Seed Dormancy

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Seed dormancy is the innate inhibition of germination of a viable seed even placed in the most favourable environment for germination. seed dormancy is also called the rest of the seed. Generally, Seeds are dormant once they leave their parent. It enables the embryo to safely pass the unfavourable part of the season and germinate when the conditions are suitable for the establishment of the seedlings.

Dormancy mechanisms are put in place by the Mother Plant to ensure that seeds will not germinate while still attached (vivipary). Domesticated species often have dormancy mechanisms bred out of them and therefore will display viviparous germination or what’s commonly known as pre-harvest sprouting. Maternal tissue surrounding the seed, such as the lemmas and paleas in grasses, play an important role in preventing viviparous germination.

Types of Seed Dormancy:

Depending upon the source of dormancy, the seed dormancy is classified into two types: Exogenous Dormancy and Endogenous Dormancy.

Exogenous Dormancy:

This type of dormancy is imposed by factors outside the embryo. In such dormancy, the tissues enclosing the embryo can affect germination by inhibiting water uptake, providing mechanical resistance to embryo expansion and radicle emergence, by limiting oxygen to the embryo, preventing leaching of the inhibitor from the embryo and supplying inhibitor to the embryo.

Endogenous Dormancy:

This type of dormancy is imposed by rudimentary or undeveloped embryo at the time of ripening or maturity.

Depending upon the surrounding conditions, dormancy can be classified as

Innate dormancy:

It is the condition of seeds which is incapable of germination even if conditions suitable for seedling growth are supplied. It may be due to the embryo being immature at the time of dispersal. Innate dormancy is present within the seed when it reaches physiological maturity and remains for some time after the seed is shed. e.g. in Heracleum sphondylium the development of embryo continues at the expense of extra-embryonic food reserves for several months after seed is shed

Enforced dormancy:

It is the condition of seeds which is incapable of germination due to environmental restraints i.e. unfavourable conditions. Unfavourable conditions include an inadequate amount of moisture, oxygen, light and a suitable temperature. This dormancy is forced upon the seed by some limitation of the germination environment. This type of dormancy disappears once the missing condition(s) are supplied. e.g. seed buried deeply in soil by tillage, etc., commonly has enforced dormancy.

Induced dormancy:

Induced dormancy has also been referred to as secondary dormancy. This type of dormancy occurs when the seed has imbibed water but has been placed under extremely unfavorble conditions for germination. When later placed under more favorable conditions the seed fails to germinate while still remaining viable.

Reasons for Seed Dormancy:

Immature Embryo:

Seeds with small and undeveloped embryos do not germinate.  If the embryo is immature at the time of seed shedding. The seed will remain dormant till the embryo becomes mature, e.g. Anemone nemorosa, Ranunculus ficaria, etc.

After-Ripening:

After-ripening is a period of quiescence that the seed must go through to finalize the separation from the Mother Plant and become autonomous and on its own. Some seeds require a period of dry storage for developing the ability to germinate, e.g. Wheat, Oat, Barley, etc.

Impermeable Seed Coat:

The seed coats of many species are completely impermeable to water and oxygen at the time of their maturity.  Due to this, the germination fails to occur until water penetrates through the seed coats.  e.g. Apple, Chenopodium.

Hard Seed Coat:

The seed coat is mechanically resistant and does not allow the embryo to grow. In such seeds, the seed-coats are so strong that they do not yield to the pressure of the expanding embryo. The embryos of these seeds have no dormant period and will grow readily if the seed coats are removed. e.g., Amaranthus, Lepidium, Brassica, Caspella, etc..

Germination Inhibitors:

Germination of seeds is sometimes prevented by the presence of compounds known as inhibitors.  They occur in the seed coats and cotyledons of the embryos. Inhibitors may be present in the embryo (e.g., in Xanthium), endosperm, (e.g., in Iris) or in the seed coat (e.g., in Cucurbita).  The important germination inhibitors causing seed dormancy are abscisic acid, phenolic acid, ferulic acid, coumarin, short fatty acids, and cyanogenic chemicals.

Advantages of Seed Dormancy:

• In temperate zones, the dormancy of seeds helps the plants to tide over a severe cold which may be injurious for their vegetative and reproductive growth. Thus the seed dormancy ensures the germination of seeds under favourable conditions only. Dormancy induced by the inhibitors present in the seed coats is highly useful to desert plants.
• In tropical regions, the dormancy of seeds resulting from their impermeable seed coats ensures good chances of survival during water stress.
• It ensures the agricultural seeds to be stored artificially. Dormancy of seeds in many cereals is of utmost importance to mankind. If these seeds would germinate immediately after harvest in the field, they will become useless to man for consumption as food. Rain, at the time of harvest or maturity, may spoil entire produce by initiating germination. Thus it enables storage of seeds for later use by animals and man.
• It also enables the proper dissemination (dispersal) seeds to distant places from the unfavorable environment.
• Dormancy help seeds to remain alive in the soil for several years and provide a continuous source of new plants even when all the mature plants of the area have died down due to natural disasters.

Methods to Break Seed Dormancy:

Mechanical Scarification:

In some cases, a seed doesn’t contain germination inhibitors, but it has a seed coat that is hard and impermeable to water  Before planting the seed its seed coat is scratched with some abrasive material or knife or razor. This process softens the coat and makes it pervious to water.

Chemical Scarification:

Dipping of seeds in strong acids like H₂SO₄or some organic solvents e.g. acetone, alcohol or dipping in boiling water breaks dormancy by weakening the seed coat. This process removes seed coat impermeability.

Stratification:

Some seeds contain germination inhibitors that break down when exposed to cold temperatures. Stratification exposes the seeds to cold temperatures in the refrigerator, simulating a short winter. When brought back to room temperature, they germinate readily. It takes between two and six weeks of cold temperatures to break down the inhibitors, depending on the type of seed.

Other Methods:

• In some instances, seeds subjected to high oxygen pressure also germinate.
• Exposure to light breaks down the germination inhibitors in some types of seeds, particularly wildflowers that produce small seeds.
• Immature embryo dormancy may be broken when embryos are allowed to complete their development.
• By use of some chemical like potassium nitrate, gibberellin, kinetin, etc.

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