Germination of a plant is a phenomenon, where a seed germinates to form a new seeding or a plant. Germination process results in the emergence of the radicle (primary root) and plumule (primary shoot). A seed without germination remains dormant in a soil. Under favourable conditions, optimum temperature, oxygen, light, water etc. a seed germinates and promotes embryo development. An embryo is present within the seed and the food reserves stored in the outer layer of seed or seed coat. A seed without the embryo refers to as “Dormant seed” which does not undergo germination.
Germination rate: The germination rate of a seed expresses in a percentage. It depends upon the growth conditions provided to seed for the germination. Germination rate up to 60% means that 60 out of 100 seeds will possibly undergo germination at a given period of time.
Germination capacity: There is another term which can define as the number of seeds capable of undergoing germination in a given population.
Content: Germination of Plant
- Definition of Germination
- Process of Plant Germination
- Conditions Necessary for Germination
- Factors Affecting Germination
Definition of Germination
Germination is a biological process, where a seed forms a germ tube that activates the embryo within the seed and promotes the growth of the new seedling. There are generally two types of germination, namely:
Epigeal germination: Here, the hypocotyl region of the shoot system elongates and goes upwards by pulling the cotyledon out of the soil.
Examples: Epigeal germination occurs in plants like cotton, papaya, onion etc.
Hypogeal germination: Here, the epicotyl region of the shoot system elongates, and the cotyledon remains inside the soil.
Examples: Hypogeal germination occurs in plants like pea, mango, rice etc.
As we can see in the image, epicotyl is the region above the cotyledons in the axis of growing seedling, whereas hypocotyl is the region below the cotyledons and above the roots.
Process of Plant Germination
The method of germination involves several developmental stages that include the following steps:
It is the first and the most crucial step for a dormant or dry seed. During the imbibition stage, a dry seed absorbs water that results in swelling. By the absorption of water, the dry seed gets rehydrated and softens the seed coat. Further, the swelling ruptures the soft or moistened testa as a result of which radicle comes out of the seed as a “Primary root”.
A seed becomes metabolically active and respires vigorously in the presence of oxygen. Seed undergoes respiration to fulfil the needs for a growing plant. During this stage, a seed exploits oxygen to convert the nutrients existing in the soil into energy. Cellular respiration in a seed involves oxidation of the respiratory substrates to release energy.
Protein, carbohydrates and fats are the usual respiratory substrates of a seed, which on oxidation releases energy in the form of ATP and carbon dioxide by the breakage of the C-C bond.
The metabolization of Reserve food
The outer aleurone layer of an endosperm produces hydrolysing enzymes via the assistance of gibberellic acid. Hydrolysing enzymes like amylases, proteases etc. helps in the metabolization of reserve food. A seed assimilates the reserve food materials like stored starch, proteins or fats by harnessing the energy released during the respiration.
The hydrolysing enzymes convert the insoluble organic matter into a soluble form and organic food into an elementary form. The simpler food is then transported to the growing epicotyl, hypocotyl, radicle, plumule via cotyledons.
Development of the Embryo Axis into Seedling
A seedling grows from an embryo axis, after food translocation from the endosperm wall to the different parts. This stage makes the cells of an embryo metabolically active. Then the cells of embryo undergo rapid cell divisions, expansion and form a seedling.
Conditions Necessary for Germination
Environmental factors like water, oxygen, temperature and light are necessary for the germination of a plant.
It is the crucial source for the germination of a seed. Water is necessary for the imbibition step to promote the emergence of a radicle and plumule. Excessive water can also inhibit seed growth. Due to excessive water, the soil becomes soggy as a result of which a plant will not get sufficient oxygen and can’t stand straight.
A seed contains reserve food material like proteins, carbohydrates, fats etc. in a dry state which cannot exploit by an embryo. Thus, for the embryo development, the dry organic food material should convert into liquid form for further growth into a seedling.
To prove the necessity of water, we can take two beakers. In beaker-A, place dry cotton wool at the bottom and add two three-gram seeds over it. In the beaker-B, place the wet cotton wool and over it add few gram seeds. Allow the seeds to germinate for 1-3 days. After 1-3 days, we will come to know that the seed germinates in the presence of water as seen in beaker-B.
Air is also a critical factor that is necessary for seed growth. To prove the importance of oxygen for the seed germination, take two beakers. In beaker-A, take wet cotton wool and place gram seeds over it and allow it to germinate.
In beaker-B, boil the water to remove the dissolved oxygen content. Then cool the water and add few gram seeds in a beaker. Over this, add little oil, which will prevent the entry of atmospheric oxygen and allow the seeds to germinate for several days.
After 2-3 days, we will see the germinated seeds in the beaker-A, not in B. The experiment proves that air is necessary for the seed germination. Oxygen helps in the seed respiration and provides the energy source for the metabolism of complex biomolecules.
Seed germination occurs within a temperature range between 0 to 50 degrees Celsius. Seed germinates best at a moderate or an optimum temperature within 25 to 30 degrees Celsius. Very low temperature reduces the rate of metabolic activity by seed and a very high-temperature destroys the embryonic tissue. To prove its necessity, again take two beakers and label it as ‘A’ and ‘B’.
In beaker-A, add some ice cubes along with few gram seeds and keep it in a box. Then take beaker-B and add few gram seeds into the water at room temperature. After 2-3 days, germination of the seed occurs in the beaker-B that place at room temperature.
Therefore, components like water, air and temperature are necessary for the seed germination. There is an experiment named “Three bean experiment” which proves the importance or requirement of oxygen, water and temperature for the germination of seed. This experiment makes the use of three-bean seeds:
- One grain sets inside the water.
- The second seed sets partially in the water.
- And, the third seed must expose to the air.
Allow the seeds to germinate and wait up to 2-3 days. This experiment proves that the seed that is partially in water will germinate as it gets air, light and water at the same time.
Light and Darkness
In addition to air, water and temperature, the presence or absence of light also plays an important role. Germination occurs in the darkness, within the soil and s seed forms a primary root. Once a seed develops a shoot, it requires the presence of light under which it gets energy from the process of photosynthesis. A shoot undergoes “Photomorphogenesis” where a seedling grows, forms leaf and turns green, under the presence of sunlight.
Factors Affecting Germination
The factors affecting the germination of the plant includes some extrinsic and intrinsic factors.
These are the external or the environmental factors which can affect the germination rate and capacity of a seed.
Water: Insufficient water supply will not allow the seed to germination as it involves the emergence of radicle and plumule.
Oxygen: Germination requires vigorous oxygen supply, and by consuming oxygen, a seed releases energy. The energy again dissipates by the seed for the growth of an embryo.
Temperature: It affects the rate of embryonic growth and the metabolism of food material by the seed.
Seed dormancy: It is a condition where the seed does not germinate and remain in a dormant state by a restricted growth of an embryo, restriction of water and oxygen uptake etc.
Seed germination also affected by the plant hormones like abscisic acid (ABA) that inhibits the germination process.