Seed germination may be defined as the fundamental process by which different plant species grow from a single seed into a plant. This process influences both crop yield and quality.
Process of Seed Germination
- Imbibition: The first step in the seed germination is imbibition i.e. absorption of water by the dry seed. Imbibition results in swelling of the seed as the cellular constituents get rehydrated. The swelling takes place with a great force. It ruptures the seed coats and enables the radicle to come out in the form of primary root. This stage is referred to as an Imbibition.
- Respiration: Imbibition of water causes the resumption of metabolic activity in the rehydrated seed. Initially their respiration may be anaerobic but it soon becomes aerobic as oxygen begins entering the seed. The seeds of water plants, as also rice, can germinate under water by utilizing dissolved oxygen. The seeds of plants adapted to life on land cannot germinate under water as they require more oxygen. Such seeds obtain the oxygen from the air contained in the soil. It is for this reason that most seeds are sown in the loose soil near the surface. Ploughing and hoeing aerate the soil and facilitate seed germination. Thus the seeds planted deeper in the soil in water-logged soils often fail to germinate due to insufficient oxygen.
- Seed Germination: This is a final stage of seed germination. In this stage, the cell of the seeds are elongated and divided, which brings out the root and radicle out of the seed and cotyledons are expanded which, are the true leaves of the new plant.
External Factors Affecting Germination
- Water: Water is a essential element, Which is required for germination. Most seeds need enough water to moisten the seeds but not enough to soak them. The uptake of water by seeds is called imbibition, When the seed imbibes water, hydrolytic enzymes are activated which break down these stored food resources into metabolically useful chemicals.
- Oxygen: It is required by the germinating seed for metabolism. Oxygen is used in aerobic respiration, the main source of the seedling’s energy until it grows leaves. Oxygen is an atmospheric gas that is found in soil pore spaces; if a seed is buried too deeply within the soil or the soil is waterlogged, the seed can be oxygen starved. Some seeds have impermeable seed coats that prevent oxygen from entering the seed, causing a type of physical dormancy which is broken when the seed coat is worn away enough to allow gas exchange and water uptake from the environment.
- Temperature: It affects cellular metabolic and growth rates. Seeds from different species and even seeds from the same plant germinate over a wide range of temperatures. Seeds often have a temperature range within which they will germinate, and they will not do so above or below this range. Many seeds germinate at temperatures slightly above 16-24 C, while others germinate just above freezing and others germinate only in response to alternations in temperature between warm and cool.