What is seed germination?
Seed germination is defined as a process by which the seed develops into a complete plant.
A seed is the product of fertilisation in angiosperms. A seed is formed when pollen grains come into contact with a female flower and fertilises the ovary. Once the embryo is formed, it develops into a seed, which develops into a whole plant after entering the soil.
Steps involved in seed germination
The entire process of seed germination can be explained using the 5 steps mentioned below.
The germination of seed starts with imbibition. Imbibition is a process where the seed swells by absorbing water. In a dry seed, all the cellular constituents become rehydrated. Therefore, when it absorbs water, the seed swells with a huge force. Due to imbibition, seed coats are ruptured and the radicle evolves out as primary toot.
All the metabolic functions of the seed are usually inactive in dry state i..e before imbibition. Once the seed absorbs water & imbibes, all metabolic activities are resumed.
During the initial days of imbibition, the type of respiration seen in seeds is anaerobic where the energy is derived by glycolysis. Gradually, as the seed is exposed to oxygen, the type of respiration changes into aerobic mode.
The seeds of underwater plants also germinate by utilising dissolved oxygen present in the water. However, the seeds of terrestrial plants cannot adapt to underwater conditions because they require large amounts of oxygen.
Light has a significant impact on seed germination and is considered a crucial element. The seeds whose germination doesn’t depend on exposure to sunlight are termed non-photoblastic seeds. The seeds whose germination is impacted by light are called photoblastic seeds. The two types of photoblastic seeds are as follows:
- Light & seed germination
Negative photoblastic seeds - These seeds germinate only in darkness. The negative photoblastic seeds do not germinate in the presence of sunlight.
- Positive photoblastic seeds - These seeds require sunlight to germinate and cannot germinate in the absence of light.
- Example: Lettuce seeds
- Example: Onion
- Plant growth regulators & seed germination
- A seed requires plant growth regulators to regulate its overall growth and development. The two plant growth regulators that play a key role in seed germination are Gibberellic acid and Abscisic acid. Gibberellic acid promotes the germination of seeds whereas Abscisic acid prevents seed germination. In the presence of Gibberellic acid and other seed germination promoting growth regulators, the seed develops into an advanced stage.
- Development of a seedling
- The seeds continue to grow, divide, and develop. Metabolic activity increases. Gradually, the embryo axis of the seed develops into seedlings.
Factors influencing seed germination
All the key factors influencing seed germination fall under two classes - External and Internal.
- Water: Water is an essential component for a seed to come out of dormancy. For a seed to be metabolically active, the amount of water present in the cell should be 75-95%. Therefore, a dormant seed (consists of 5-15% water) needs water to perform all metabolic functions.
- Oxygen: Oxygen is essential for a seed to respire aerobically. The seed derives all the energy essential for growth through aerobic respiration only.
- Optimum temperature: Temperature is known to influence the metabolism and growth rate of seeds. An optimum temperature of 25-30°C is essential for the seed to germinate.
- Seed dormancy: Seed dormancy is a state where the seed remains inactive. Seeds become dormant due to a variety of reasons such as:
- Inadequate water supply
- Insufficient minerals
- Immature or undeveloped embryos
- Resistant or impermeable seed coats
- Presence of ABA (plant growth inhibitor)
- Inadequate availability of oxygen
Conditions for seed germination
The three conditions for seed germination are sufficient water availability, adequate oxygen availability, optimum temperature, and light.
- Sufficient water availability: Seed germination is possible only if the seed comes out of the dormant state. Imbibition is the only way for the seed to come out of dormancy. As discussed above, cells need at least 75 - 95% of water to carry out various metabolic functions. A dormant seed needs water to become metabolically active.
- Adequate oxygen availability: Seeds require a continuous supply of oxygen to respire. Similar to human beings, respiration produces energy that is utilized for seeds to carry out metabolic activities.
- Optimum temperature: A moderate temperature of 35 to 45°C is essential for seed germination. Any temperatures above and below the optimum range do not promote seed germination.
- Light: Light is another major factor influencing seed germination. A few seeds mandatorily need sunlight to germinate whereas a few germinate in the presence of darkness only.