Seed Dormancy

Some of the most common factors affecting seed dormancy are listed below:

• Light: Seeds of most plants germinate equally well in light and darkness. Seeds of plants such as lettuce remain dormant during darkness and germinate if they are exposed to light. Thus, darkness inhibits germination in those plants. In contrast, seeds of Nemophila and Silene species remain dormant when exposed to light. They germinate only in total darkness.
• Temperature: Normally, low temperature promotes germination, and high temperature induces dormancy in most plants. Seeds of temperate plants such as apple, walnut, and pinus remain dormant if not exposed to chilling temperature (1°- 5°C) over an extended period (nearly a week). Exceptions like desert plants require extreme high temperature (60°-70°C) during daytime and shallow temperature (5°-10°C) during the night to start germination.
• Presence of Tough Seed Coat: Seeds of leguminous plants such as pea, beans, clovers, alfalfa, water lotus, and morning glory have tough seed coats. Seeds of such plants prevent the entry of water inside the seed, thus preventing their germination. Also, a hard coat obstructs the exchange of gases like oxygen, necessary for seed respiration.
• Immature (Rudimentary) Embryo: Seeds of orchids such as ginkgo (Ginkgo biloba), European ash (Fraxinus), and holly (Ilex) has immature embryo when the seed is shed. Such plants do not germinate until they attain full maturity.
• Presence of Inhibitory Chemicals: Several naturally occurring compounds like ascorbic acid, ferulic acid, and coumarin are found in the embryo (e.g., in Xanthium), endosperm (e.g., in Iris), or in the seed coats (e.g., in Cucurbita) induces seed dormancy.  

Based on the Source

Depending on the source of dormancy, seed dormancy is classified into two types:

1) Exogenous Dormancy

Factors outside the embryo induce this type of dormancy. Here, the tissues enclosing the embryo affect germination by preventing water uptake, limiting the entry of oxygen to the embryo, providing mechanical resistance to embryo expansion and the emergence of radical, supplying inhibitors, and preventing the leaching of the inhibitors.

2) Endogenous Dormancy

This type of dormancy is induced because of chemical changes within the seed’s embryo. One reason for a plant to not germinate during endogenous dormancy is that the embryo is underdeveloped at maturity.

Based on the Surrounding Conditions of the Seed

1) Innate Dormancy: It is a condition when seeds cannot germinate even when exposed to favorable growth conditions. The inability is sometimes due to the embryo being immature at the time of germination. Innate dormancy can also be imposed chemically by inhibitory compounds in the seed coat or the embryo.

2) Enforced Dormancy: Found when seeds are deprived of favorable growth conditions such as sufficient moisture, oxygen, light, and suitable temperature. In enforced dormancy, no physiological mechanism is involved in inducing dormancy. The seeds are said to be in the ‘quiescent’ stage.

3) Induced Dormancy: Caused by extremely unfavorable growth conditions. Such plant species have no innate dormancy but acquire them with time. After a certain period, the seeds permanently fail to germinate.

There are different treatments done to overcome seed dormancy. The methods are given below:

1) Methods to Break Dormancy of Seeds with Hard Seed Coat

1. Scarification: It is the process of removing the seed coat by mechanically rupturing them. These make a hard seed coat permeable to water and gases.
2. Treating seeds with hot water or concentrated sulfuric acid for a short duration makes the seed coat soft, allowing water and oxygen to enter.

2) Methods to Break Dormancy of Light Sensitive Seeds

1. Exposure to white light: Seeds of tobacco and tomato revokes dormancy when exposed to white light after the seeds have imbibed 30-40% moisture.
2. Exposure to red light: Exposure to a very low intensity of light is sufficient to overcome dormancy. Even a brief exposure to the red light of wavelength 660u is effective for germination overcoming dormancy. In contrast, exposure to far-red light of wavelength 730u induces dormancy and thus prevents germination.
3. Treatment with plant growth hormones: Application of gibberellic acid and kinetin can substitute to the red-light requirement of lettuce seeds. Thus, seeds can germinate in the presence of these hormones in total darkness.

3) Methods to Break Dormancy of Temperature Sensitive Seeds

1. Stratification: It is also known as vernalization. It is the process of incubating seeds at an appropriate low temperature to break their dormancy. Before exposing to a low temperature, such seeds are allowed to imbibe water.
2. Presence with gibberellic acid: Dormant seeds contain an inhibitory hormone called gibberellic acid, which disappears during stratification. At this point, gibberellic acid is secreted, which promotes seed germination. Thus, the chilling requirement can be replaced through the application of gibberellic acid.
3. Use of growth-promoting chemicals: Chemicals such as potassium nitrate, thiourea, and ethylene break seed dormancy and induces germination. Similarly, the application of cytokinin and ethylene also promotes seed germination.

Importance

• Helping seeds overcome adverse growth conditions (such as severe cold and water stress) is injurious for their vegetative and reproductive growth. Seed germination is thus is a survival strategy in plants.
• Ensuring seeds to germinate only under favorable growth conditions
• Storing seeds without losing their viability for later use. Dormancy helps seeds to remain viable in the soil for several years, providing a continuous source of the new plant even after natural calamities such as flooding, earthquakes, or draught.
• Preserving the gene pool thus preventing plant species from getting extinct
• Preventing in-situ germination (vivipary) of seed. Dormancy is necessary for cereal crops. If they germinate immediately after harvest, they will become unfit for human consumption
• Ensuring proper dispersal of seeds to distant places

Disadvantages

• Preventing germination of seeds immediately after attaining maturity
• Preventing uniform emergence of seedlings
• Creating difficulty to maintain plant population
• Interfering with seed-testing procedures, thus causing problems to seed analysts
• Allowing noxious weed seeds to germinate that remain dormant for many years in the soil, thus interfering with the growth of crop plants
• Affecting uniform germination in dormancy revoked seeds