Seed Dormancy,Overcoming seed dormancy

Seed dormancy

        

Seed dormancy usually develops as the result of the action of two kinds of processes; one within the embryo and one in the seed coat and other tissues external to the embryo. So, Crocker (1916) divided seed dormancy into seed-coat induced and embryo-induced.

   

1. Seed-coat induced dormancy : The dormancy of seeds due to the extreme hardness of seed-coat is called seed-coat induced dormancy. The hard seed-coat checks the entry of water, exchange of gases and expansion of embryo.

   

2. Embryo-induced dormancy : The dormancy of seeds due to rudimentary or complete dormant embryo is called embryo-induced dormancy. In this case the rudimentary embryo is incompletely developed and requires a rest period for complete development while complete dormant embryo shows dormancy due to physiological conditions.

Other types of dormancy may be:

   

3. Secondary dormancy : when the seeds become dormant again after breaking the dormancy, it is called secondary dormancy. It may be due to combination of different kinds of dormancy in a single seed, e.g., Xanthium pennsylvanicum.

 4. Special type of dormancy : The failure of seedling development is not always traceable due to dormancy of seed itself. In many of the spring wild plants the germination of seed takes place but the growth is restricted due to establishment of young roots. Sometimes the system of epicotyl fails to germinate. In some cases, the epicotyl may be pushed through the seed-coat but remains dormant. This dormancy is often broken by exposure to low temperature.

CAUSES OF SEED DORMANCY-

The dormancy of seeds may be either due to single or a combination of many different factors. There are many causes for seed dormancy-

Seed-coat Induced Dormancy-

      

The seed-coat of most of the seeds is formed by the integumentary layers of ovules. Chemically it is composed of a complex mixture of polysaccharides, hemicellulose, fats, wax and proteins. During seed ripening, the chemical components of the seed coat become dehydrated and form a hard and tough protective covering around the embryo.

The seed coat induced dormancy may be due to following causes-

Water impermeability- the seed-coats of many plants are completely impermeable to water for e.g. seeds of families Leguminosae, Malvaceae, Chenopodiaceae, Convolvulaceae, Solanaceae and Nympheaceae etc. Here   the germination fails to occur until water penetrates through seed-coat. In many such seeds permeability of coat to water increases slowly in dry stage. The action of bacteria and fungi also increases the permeability of seed-coat to water and shortens the dormant period of seeds. In many plants seeds have waxy coating.

Gas impermeability – The seed-coats of certain seeds are impermeable to gases such as oxygen and carbon – di- oxide. Since oxygen is required for early respiratory activity in germinating seeds, the seeds fail to prolong germination. E.g. grasses, Compositae.

Mechanical resistance of seed-coat to the growth of embryo- The seeds of some common weeds such as Alisma, Amaranthus and Capsella etc. have such hard and tough seed-coat that it prevents any expansion of embryo. Thus, they remain dormant. However, if the seed-coats become dry and then again become saturated with water, they are no longer able to resist the expansion of embryo. The seed-coat ruptures and germination takes place.

Embryo-induced Dormancy-

Embryo-induced dormancy may be of two kinds.

Rudimentary and poorly developed embryo- In many plant species like Anemone nemorosa, Fraxinus excelsior, Ginkgo biloba, and members of Orchidaceae, Orobancheae etc., and the seed dormancy may be due to immature and rudimentary embryo. In such seeds the embryo does not develop as rapidly as surrounding tissues. Thus, when the seeds are shed, they are still imperfectly developed due to incomplete embryo. Thus, when the seeds are shed, they are still imperfectly developed due to incomplete embryo. The germination of such seeds takes place only after a period of rest (dormancy) during which the further development of embryo is completed.

Embryo fully developed but unable to resume growth- In many species,e.g. seeds of apple, peach, Iris, Hemlock, peas, cherry, etc., although the embryos are completely developed in ripe seeds but the seeds fail to germinate even when the environmental conditions for  germination are favourable. Dormancy of such seeds is due to the physiological reasons of embryo. The germination in such seeds can be induced if they are stored in moist, well aerated and low temperature conditions.

Dormancy due to Specific Light Requirement

              

The seeds of Lactuca sativa, Lythrus salicaria, Nicotiana tabacum etc. have a specific light requirement for germination. In imbibed Lactuca sativa seeds the germination is stimulated by red light of 660 nm wavelength, while it is inhibited by far-red light of 730 nm wavelength. The germination of Bignonia requires a photoperiod of 12 or more hours for seed germination. The light sensitive seeds are called photoblastic.

Dormancy due to Germination Inhibitors-

Sometimes dormancy occurs due to presence of germination inhibitors in seed-coats, endosperm, embryos or structures surrounding them such as the juice or the pulp of the fruit e.g. in tomatoes, and in glumes. A number of chemical substances such as organic acids, phenolics, tannins, alkaloids, unsaturated lactones, mustard oil ammonia releasing substances, cyanide releasing substances, indoles and gibberellins etc. have been isolated from the seeds which act as germination inhibitors. Besides, the other natural inhibitors are coumarin, parascorbic acid, ammonia, phthalides, ferulic acids, and abscisin II etc. If these inhibitors are leached out, the germination of seeds takes place.

METHODS OF BREAKING SEED DORMANCY

   

The dormancy of seeds can be broken and the dormant seed can be induced to germinate by one or more methods described below-

Scarification

      

This method is used for breaking dormancy of seeds caused by hard seed coats which becomes impermeable to water and gases etc. In this method the seed-coat is made permeable either by mechanical methods or chemical treatments.  The seed-coat becomes soft and weak by this treatment. When mechanical breaking of seed-coat is done at one or more places, it is called Mechanical Scarification. The treatment of seed-coat with strong mineral acids or other chemicals is called chemical Scarification. Mechanical Scarification is done by shaking the seeds with sand or by nicking the seed-coat with knife. Chemical Scarification is usually done by dipping seed into strong acids like H2SO4 or into organic solvents like acetone or alcohol. It can also be done by boiling the seeds in water. Under natural conditions in the soil, micro-organisms like bacteria and fungi act upon the seed-coat to decompose it.

Stratification

               

This method is used to break the embryo – induced dormancy. In this process the seeds are exposed to well aerated, moist conditions under low temperature (0° to 10°C) for weeks to months. This treatment is called Stratification or after- ripening. During stratification some chemical changes occur in the immature embryo of seeds which are necessary for seed germination. These changes are-

The concentration of nitrogen and phosphorus are shifted to the various parts of the seeds.

Various constituent amino-acids, organic acids and enzymes are also shifted.

Cyanogenic glycosides are decomposed.

The concentration of various growth regulators is changed.

Alternating temperature

               

In some seeds, e.g. Poa pratensis the seed dormancy is broken by the treatment of alternating low and high temperatures. The difference between the alternating temperatures should not be more than 10-20°C. his method is beneficial in those seeds in which the dormancy is due to immature embryos. Alternating temperature of 15°C and 25°C is useful in breaking the dormancy of photoblastic seeds like Rumex crispus.

Light

              

The light sensitive seeds are called photoblastic which may be of following three types:

Positive photoblastic seeds : the seeds requiring single exposure of light for germination  are called positive photoblastic seeds, e.g., Lactuca sativa.

Negative photoblastic seeds :

The seeds requiring complete darkness  for germination  are called negative photoblastic seeds,

Non-photoblastic seeds :

The seeds requiring either light or darkness for germination  are called non-photoblastic seeds,

The dormancy of Positive photoblastic seeds can be broken by exposing them to red light (660nm). Far –red light inhibits the seed germination indicating the involvement of photoreversible pigment phytochrome in the process of seed germination. This pigment occurs in two forms, one red light absorbing and other far- red light absorbing. Both these forms are photochemically interconvertible. The red light absorbing form (PR) is converted into far-red form (PFR) after absorbing the red light. The far-red form absorbs the far-red light and is converted back into red absorbing form of the pigment.

It is supposed that in positive photoblastic seeds, the far- red absorbing form of the pigment is stimulatory to seed germination while red absorbing form is inhibitory to seed germination. 

         

Pressure

           

The seed germination in certain plants like sweet clover (Melilotus alba) and alfalfa (Medicago sativa) can be greatly improved after being subjected to hydraulic pressure of about 2000 atm. at 18°C for about 5-20 minutes. This pressure changes the permeability of seed coat to water resulting into seed germination.

Growth regulators

    

Growth regulators are used to hasten the development of roots or cuttings and to increase the number of roots. Kinetins and gibberellins have been used to induce germination in positively photoblastic seeds. Besides, a number of chemicals such as KNO3, thiourea and ethylene etc. have also the capacity to induce seed germination.