Research Background

     Factors That Influence Seed Dormancy:

     Seed germination is the process by which the uptake of water leads to the rupture of the seed coat by the radicle or the shoot. The blockage of any one of the steps leading to the germination can, and likely will, cause a state of dormancy (a failure to germinate) in the seed. There are many factors that play a role in the germination of seeds.

     Seed germination may be blocked by the absence of some external factors. External factors include water, proper temperature, proper mixture of gases, and proper light requirements. However, some seeds may fail to germinate even if external conditions are favorable due to some internal factor.

     A hard seed coat is one internal factor. A hard seed coat can cause dormancy by depriving the seed of water and gases, and it can even mechanically restrict the growth of the embryo. The Leguminosae family has the largest number of species of plants which produce seeds with hard seed coats. Some seeds have a one-way valve that lets moisture out but it does not take moisture in. The dormancy of seeds with these characteristics can be broken artificially by scarification.

     Scarification is the process by which a seed coat is made permeable to water and /or oxygen, and the seed coat is weakened so the expansion of the embryo is not restricted. Scarification can be done mechanically by cracking or scratching the seed coat with a knife or other abrasive materials. In nature this can be done by the mechanical abrasion of rock particles, by alternate freezing and thawing, or by bacterial action. Chemical scarification can be accomplished by dipping seeds in strong acid, such as sulfuric acid, or strong solvents, such as acetone or alcohol for seconds up to several minutes. In nature this can happen when animals eat and expel seeds. Acids form the animals' stomachs can weaken the seed coats.

     Other factors influence the dormancy of seeds. Growth inhibiting substances can be present in the seed coat, seed interior, or in tissues surrounding the seed that prevent a seed from germinating. For example, pears, apples, citrus fruits and other fleshy fruits contain inhibitors to keep the seed from germinating while still in the fruit. They will germinate only after being removed and washed. Also, many desert plants have inhibitors in their seed coats that prevent a seed from germinating unless there has been sufficient rainfall for the seedling to become established.

     Many plants produce seeds that must go through a period of after ripening before they are able to germinate. This is where the seed remains dormant from the time the seed falls to the ground in autumn until its germination in the spring. Meanwhile, the seeds are covered by leaves and winter snows. During this time, the seed's embryo fully develops with aid from the food materials stored in the endosperm.

     The amount of time that a seed will remain viable (capable of germinating) can range from days to many years depending on the type of seed and the conditions of storage. Some seeds such as certain willows, orchids, and cottonwoods remain viable for only a few days or weeks. Seeds of the Pumpkin Family can remain viable to several years. Wheat seeds that have been properly stored have been reported to have been better than 30% viable for more than thirty years. On the contrary, a few species of both monocots and dicots produce seeds with no dormancy period. The red mangrove, a tropical tree, is one example.

     There are many factors that affect the dormancy of seeds. External factors, as well as internal factors, play very important roles in the germination of seeds. Favorable environmental conditions alone are not enough for some seeds to sprout into beautiful plants. Dormancy is nature's way of giving plants a better chance to survive.