Question

Water impermeable seed coat is found in ___________.

• A. Malvaceae
• B. Chenopodiaceae
• C. Solanaceae
• D. All of the above

Answer 1

The correct option is D All of the above

Two types of seed dormancy have been recognized, coat-imposed dormancy and embryo dormancy. Coat-imposed dormancy is dormancy imposed on the embryo by the seed coat and other enclosing tissues, such as endosperm, pericarp, or extra floral organs. The embryos of such seeds will germinate readily in the presence of water and oxygen once the seed coat and other surrounding tissues are either removed or damaged. All the families listed in the options above are good examples of coat dormancy. There are five basic mechanisms of coat-imposed dormancy:

1. Prevention of water uptake - Prevention of water uptake by the seed coat is a common cause of seed dormancy in families found in arid and semiarid 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 breaking through the seed coat. In some cases, however, the seed coat may be too rigid for the radicle to penetrate. Nuts with hard, lignified shells are examples of dormancy caused by mechanical constraint. Such shells must be broken by biotic or environmental forces for the seed to germinate. Even nonlignified tissues, such as the endosperm of lettuce seeds, can suppress expansion of the embryo. 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 - Some seed coats are considerably less permeable to oxygen than an equivalent thickness of water is e.g., less by a factor of about 104 in seeds of cocklebur (Xanthium pennsylvanicum). This lowered permeability to oxygen suggests that the seed coat inhibits germination by limiting the oxygen supply to the embryo. In support of this idea, investigators can break the dormancy of such seeds either by making a small hole in the coat with a pin (without weakening the coat mechanically) or by treating the coat with concentrated oxygen. However, other studies suggest that the oxygen consumption of the embryos from such seeds is considerably less than the amount of oxygen able to penetrate the seed coats under normal aerobic conditions. Thus the role of oxygen impermeability in seed coat dormancy remains unresolved.
4. Retention of inhibitors - The seed coat may prevent the escape of inhibitors from the seed.
   For example, growth inhibitors readily diffuse out of isolated Xanthium embryos, but not from intact seeds.
5. Inhibitor production - Seed coats and pericarps may contain relatively high concentrations of growth inhibitors that can suppress germination of the embryo. ABA is a common germination inhibitor present in these maternal tissues. In certain cases where repeated washing (leaching) removes dormancy, the effect is thought to be due to the loss of such inhibitory compounds.
   Therefore, the correct answer is option C.