Seed Germination is the process in which dormant embryo of seed resumes active growth and forms a seedling. When all the necessary conditions for germination have been supplied, the seed may start to sprout. After the seeds have imbibed water, there is significantly increased in the physiological activity. By enzymatic action, the stored food is digested to the soluble form and can be used by developing an embryo. Through the transfer of soluble food, radicle and plumule begin to grow. The radicle usually grows first. In some seeds like the bean, the hypocotyl as a whole elongated carrying the cotyledons and plumule out of seed and soil, this type of germination is called epigeal. In the hypogeal type of germination, as in pea, hypocotyls does not elongate, but the plumule itself elongates and leaving the cotyledons inside the seed coat.
Seed germination lima bean
In the bean seed, food is stored in the cotyledons. After the digestion, it is translocated to hypocotyl, the radical and the plumule.The radical emerges first, after which the hypocotyls elongates, carrying two cotyledons and plumule outside the soil The cotyledons after coming above the ground become flat and green in appearance. During this time, plumule develops fast and forms two first functioning foliage leaves. Radicle bends downwards in the soil and forms root. By the time plumule has become an actively growing vegetative shoot, the hypocotyls functioning stem and the radical, a root.
Seed germination Maize
Maize is endospermic grain. At the time of germination, cotyledon and endosperm do not come out of the soil.Germination, therefore, is hypogeal. Cotyledon or scutellum absorbs the food material stored up in the endosperm. All growth of seedling occurs from the elongation of the radical and the plumule. Radicle emerges first but followed almost immediately by plumule.The radical gradually breaks through the coleorhizae and forms the primary root. Plumule breaks through the upper distinct cylindrical portion of the sheath, called the plumule sheath or coleoptiles. Adventitious roots begin to develop at once from the point of origin of the radical on the hypocotyls. The primary root soon perishes. The plumule grows upwards. The first leaf soon emerges out of the plumule sheath and others follow in succession. When the germination is complete, the seedling becomes independent of endosperm.
The conditions necessary for seed germination are:
External conditions for seed germination
- water – Uptaking water, the seed swells, and seed coat becomes soft and permeability increases. Increasing permeability allows better exchange of gases Water creates sufficient turgor pressure inside the seed, cracks the seed coat and enables the embryo to come out easily. Water makes the protoplasm active and converts the insoluble food present in cotyledons and endosperm into soluble forms. Activated protoplasm secretes enzymes which digest food. This digested food is used for respiration and supplies energy to the growing embryo. Water also helps in the transport of digested food from the endosperm and cotyledons to the growing embryo.
- Oxygen – It is mainly essential for respiration which releases energy. Energy is required to complete different stages of germination.
- Temperature– Different kinds of seeds require different temperature ranges which are suitable for germination. All vital activities start within the limit of suitable temperature, not less than 0oC and above 40oC. So optimum temperature (25oC to 30oC) favors more seed germination than the minimum and minimum temperature.
- Light – According to German investigator, Kinzel light induces seed germination. In some cases that is possible whereas in other cases it shows inhibiting influence on germination. When the reserve food material is exhausted, then the light is required for the development of chlorophyll and new young leaves for photosynthesis.But this stage is followed by germination, so light is not that essential like water, oxygen, and temperature.
Internal Conditions for seed germination
- The viability of seeds – Upto a particular period a seed can germinate i.e. known as the viability period. This period varies from one seed to other.
- Completion of dormancy – Before the completion of rest or dormant or inactive period, seed can not germinate.
- Growth hormones – These hormones are required for the growth of embryonic axis of germination.
- Food – Stored food in the cotyledons and endosperm and the enzymes secreted by active protoplasm help in the process of germination.
- Seed coat – The testa is permeable to water, the seed swells, and germinate. When a seed absorbs moisture and swells the radicle of embryo axis comes out through the micropylar end, grows downward into the soil known as primary root.
Then the seed coat ruptures due to excessive swelling and the plumule grow upwards opposite to soil surface and become young shoot. The two cotyledons on both the lateral side of the axis either come out of the soil or remain below the ground surface.
Depending upon the position of the cotyledons, the seed germination is of two types, and the third type is germination of seed inside the fruit.
- Epigeal germination
- Hypogeal germination
- Viviparous germination (germination inside the fruits)
Epigeal seed germination
Cotyledons of seeds are pushed out of the soil due to rapid elongation of the hypocotyl. Cotyledons are green, example – Ricinus, Dolichos, Helianthus, Allium, Curcubita.
Hypogeal seed germination
Here the cotyledons remain inside the soil or just above the soil surface but non-green. The portion of the axis in between the cotyledons immediately elongates, known as Epicotyle. As the cotyledons remain inside soil surface, the germination is hypogeal.They dry up gradually and fall off. It is found in dicotyledonous seeds of Pisum, Cicer, Magnifera and monocotyledons seeds of Zea mays and Oryza sativa.
Viviparous seed germination
This is a special type of seed germination found in plants growing in seacoasts and salt lakes.
The seed germinates here while inside the fruit and attached to the parent plant. The seeds do not have any dormancy period. The radicle comes out piercing the fruit wall and elongates.
Due to the heavy weight of the seedling, the fruit gets detached from the parent plant and falls on the soil surface vertically. Then the radicle develops lateral roots and plumule gives rise to the shoot.
This type of germination is mostly found in mangrove plants.
The seeds of most of the plants germinate when they are provided with favorable environmental conditions e.g. moisture, air suitable temperature etc. There is, however, a more numerous group of plants whose seeds do not readily germinate even though they are placed under suitable conditions. Germination of seeds may be delayed for days, weeks, months or even years. Quiescent seed can live for years but germinate when soaked in water under suitable temperature and in the presence of oxygen. The dormancy of seed thus may be defined as the condition of seed when it fails to germinate even though the environmental conditions, usually considered favorable for active growth are present.
Factors causing dormancy of seeds
Dormancy due to seed coats
At the time of seed ripening the chemical components of seed coat become dehydrated and form a hard, protective layer around the embryo. Several different kinds of seed coat effects have been noticed e.g. water impermeability, gas impermeability, mechanical resistance and seed coats containing inhibitors.
Dormancy due to condition of embryo
The embryo in the seed may be immature or rudimentary and poorly developed. Further development of these embryos occurs during the period of dormancy.
Dormancy due to specific light requirement
The response of seeds to sunlight falls under three categories:
- Seeds germinate on exposure to light (photoblastic seeds).
- Seed fail to germinate when exposed to light (negative photoblastic).
- Deeds germinate in light as well as in dark (Non-photoblastic).
Dormancy due to germination inhibitors
Presence of certain inhibitory substances such as abscisic acid (ABA), coumarin, phenolic acids in the embryo, endosperm or other tissues of the seed or fruit is another cause for seed dormancy. It has been shown that inhibitory substances present in the seed covering, block the growth of embryo.
Methods of breaking dormancy
- The dormancy can be removed by breaking, softening or weakening of seed coats. This can be done by microbial action in the soil, cutting of seed coat, use of organic solvents. Scratching of the seed coat (scarification) has the same effect. Acid treatments are also sometimes recommended to break the dormancy caused by hard seed coats.
- In seeds like Crotalaria, Melilotus etc., water and oxygen fail to enter into seed due to blockage of opening in the seed coat. The seeds are shaken to remove the plug.This type of treatment is called impaction.
- Alternating daily temperature strongly promotes germination of some seeds.
- The concentration of inhibitors can be reduced by exposure to alternating temperature, chilling treatment, placement in running water, treatment of hydrated seeds with oxygen, nitrite, thiourea, gibberellins, cytokinins, and ethylene.
Advantages of seed dormancy
- Seed dormancy is an adaptation to ensure seed germination only under favorable conditions Thus it enables the successful establishment of seedlings.
- It enables seeds to be disseminated in time and space germination of different seed take place under specific seasons and conditions.
- To ensure agricultural security, see can be artificially stored.
Chromosomes present in different parts of plants:-
|Microspore mother cell||2n|
|Megaspore mother cell||2n|
|Leaf, Root, Stem||2n|
|Male gamete n||n|
|Polar nuclei||n + n|