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Topic 23.17

The Types of Coat-Imposed Seed Dormancy

1. Prevention of water uptake. This type of coat-imposed dormancy is common in plants found in arid and semi-arid regions, especially among legumes, such as clover (Trifolium spp.) and alfalfa (Medicago spp.). Waxy cuticles, suberized layers, and lignified sclereids all combine to restrict the penetration of water into the seed.
2. Mechanical constraint. The first visible sign of germination is typically the radicle (embryonic root) 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 partly 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.

Some components of the seed coat can contribute to several aspects of dormancy. For example, pigments not only filter light, but become cross-linked to other wall constituents thereby increasing mechanical constraints and decreasing permeability to water, gases, and hormones.