TYPES OF SEED DORMANCY

Definition of seed dormancy “The inability of viable seed to germinate even under optimal conditions provided for germination is called Dormancy“.

In general definition: Seeds of many plant species do not germinate when freshly harvested even under favorable conditions, they need a period of rest or storage before they become capable of germination.

SEED DORMANCY TYPES

Below, you will find a list of various types of seed dormancy with examples.

Types of seed Dormancy: Harpen (1957) and Roberts (1972) have divided seed dormancy into three types.

1. INNATE DORMANCY: The inability of seeds to grow due to chemical or anatomical features of the seeds or organs themselves has been variously termed primary dormancy, true dormancy, innate dormancy, constitutive dormancy, or organic dormancy.
2. ENFORCED DORMANCY: The failure of seeds to grow under unfavorable environments moisture, oxygen, light, and temperature) has been referred to as enforced dormancy or imposed dormancy.
3. INDUCED DORMANCY: Dormancy is induced in normally germinable seeds due to their exposure to one or more unfavorable conditions, such as excessive moisture, adverse light, or temperature regimes, this phenomenon is referred to as Secondary dormancy or induced dormancy.

Note: In a germination test, the presence of hard seeds may from two potential causes. In Case 1, the seeds may be alive but in a dormant stage. In Case 2, the seeds could be dead or non-viable. Both scenarios can be verified through the Tetrazolium test or TZ or Seed Viability test.

Types of seed Dormancy: Nikolaeva (1977) has proposed a more detailed classification of dormancy

EXOGENOUS DORMANCY

Dormancy arises from external factors located outside the embryo.

A. Physical dormancy: physical impermeability of seed coat to water:

Dormancy is due to a hard seed coat (Hard testa) structure, which is hard enough to disallow the entry of moisture into the seeds, thereby preventing seed germination.

• Example- Seeds of Leguminoseae, Malvaceae, chenopodiaceae, convolvuaceae, Solanaceae, Liliaceae etc

B. Physical dormancy: Impermeability of seed coat to gases:

Dormancy is related to the insufficient intake by seeds of atmospheric gases, especially oxygen, due to the impermeability of seed structures enclosing the embryo.

• Examples– Gramineae, Leguminoseae, fruit crops, forest trees, etc.

C. Inhibitors present in seed coat/Endosperm:

Some biochemical substances present in seed coat or endosperm block the germination of an embryo.

A list of inhibitors found in different plant species

Inhibitors	Plant species where reported
Ferulic and Caffeic acids	Tomato
Aflatoxin, Abscisin	Barley
3-dibromo-4-hydroxybenzonitrilc acid	Squash, Barley
Coumarin	In most of the species

ENDOGENOUS DORMANCY

When factors responsible for the dormant state reside within the embryo, is known as endogenous dormancy.

A. Morphological dormancy: Incomplete embryo development:

Dormancy is due to incomplete development of the embryo (underdeveloped embryo or dormant embryo). Under such circumstances, germination remains dormant until the embryos reach their usual size.

• In the members of the families Palmaceae, Magnoliacea, Azaliaceae, Paeoniaceae, etc.

B. Physiological seed dormancy: Inhibitors present within the embryo: 

In such cases, germination can commence only when these inhibitors are leached out of the embryo.

• Example: In seeds of Xanthium, Fraximus, etc.
• Photoblastism: The phenomenon of improvement of germination by exposure to light is known as photoblastism. Seeds showing improvement in germination in response to light are called Positively photoblastic. (ex: Nicotiana tobacum, Lactuca sativa). While those exhibiting inhibition are referred to as Negatively photoblastic.
• Stratification: Seeds of many species remain dormant unless they are exposed to low temperatures for a certain period. Ex: members of family Rosaceae, Olearaceae, Aceraceae, Celastraceae, etc.

3. Combined dormancy: Dormancy is produced by a combination of two or more factors, like a combination of immature embryos, restriction on oxygen supply, and a chilling requirement.

METHODS OF BREAKING SEED DORMANCY

• Natural Breakdown of Dormancy: The impermeable seed coats present in many species become permeable due to the rupturing action of natural agents like microorganisms (like Rhizoctonia spp damage the seed coats of Albizzia julibrissin), high or low temperature, humidity, fire and abrasion due to wind or digestive tracts of birds and animals, which feed on these seeds.
• Seed Coat Treatment: These treatments are either physical or chemical in nature, and aim at making hard seed coats permeable to water or gases by either cracking or softening them.

Dry treatment: Mechanical or Thermal action

• Scarification: Rubbing the seeds on sandpaper manually or by using a mechanical scarifier. Care should be taken to avoid damage to the embryonic axis. This treatment is effective in species like Subabool. Green gram etc.
• The seed coat may be pierced by a needle or a small incision may be made in the abaxial end of the seed. Example: in Bitter Gourd (Memordica charantia).
• In some cases, complete removal of the seed coat in certain crops. Example in Rubber (Hevea spp)
• In some cases in Yam tuber, forest tree spp, and some gourds, we practice vibration techniques to loosen the seed coat.

Wet treatment: Thermal or Chemical action

• Seeds are soaked in a concentrated or dilute solution of Sulphuric acid for 1 to 60 minutes, after that seeds are washed with tap water to remove all traces of acid. Example in Cotton (Gossypium spp).
• In some species, for Example in Lentil, Bengal gram, etc, soaking the seeds in hot water (80^oC) for 1-5 minutes effectively softens their seed coats. Care should be taken because some species are highly sensitive.

Embryo treatment: Dormancy is due to factors located within the embryo.

• Stratification: This is the incubation of seeds at a suitable low temperature (usually 0-5^o C) over a moist substratum before transferring them to a temperature optimum for germination. It is more commonly used in crops like Cherry, mustard, Rosaceae.
• High-temperature treatment: In some species, incubation at 40-50 o C for a few hours to 1-5 days may be effective in overcoming dormancy. But seed moisture should be less than 15 %. Example in rice (Oryza sativa)
• Chemical treatment: Growth regulators or other chemicals may be applied to induce germination. The growth regulators most commonly used for this purpose are GA3 (100 ppm is the most commonly used concentration) and Kinetin (Concentration range, usually, 10-15 ppm). Other widely used chemicals are potassium nitrate (0.2%) and thiourea (0.5 to 3%). In the case of Wheat, Oats, Barley, Tomato, etc.

Also check out Seed pelleting, Steps in Seed pelleting, Types of Seed pelleting, Procedure of Seed pelleting, Advantages, and Disadvantages

CAUSES OF SEED DORMANCY

• Seed dormancy due to hard seed coat (prevent entry of water and gas).
• Due to immature or incomplete development of the embryo.
• Due to light-sensitive seeds (positive photo blastic or seed dormant at dark conditions and negative photoblastic or seed dormant at light conditions).
• Dormancy due to inhibitors present in the seed, like Coumarin, Absicic acid (ABA), Phenols, Cresols, and Cyanides.
• Due to temperature.

ADVANTAGES OF SEED DORMANCY

• Seedling Survival: Seed dormancy helps the chances of seedling survival by allowing seeds to remain quiescent until conditions become favorable for growth.
• Creation of a seed bank
• Synchronized germination ensures a uniform stage for the entire plant population.
• Development for outcrossing: Seed dormancy can promote outcrossing and genetic diversity by delaying germination until pollen from other plants is available and reducing the chances of inbreeding.
•  Seed dispersal involves the seed coat being scarified as it passes through the digestive tract of a bird.

DISADVANTAGES OF SEED DORMANCY

• Poor germination: Seed dormancy can lead to a reduced germination rate and affect crop yields.
• Time taken: Dormant seeds delay in germination.
• Storing dormant seeds for extended periods can incur costs associated with maintaining proper storage conditions, such as temperature and humidity control, which may strain resources.
• The additional cost of breaking dormancy.
• Not desired in the processing industry: Unpredictable germination rates or delays in the germination affect the seed industry market.

FAQ

Factors affecting seed dormancy?

• Physical impermeability of seed coat, water, and gas
• Inhibitors present in seed coat/Endosperm
• Temperature
• Light
• Period after ripening
• Hard seed coat

Seed dormancy?

Seeds do not germinate when freshly harvested even under favorable conditions, they need a period of rest or storage before they become capable of germination.

Scarification in seed dormancy?

Rubbing the seeds on sandpaper manually or by using a mechanical scarifier or piercing by a needle or complete removal of the seed coat in certain crops, Care should be taken to avoid damage to the embryonic axis. This treatment is effective in species like Subabool, Bitter gourd, and Rubber respectively.

Causes of seed dormancy?

• Seed dormancy due to hard seed coat
• Due to an immature embryo
• Due to light-sensitive seeds
• Due to inhibitors present in the seed
• Due to temperature

Methods of breaking seed dormancy?

• Dry treatment like Scarification, pierced by a needle, complete removal of the seed coat and vibration technique, etc.
• Wet treatment like Stratification, Sulphuric acid treatment, hot water treatment, chemical treatment (GA3), etc.

Which hormone breaks seed dormancy?

GA3, Kinetin, ethylene, and cytokinin are Other widely used for breaking seed dormancy.

Stratification seed dormancy?

Seeds of many species remain dormant unless they are exposed to low temperatures for a certain period. Ex: members of family Rosaceae, Olearaceae, Aceraceae, Celastraceae etc.

Three types of seed dormancy?

• Physical dormancy
• Physiological dormancy
• Morphological dormancy

What are the four types of seed dormancy?

• Physical dormancy
• Physiological dormancy
• Morphological dormancy
• Combined dormancy

What are the 5 types of seed dormancy?

• Innate dormancy
• Enforced dormancy
• Induced dormancy
• Physical dormancy
• Physiological dormancy

Significance of seed dormancy?

Dormancy helps to create a seed bank, it lets them stay alive in the ground for many years.

Physiological seed dormancy?

Dormancy due to Inhibitors present within the embryo or underdeveloped embryo development is known as physiological seed dormancy.

Physical seed dormancy?

Dormancy is due to a hard seed coat (Hard testa) structure, which is hard enough to disallow the entry of moisture and gas into the seed is known as physical dormancy.

Types of seed dormancy with examples?

Type of seed dormancy	Examples
Physical Dormancy	Leguminoseae, Malvaceae, chenopodiaceae, convolvuaceae, Solanaceae, Liliaceae
Physiological Dormancy	Xanthium, Fraximus etc
Morphological Dormancy	Palmaceae, Magnoliacea, Azaliaceae, Paeoniaceae etc.