Seed Dormancy

Introduction

Seed dormancy is the intrinsic property of a seed that determines the environmental conditions for the seeds to germinate. Seed dormancy can be defined as a condition in which seeds are prevented from germinating under favourable conditions such as temperature, light, water, gases, seed, coats, etc. Seeds require an ample amount of time before they are fully capable of germinating. The time may differ from several days to months, and it may take years. These factors include hormones, heat, water, light, etc. Seed germination is the growth of a seed into a young plant.

Seed Dormancy

Seed dormancy is when seeds do not germinate if environmental conditions such as temperature, air, and water are favourable for germination.

Under optimum conditions like moisture, temperature, and aeration, some fruits like mango and citrus germinate from the fruit. But, in some cases; fruits like apple, pear, cherry germination do not occur if favourable conditions are provided. This phase is called ‘dormancy’. Dormancy acts as the survival mechanism as these species do not germinate until adverse conditions end. It has been observed that in some species, low temperature leads to the termination of the dormancy period. Dormancy is characterised by many factors and can persist indefinitely unless specific treatment is given.

Types of Dormancy

Different types of include-

Exogenous Dormancy

Factors outside the embryo characterise exogenous dormancy. In this type of dormancy, germination is affected by the tissue enclosing the embryo by inhibiting water intake and providing mechanical resistance to embryo expansion, preparing for gaseous exchange by limiting oxygen to the embryo, leaching of inhibitors are prevented from the embryo and thus supplying inhibitor to the embryo.

Further, it is classified into three types-

  1. Physical dormancy (seed coat dormancy): During dehydration and ripening, seed coats may become hard, fibrous, or gummy, i.e., adhesive gum, due to which they become impermeable to water and gases, thus preventing the physiological process of initiation of germination. This is a common type of dormancy generally seen in drupe fruits having hard endocarps like olive, peach, plum, cherry, apricot, etc., and walnut, pecan nut having surrounded shell. The outer seed coat of the leguminosae family is hardened, becomes impervious to water, and gets suberised.
  2. Mechanical dormancy: In some cases, fruit coverings hinder the growth of the radicle and result in the dormancy of seeds. Seed covering like shells of walnut, pits of stone fruits, and stones of olive are too strong to allow the dormant embryo to expand during germination. In these types of seeds, germination does not occur until seed coats get softened either by creating warm or moist conditions during storage or by some microbial action.
  3. Chemical dormancy: This type of dormancy is quite common in fleshy fruits, citrus fruits, cucurbits, stone fruits, pear, grapes, and tomatoes. This dormancy is determined by phenols, coumarin, and abscisic acid factors. These substances are strong inhibitors of seed germination.

Endogenous Dormancy

Endogenous dormancy is imposed by a rudimentary or underdeveloped embryo occurring at the time of maturity. 

Further, it is of the following types- 

  1. Morphological dormancy (rudimentary and linear embryo): In this type of dormancy, the embryo is not fully developed at the time of seed germination. Seeds do not germinate if planted immediately after harvesting. Families such as Ranunculaceae (Ranunculus) Papaveraceae (poppy) have a rudimentary embryo.

Physiological Dormancy

  1. Non-deep physiological dormancy: This type of dormancy is transitory and disappears during dry. Temperate fruits like apple, pear, cherry, peach, and apricot, vegetables and flower crops, cultivated cereals have this type of dormancy.
  2. Photo dormancy: Seeds that require light or dark conditions. It is mainly due to photo-chemically pigment called phytochrome present in plants.
  3. Thermo dormancy: Seeds that require a specific temperature for their germination are called thermo dormant. Seeds of celery and pansy do not germinate when temperatures are below 250 ℃.

Double Dormancy

This type of dormancy is due to hard seed coats. 

For example- seed coats of legumes are impervious, and embryos are dormant.

Secondary Dormancy

This type of dormancy is due to germination conditions. These conditions include unfavourably high or low temperature, darkness, and water stress. It is categorised into two types-

  1. Thermo dormancy
  2. Conditional dormancy

Advantages of Seed Dormancy

  1. Germination occurs only when there are favourable conditions present for seedlings.
  2. Helps in creating a seed bank.

Methods of Breaking Seed Dormancy

There are several methods used for breaking seed dormancy. These are-

  1. Softening seed coat and other seed coverings; helps in better absorption of water and gases that lead to the better germination of seeds. This is achieved by scarification.

Scarification: Process of breaking, scratching, mechanically altering, or softening the seed covering to make it permeable to water and gases. Generally, three types of treatments are commonly used as scarification treatments. These are- mechanical, chemical, and hot water treatments.

Mechanical scarification: This is simple and effective. Hard seed coat rubbed on sandpaper or cutting with a file or cracking with a hammer are simple methods for a small amount of relatively large seeds. On a large scale, mechanical scarifiers are used.

Acid scarification: Dry seeds are placed in the ratio of one part of the seed to two parts of acid (HCl or H2SO4) in the containers of glass, earthenware, wood, non-metals or plastic. Time varies depending upon the species, from 10 minutes to 6 hours. The acid is poured off, and the seeds are washed to remove the acid in the last step. Seeds of legume species, brinjal, and tomatoes are treated with this type of treatment.

Hot water scarification: Seeds are dropped into the hot water with temperatures ranging from 77 to 100 ℃. Seeds are soaked in cooking water after subsequent removal of the heat source. With the help of a suitable screen, unswollen seeds are removed from swollen seeds.

Warm moist scarification: Seeds are placed in a warm medium to soften the seed coat. This treatment is beneficial for seeds having double seed dormancy.

Stratification: It is a method of handling dormant seeds in which seeds are subjected to a period of chilled temperature after the embryo gets ripened. It can also be termed as moist chilling.

It is of the following types-

  1. Outdoor stratification: Refrigerated facilities are not available. These are provided by storing seeds in open field conditions in deep pits or wooden frames. This pit or tray provides moisture to the seed if irrigated at regular intervals.
  2. Refrigerated stratification: Acts as an alternative to outdoor stratification. Useful for tiny seeds that require special handling. Dry seeds are imbibed with water to refrigerated stratification for twelve to twenty four hours. Twenty-four hours of soaking is sufficient for seeds without hard seed coats. In this, seeds are treated at low temperatures. The stratification temperature is usually 4-7 ℃.  At higher temperatures, seeds sprout prematurely and low temperature delays sprouting.
  3. Leaching of inhibitors: Some inhibitors of phenolic compounds present in seed covering inhibit germination so leaching requires washing them with fresh water.
  4. Pre-chilling: Dormancy can be overcome by pre-chilling treatment. Seeds soaked at 5-10 ℃ for 5-7 days are soaked immediately in the field.
  5. Pre-drying: Acts as a valuable method to overcome dormancy. Seeds are subjected to a temperature of 37-40 ℃ for 5-7 days before sowing.
  6. Seed priming: freshly harvested fruits overcome dormancy by this method. This is the most widely used method. This method used procedures like osmo- conditioning, infusion and fluid drilling.
  7. Hormonal treatment: Among various hormones, GA is used for breaking seed dormancy in different seeds. Cytokinin is another hormone that breaks physiological dormancy and stimulates germination in seeds. Kinetin and BAP (Benzylamino Purine) have commercially used cytokinin for breaking seed dormancy.

Conclusion

We can conclude from the above discussion that seed dormancy allows seeds to overcome unfavourable conditions for seeds that hinder their growth. Seed dormancy finds essential applications in plant ecology and agriculture. Several processes and methods we discussed helped us overcome the period of dormancy and thus switch from the dormant to the germination stage.

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