Seed Dormancy

CHAPTER: 14(B)

Seed Dormancy

Seed Dormancy:

•      The growth of  a seed is completely arrested after it is fully developed.

•      In some  cases the growth is suspended because of the lack of favorable conditions in the environment.

•      Such seeds will germinate if they are supplied with water and suitable temperature.

•      In many other cases, however , the seeds would not germinate, even if they are provided with water etc.

•      This failure of seeds to germinate even under favorable conditions is termed as dormancy.

•      In such cases , the completely dry ripe seed is physiologically inactive and is said to be in a resting stage. This seed is called dormant and the phenomenon is termed dormancy.

•      Dormancy incidentally helps in overcoming unfavorable conditions.

•      In majority of cases, true dormancy helps the seed to survive through an unfavorable season e.g., a very cold winter or a very hot summer.

Types of dormancy:

1. Imposed Dormancy/ Quiescence:

•      When the suspension of active growth results due to the effects of favorable environmental conditions is called imposed dormancy.

•      Such seeds which just wait for suitable environmental condition to germinate are said to be quiescent (inactive).

2. Innate Dormancy:

•      When the active growth is arrested even under favorable environment condition due to some internal factors is called innate dormancy.

3. Secondary Dormancy:

•      Many seeds are capable of germinating immediately after they are shed. Such seeds can, however, pass into a dormant stage, if they are placed in favorable conditions for some time.

•      This type of induced dormancy is termed as secondary dormancy.

•      Absence of light or presence of very low temperature or very high concentration of CO₂ may induced secondary dormancy.

•      In most cases, secondary dormancy can be overcome by chilling treatment.

Advantages of Dormancy:

1. Easy to handle during post harvest preservation.
2. Prevents plant from adverse environmental conditions
3. If the cereals do not have dormancy, seeds germinate immediately after harvesting. So, otherwise it might have been difficult to preserve cereals.
4. Insitu preservation is possible.
5. Temporary dormant period experienced by many cereals grains allow easy harvest and storage. Otherwise these grains would germinate in the field and be useless to man.

Disadvantages of Dormancy:

1. The ability of certain weed seeds to lie dormant for many years in the soil proves to be a great inconvenience. During ploughing, the dormancy of many of these seed will be broken, thereby allowing to compete with any economic crop sown in the area.
2. Delays germination can not germinate seed at the desired time.

Causes of Dormancy ( Factors Responsible for Dormancy):

Ø  Seed dormancy may be due to any one of the causes or a combination of two or more causes. The causes of seed dormancy are describe below:

1. Impermeability of seed coats to water:

•      Seeds of many plants including the leguminosae, malvaceae, and solanaceae have very hard seed coats. Such seed coats are impermeable to water.

2. Impermeability of seed coats to oxygen:

•      In some species the hard seed coat is completely impermeable to oxygen. Such seeds would not germinate unless they have undergone the period of dormancy during which the permeability of the seed coats will gradually increase.

3. Mechanically resistant seed coats:

•      In certain wild plants the hard seed coat acts as a physical barrier to the expansion of the embryo.

4. Tough Seed coats:

•      In some seeds like mustard, the seed coats are so strong that they do not yield to the pressure of the expanding embryo.

•      The failure of the seed coats to burst under this pressure is responsible for their dormancy.

5. Immature Embryo:

•      In some cases the ripe seeds, when shed, contain only a rudimentary or imperfectly developed embryo. Such embryos may consist only of the fertilized egg or in other cases they may be in a partial state of development or the development may be nearing completion.

•      Germination is naturally delayed till the embryo is completely developed.

6. Dormant Embryo:

•      Seed of some plants will not germinate even though the embryos are completely developed and all the conditions necessary for germination are provided.

•      No germination occurs even if the seed coats are removed.

•      Dormancy in such cases is due to physiological conditions of the embryo.

•      Some physiological changes called after ripening occur in the embryo before seed is capable of germination.

7. Inhibitors of Germination:

•      Ammonia of halides, coumarin and Paraascorbic acid , Ferrulic acid, Cyanides may present in seed may cause seed dormancy.

Methods of Breaking Seed Dormancy:

•      The dormancy of seeds though very useful to man is not likely by the farmers who would like the seeds to germinate soon after they have harvested.

•      A number of methods are, therefore, employed for the breaking of dormancy.

•      The methods of breaking seed dormancy are described below:

1. Scarification:

•      This the method by which the hard or tough seed coats ruptures or weakened to facilitate their germination . It can be performed by two ways:

1. Mechanical Scarification:

•      This method is carried out in such a way that the embryo remain uninjured or undisturbed. It can be done by using machine, hands , hammer, knife of Scalpel or sand paper or rough stone.

2. Chemical Scarification:

•      This method is carried out by keeping the seeds in strong mineral acids like KCl, nitiric acid, Sulphuric acid or organic solvents.

•      Care should be taken during chemical scarification so that the embryos are not broken or injured or damaged.

2. Stratifiction:

•      The dormant seeds are treated at low temperature under moist condition. This method helps to decrease acidic inhibitors action and increase in growth activator content and mobilized food materials.

3. Impaction:

•      In case of legumes there is formation of plugging then after shock and jerking i.e. mechanical power, the seed germinates. This method is called impaction.

•      The impaction increases the absorption of water to the plugged seeds.

4. Chilling Treatment:

•      In many woody and certain herbaceous species the dormancy can be broken by chilling treatments, just above freezing 0^oC - 5^oC.

•      Under field conditions this chilling requirement is normally met by natural winter temperatures. This is why such seeds do not germinate in the autumn when they are shed and their germination is delayed until the following spring.

5. Alternating temperature:

•      In some cases, subjecting the seeds to alternately low and high temperature also breaks the dormancy of the embryo. The difference between low and high temperature should be about 10^oC.

•      More favorable results are obtained if the low temperature period is long and high temperature period is short.

6. Pressure:

•      Subjecting the seeds of some plants, for e.g., alfaalfa, to high hydraulic pressure at a medium temperature increases the permeability of the seeds coats to water and thereby greatly improves the germinability of seeds.

7. Exposure to light:

•      Red light fasten germination under humid condition in positive photoblastic seeds.

•      Photoblastic seeds with light requirement can be made to germinate by proper exposure to light.

•      Red and far light play some very important role in germination. Phytochrome regulates several light dependent developmental processes in plants.

8. Chemical Treatment:

•      The urea, Potassium nitrate, ethylene, purine, pyrimidines, Gibberllins, Cytokinins, Ethylene are the well known chemicals which help to break seed dormancy.