Types of dormancy:

Different types of dormancy include

1. Exogenous Dormancy

• This type of dormancy is imposed by factors outside the embryo.
• In exogenous dormancy, the tissue that surrounds the embryo prevents water absorption, provides mechanical resistance to embryo expansion and radical emergence, modulates gaseous exchange (limiting oxygen to the embryo), inhibits leaching of the embryo from the embryo can affect germination by inhibiting and supplying an inhibitor to the embryo. It is of three types:

i) Physical dormancy (seed coat dormancy): during drying and ripening the seed cover may become hard, or fibrous or mucilaginous (adhesive gum), as a result of which they become impermeable to water and gases, which prevents the physiological processes initiating germination. Dormancy is very common in drupe fruits such as olives, peaches, plums, apricots, cherries (hardened endocarp), walnuts, and pecan nuts. In various plant families, such as Leguminaceae, the outer seed coat hardens and becomes impervious to water.

ii) Mechanical dormancy: In some fruits the seed cover inhibits radicle growth, resulting in the dormancy of the seeds. Some seed-covering structures, such as walnut shells, stone fruit kernels, and olive stones, are so strong that dormant embryos do not allow expansion during germination. Water can be absorbed but the difficulty arises due to cementing materials like walnut. Germination in such seeds does not occur unless the seed cover is softened by creating moist and warm conditions during storage or by microbial activity.

iii) Chemical dormancy: Chemicals in seeds of some fruits accumulate in the tissues covering the fruit and seed during development and remain with the seed after harvesting. It is very common in fleshy fruits or fruits whose seeds remain in juice, such as citrus, cucumber, stone fruit, pear, grape, and tomato. Some of the chemicals that cause inhibition are phenol, coumarin, and abscisic acid. These substances can strongly inhibit seed germination.

2. Endogenous dormancy

This type of dormancy occurs at the time of ripening or maturation by a rudimentary or underdeveloped embryo. It can be of different types such as morphological, physiological, double dormancy, and secondary dormancy.

i) Morphological dormancy (Rudimentary and linear embryo): Some seeds have dormancy in which the embryo is not fully developed at the time of seed propagation. Such seeds do not germinate when planted immediately after harvesting. Plants with a rudimentary embryo produce slightly more seeds than a pro-embryo embedded in a large endosperm at the time of fruit maturity. The size of the embryo increases after the seed has taken up water but before germination begins. The formation of rudimentary embryos is common in various plant families such as Ranunculaceae (Ranunculus), Papaveraceae (Poppy).

ii) Physiological dormancy

a) Non-deep physiological dormancy: Seeds require time in dry storage to lose dormancy after ripening. This type of dormancy is often transitory and disappears during dry storage. Temperate fruits such as apples, pears, cherries, peaches, plums and apricots, cultivated cereals, vegetables, and flower crops have this type of physiological dormancy that can last one to six months and disappear with dry storage.

b) Photo dormancy: The seeds which require light or dark conditions to germinate are called photo-dormant seeds. This is due to a photo-chemically reactive pigment called phytochrome, widely present in some plants. When imbibed seeds are exposed to red light (660–760 nm), the phytochrome changes to the red form (PFR), thereby altering the germination process. However, when seeds are exposed to far-red light (760–800), Pfr is converted to Pf which inhibits the germination process.

c) Thermo dormancy: Some seeds require a specific temperature for germination, otherwise, they remain dormant. Such seeds are said to be thermo-dormant seeds. For example, lettuce, celery, and pansies do not germinate if the temperature is below 25⁰

Physiological dormancy is of 3 types:

I) Intermediate physiological dormancy: Seeds of some species require a specific period of chilling of one to three months, while in imbibed and aerated state, commonly referred to as moist chilling. For example, seeds of most temperate fruits require moist chilling to remove seed dormancy. This requirement led to the standardization of the world-famous, horticultural practice of stratification. In this process, seeds are placed in boxes between layers of moist sand and exposed to chilling temperatures (2 to 7⁰C) for a period of 3-6 months to remove dormancy.

II) Deep physiological dormancy: Seeds, which typically require a relatively long (>8 weeks) period of moist chill stratification to overcome dormancy, like a peach.

III) Epicotyl dormancy: Seeds having different dormancy positions for radical hypocotyl and epicotyl are called epicotyl dormancy. Lilium, Hepatic Antiloba, and Trillium.

iii) Double dormancy

• In some species, seeds are dormant due to hard seed coats/sheaths and dormant embryos.
• For example, the seed coat of some tree legumes is impermeable and at the same time, their embryos are dormant.
• Such seeds require two years to break dormancy in nature. First, in spring, microorganisms act on the seed to make it weak and soft, and then in winter the following year cold temperatures break the dormancy of the embryo.
• The combination of two or more types of dormancy is known as ‘dual dormancy’. It can be morpho-physiological i.e. a combination of less developed embryonic and physiological dormancy or exo-endodormancy i.e. combination of exogenous and endogenous dormancy conditions i.e. hard seed coat (physical plus intermediate physiological dormancy).

iv) Secondary dormancy

Secondary dormancy is due to germination conditions. This is another adaptation to prevent the germination of seeds if other environmental conditions are not favorable. These conditions can be caused by unfavorably high or low temperatures, prolonged darkness, and lack of water. It is of 2 types:

I) Thermo dormancy: High temperature-induced dormancy.

II) Conditional dormancy: Change inability to germinate related to the time of the year.

Advantages

1. The seed is allowed to germinate only if the environmental conditions are in favor of the survival of the seedling as in the fruit plants of the temperate zone.
2. Helpful in the creation of the “Seed Bank”
3. Dormancy can also synchronize germination at a particular time of year.
4. Seed disposal can be facilitated in a particular dormant stage. For example the modification of the seed cover through the digestive system of a bird or other animal.

 

Methods of Breaking Dormancy

Several methods are used to break the seed dormancy of horticultural crops. These are briefly described here:

1. Softening of seed cover and other covering: It helps in better absorption of water and gases, which ultimately leads to better germination of seeds. This can be achieved by the following methods.

a) Scarification:

Scarification is the process of breaking, scraping, mechanically altering, or softening the seed cover to make it permeable to water and gases. Three types of treatments are commonly used as scarification treatments. These include mechanical, chemical, and hot water treatments.

i) Mechanical scarification

• It is simple and effective when the appropriate equipment is available.
• Scrubbing seeds with hard seed coats with sandpaper, cutting with a file, or cracking with a hammer are simple methods useful for small amounts of relatively large seeds. The sand gravel should be of a different size than the seed to facilitate subsequent separation.
• This treatment should not proceed to the point at which the seed is injured and the inner part of the seed is exposed.

ii) Acid scarification

• Dried seeds are placed in containers and concentrated sulfuric acid (H2SO4) or HCl in the ratio of one part seed to two parts acid is added to the seeds.
• The quantity of seed treated at any point in time should not exceed 10 kg to avoid uncontrolled heating.
• The containers must be of glass, earthenware or wood, non-metal or plastic. During the treatment, the mixture should be stirred carefully at intervals to give a uniform result.
• Time may vary from 10 minutes to 6 hours depending on the species.
• For thick coat seeds that require a longer duration, the process of scarification can be estimated by sampling at intervals and checking the thickness of the seed coat. When it becomes paper-thin, the treatment should be stopped immediately.
• At the end of the treatment period, the acid is removed and the seeds are washed to remove the acid.
• Acid-treated seeds can be sown immediately either wet or dry and can also be stored for later planting. Seeds of most legume species, large seeds, brinjal, and tomato seeds show good results with simple sulfuric acid treatment.

iii) Hot water scarification

• Put the seeds in hot water of 77 to 100⁰C temperature 4-5 times according to their quantity.
• The heat source is immediately removed, and the seeds are soaked in boiling water for 12 to 24 hours. The swollen seeds can then be separated from the non-swollen seeds by a suitable screen.
• Seeds should be sown immediately after hot water treatment.

iv) Warm moist scarification

• The seeds are kept for several months in a moist warm medium to soften the seed cover and other covering through microbial activity. This treatment is highly beneficial in double dormant seeds.
• Hard seeds are planted in summer or before winter when soil temperatures are high, which usually helps in germination.
• For example, stone fruits, including cherries, plums, apricots, and peaches, show increased germination when planted early enough in summer, or germinate early by providing warm temperatures one to two months before chilling.

b) Stratification:

• Stratification is a method of dormant seed handling in which imbibed seeds are given chilling treatment to ripen the embryo in alternating layers of sand or soil for a specified period. It is also known as moist chilling.
• However, in temperate species, epicotyl dormancy (such as fringed trees) or hollies require moist chilling stratification after several months of warm stratification.
• Many tropical and subtropical species (eg palm) require a period of warm stratification before germination so that embryo development can continue after fruit drop.
• Seeds can be sown after fruit drop or seeds can be sown immediately after stratification in the field.
• Seeds with hard endocarps, such as Prunus species (stone fruit including cherry, plum, apricot, and peach) show increased germination when planted early in summer or provided warm temperatures one to two months before the start of chilling.

i) Outdoor stratification

• If refrigerated storage facilities are not available, outdoor stratification can be done either by storing seeds in deep pits in open field conditions or in raised beds enclosed with wooden frames.
• Although there is a possibility that seeds may be destroyed in outdoor stratification by excessive rain, freezing, drying, or by rodents. The seeds are placed in alternating layers of sand in the stratification pit to provide low temperature and proper aeration. The top is covered with sphagnum moss to maintain the moisture level.
• The pit or tray is irrigated at regular intervals to maintain suitable moisture conditions.

ii) Refrigerated stratification

• An alternative to outdoor stratification is refrigerated stratification.
• It is useful for small seed lots or valuable seeds that require special handling.
• Dried seeds should be completely absorbed in water before refrigerated stratification. Twelve to 24 hours of soaking at warm temperatures may be sufficient for seeds without a hard seed coat.
• After soaking, the seeds are usually placed in a conveniently sized box in alternating layers of well-washed sand, peat moss, or vermiculite.
• A good medium is a mixture of one part coarse sand to one part peat, which is moistened and allowed to sit for 24 hours before use. The seeds are placed in alternating layers of sand or medium.
• The normal stratification temperature is 4-7⁰ C. Seeds germinate prematurely at higher temperatures and delayed germination at lower temperatures.
• The medium should be re-wetted. Stratification seed is separated from the medium before sowing in nursery beds.
• Stratification of seeds results in quick and uniform germination and hence seed stratification should always be done before sowing under all conditions.

Table: –  Effect of Seed Stratification Period on Percent Germination of Important Temperate Fruits

Kind of fruit	Stratification period (days)	% germination
Apple	70-75	70-75
Kainth (Pyrus pashia)	30-35	90-95
Peach	60-70	55-60
Apricot	45-50	75-80
Almond	45-50	85-90
Walnut	95-100	80-85
Pecan	70-75	75-80

 

2) Leaching of inhibitors: It is an established fact that the seed cover of many species contains some inhibitors and phenolic compounds, which inhibit germination. Therefore, soaking the seeds in running water for 12-24 hours or keeping them in water for a few hours helps in the leaching of inhibitors and phenolic compounds, which help in easy seed germination.

3) Pre-chilling: In seeds of some plant species, dormancy can be removed by pre-chilling treatment. In this treatment, imbibed or soaked seeds are kept at a temperature of 5-10⁰C for 5-7 days before sowing. After that, the seed can be sown in the field immediately.

4) Pre-drying: This is also a useful process in some seeds to remove seed dormancy. In this treatment, the dried seeds are kept at 37-40⁰C temperature for 5-7 days before sowing. After this, the seeds can be sown in the field.

5) Seed priming: Seed priming refers to the procedures followed to remove dormancy in freshly harvested fruits. The most widely used seed priming processes are osmo-conditioning, infusion, and fluid drilling.

• In Osmo-conditioning, seeds are placed in a shallow layer in a container containing a 20-30% solution of polyglycol (PEG). Depending on the size of the seed and the species of the plant, the seeds are incubated at 15-20⁰C for 7-21 days.
• Various hormones and fungicides may also be added to protect the seeds from pathogens. Thereafter, the seeds are washed and dried at 25⁰C and stored until use.
• In infusions, hormones, fungicides or insecticides, and antidotes are injected through organic solutions into dormant seeds. In this process, the seeds are placed in a solution of acetone or dichloromethane containing chemicals that are used for 1-4 hours.
• Afterward, the solvent is allowed to evaporate and the seeds are dried slowly in vacuum desiccators for 1-2 hours. When soaked in water, the seeds directly absorb the infecting chemical in the embryo.
• In fluid drilling, seeds are suspended in a special type of gel before planting. There are different types of gels available in the market nowadays but sodium alginate, guar gum, and synthetic clay are widely used in drilling fluids.

6) Treatment with chemicals: Some compounds other than hormones are also used to break dormancy but their role is unclear. Thiourea is an example that is known to stimulate germination in some types of dormant seeds. The seeds are soaked in a 0.5 – 3 percent solution of thiourea for 3-5 minutes. Afterward, the seeds are washed with water and sown in the field. Similarly, potassium nitrate and sodium hypochlorite also stimulate seed germination in many plant species.