Following methods are used to break the seed dormant:
- Methods of breaking dormancy of hard seed coat:
The dormancy of hard seed coal can be broken by following methods:
(a) Seed coat can be removed from such dormant seed.Thus they can germinate under favourable conditions. But removal of the entire seed coat is not possible. It can injure the embryo. Therefore the seed coat is punctured or mechanically ruptured by rubbing. This process is called scarification.
(b) The seed is treated with hot water or concentrated sulphuric acid for very short duration. It makes the seed coat soft and dormancy of seed is broken.
(c) However. in nature seed remain buried in the soil. The action of soil micro-organisms makes its hard wall soft. Therefore, the dormancy of seed is broken.
- Method of breaking dormancy of light sensitive seeds
The dormancy of light sensitive seeds can be broken by following methods:
(a) Exposure to white light: The exposure to light promotes this process. Its other examples are seeds of tobacco. tomato, Betula and Digitalis. Such seeds respond to a light only after imbibing 30-40% moisture. Light is ineffective in the dry condition.
(b) Exposure to red and far red lights: The exposure to very low intensity of light for short duration of 1-2 minutes is sufficient to overcome dormancy . The red part of white light of wavelength of 660u is very effective for germination. If the seeds is exposed to far red radiation (730u) germination is inhibited. If the seed is first exposed to red light and immediately afterwards to far red, then again germination is inhibited. It means that the effect of red light is cancelled by far-red. The far-red is immediately followed by red light. now the seeds germinate. It means the effect of far-red is reversed by red light. The seed can be exposed to red and far red a number of times. The effect of red is cancelled by far red and vice versa. The behaviour of the seed depends upon the last radiation given. If the last installment given is red, then the seed germinates. If it is far red, then the seed fails to germinate.
(c) Role of phytochromes: Borthwick and Handricks were working in the U.S. Department of Agriculture.They discovered new pigment in plants. This pigment was named phytochrome. Phytochrome is responsible for absorbing red and far-red radiations. They control sereral morphogenetic phenomena like seed germination and flowering.
(d) Use of– plant hormones: The application of the hormones. gibberellic acid and kinetin can replace the red light requirement or germination of lettuce seeds. Therefore. seeds can germinate in the presence of these hormones in the total darkness.
(e) Role of darkness: The red far-red reversibility and phytochrome does not play any role in negative photoblastic seeds. Therefore, the dormancy of negative photoblastic seed can be broken by placing them in more than critical dark period.
- Method of breaking dormancy of chilling requiring seeds
There are two methods to break the dormancy of chilling requiring seeds:
(a) Stratification: The dormancy of chilling required seed is broken by stratification. Low temperature requirement given to the seeds to break their dormancy is called stratification. Dry seeds cannot be stratified. A minimum amount of moisture is required. Therefore, such seeds are allowed to imbibe water. Then they are exposed to low temperature.
(b) Use of hormone gibberellic acid: Such seeds contain an inhibitory hormone namely abscisic acid. This hormone disappears during stratification. Now gibberellic acid appears. It promotes growth in seed. Therefore, dormancy due to chilling requirement is under the control of promoter and inhibitor hormones. Thus the chilling requirement can be replaced by applying gibberellic acid. Therefore, such seeds can germinate in the presence of this hormone. They do not require chilling temperatures.
- Use of growth promoting substances for breaking seed dormancy
Certain chemicals promote the seed growth. Potassium nitrate, thiourea and ethylene ehlorhydrin are the most commonly used germination promoters. Similarly, the application of some of the plant hormones like gibberellic acid, cytokinin and ethylene also promotes