Germination of seeds
In a dry seed the embryo is alive and all the vital activities continue in the cells but, at a slow rate. Thus the embryo remains in a relatively inactive or dormant state but as soon as the seed is soaked in water it gets activated and the metabolic processes are accelerated rapidly.
The process by which the dormant embryo of the seed becomes metabolically active and forms a seedling is known as germination.
Process of germination:
Under suitable conditions, the seeds absorb water from the soil through the micropyle. This leads to the swelling of the seed and softening of the seed coat. The seeds store food material in the form of proteins, carbohydrates and lipids. Food material is stored either in the cotyledons or in the endosperm. The seeds also contain high level of minerals and vitamins.
The absorption of water causes a series of physiological changes. The seeds absorb water by imbibition and osmosis and consequently, the embryo becomes hydrated. The increased physiological activity in the seeds is indicated by the higher rate of respiration.
The enzymes are synthesized by the amino acid provided by the proteins that are stored in the seeds. The germinating seed has two centres of activity – the growth centres of the embryo and the stored food material.
The enzymes act on the stored food in the seed. The complex carbohydrates, proteins and fats are hydrolyzed into simple molecules which result in the production of energy as well as the building material for the new cells and the tissues. Glucose, amino acid and fatty acid are water soluble substances.
They are translocated to the growth centres of young embryo. They may be used as a substrate for respiration in both the storage and growth centres. At the growth centres, these substances provide structural components. For example, a major use of the glucose is in the synthesis of cellular and other cell wall material.
Amino acids are utilized for protein synthesis which provide important enzymes and structural components of protoplasm. Fatty acids and glycerol formed by the hydrolysis of lipids are either used in respiration or converted into sucrose.
Cell division starts in the growing parts of the embryo. Radicle is the first part of the embryo to come out of the seed coat. It is positively geotropic and soon grows towards the soil. With the expansion of the embryo, the seed coat ruptures and the plumule lying between the cotyledons comes out and the plumule forms the shoot.
On the basis of the behaviour of cotyledons, germination may be of the following types:
Epigeal germination:
During the germination of seeds, the cotyledons are raised above the surface of the soil due to rapid elongation of hypocotyl. This type of germination where the cotyledons are raised above the soil surface is known as epigeal germination. Epigeal germination is found in the seeds of Dolichos, castor, Cucurbita, tamarind, Helianthus, Carica papaya, sunflower, onion etc.
Epigeal Germination
Hypogeal germination:
In some seeds and grains, the cotyledons are not pushed upwards. They remain under the surface of the soil during germination. In this case, the epicotyl that is the portion of the axis just above the cotyledons grow faster and push the plumule upwards.
As the cotyledons remain under the surface of the soil, they do not turn green but they supply food material to the growing seedling from the reserve stock. This type of germination is called hypogeal germination. Hypogeal germination is found in dicotyledonous seeds of Cicer, Pisum, Mangifera, groundnut etc., and in many monocots like wheat, rice, maize, coconut, date palm, etc.
In monocotyledonous seeds and grains, the germination is of hypogeal type. The radicle grows out piercing through the coleorhiza and enters into the soil as the tap root but its growth is checked soon and a number of fibrous roots are developed.The plumule grows upwards being covered by the coleoptile. It ruptures the coleoptile and develops into a shoot.
Hypogeal germination
Viviparous germination:
Some plants, which grow in sea coasts show a special kind of germination known as vivipary. In this case, the seed germinates while it is inside the fruit, which remains attached to the plant. The radicle of the soil grows rapidly and comes out piercing the fruit wall. It elongates and becomes stout.
Vivipary
Later on, due to its weight, the fruit gets detached from the plant and falls vertically down. The elongated portion of the radicle pierce into the soil and soon develops lateral roots which gives anchorage to the growing seedling. The plumule grows upwards and gives rise to the shoot for example, Rhizophora etc.
Condition necessary for germination of seeds:
The Germination of seeds involves the activation of the cells of the embryo for their metabolic processes and cell division. The following few suitable conditions are, however, necessary for the activation of the cells leading to the germination of the seeds.
Moisture: For the germination of seeds and grains, availability of water or moisture is the most important condition. The protoplasm of the cells of the seed remains almost inactive or in a dormant state at a very low moisture content.
As soon as the seed is soaked, it absorbs water and swells. When the moisture content of the soil is increased, their vital activity is also accelerated. The reserve food materials present either in the endosperm or cotyledons of the seeds are hydrolyzed to simpler forms and are made available to the cells of the embryo. This also supplies energy and initiates the embryo to grow by cell multiplication and cell enlargement.
Oxygen: Oxygen is essential for respiration of the growing cells. Respiration helps in the liberation of a great amount of energy required for the various growth processes of the embryo.
Temperature: Suitable temperature is necessary for the germination of seeds. A number of physiological processes occur within the seed during germination of the seeds. Therefore, suitable temperature is always necessary for the germination of seeds.
At a very low or high temperature, respiration and other vital processes are retarded or completely checked. As a result, the supply of energy becomes insufficient, and the growth of the embryo is also checked. At extreme temperature, the cells lose vitality and die. Most of the seeds fail to germinate below 0 degree centigrade and above 48 degree centigrade.