripening.
Low temperature, or chilling, can release seeds from dormancy. Many seeds require a period of cold (0–10°C) while in a fully hydrated (imbibed) state in order to germinate.
Many seeds have a light requirement for germination, which may involve only a brief exposure, as in the case of lettuce, an intermittent treatment (e.g., succulents of the genus Kalanchoe), or even a specific photoperiod involving short or long days.