Seed Germination Germination is the process by which an organism grows from a seed or spore. Parts of a seedling Conditions Necessary for Seed Germination The embryo inside the seed remains dormant even if the seed is fully mature. It activates when it meets all the conditions needed for germination such as water, oxygen, temperature and light. Let’s see how these things help in the process of germination: 1.Water: Water holds a very important role in the process of seed germination. Most of the seeds are dry and need a significant amount of water, depending on the dry weight of the seed to activate the germination process. Water provides hydration for the essential activities of protoplasm, provides dissolved oxygen for the growing embryo, makes the seed coats soft and enhances the seed permeability. Apart from this, water also helps in the conversion of insoluble food to a soluble form, translocation of food materials and rapturing of seeds. 2.Oxygen: Seeds require oxygen for aerobic respiration till the time they develop leaves. Oxygen acts as a source of energy for seed growth and plays an important role in the germination of seeds for metabolism. 3.Temperature: Temperature is also an important factor in activating germination as protoplasm cannot carry on vital activities at a very low or very high temperature. Different types of seeds have different temperature requirements; most of them require approximately 25-30°C 4.Light: Most of the seed requires sunlight to germinate into a seedling. Epigeal Seed Germination During epigeal, the cotyledon is pushed out of the soil. This happens due to the rapid growth and elongation of the hypocotyl. E.g., castor and bean. Hypogeal Seed Germination During hypogeal germination, cotyledons remain below the soil due to the rapid elongation of epicotyl. It mostly occurs in monocotyledonous seeds. E.g. Maize. Process of Germination Water uptake (Imbibition): It is the process of absorption of water by dry seeds. Imbibition leads to swelling of the seeds. Absorption of water leads to rupturing of the seed coat. Respiration: Imbibition of water stimulates metabolic activity in the seed. Initially, seeds undergo anaerobic respiration as energy is provided by glycolysis; as oxygen starts entering the seed, they perform aerobic respiration. Plants that grow on land acquire oxygen from the air present in the soil. This is the reason we plough and loosen the soil before sowing them. Seeds of water plants use oxygen dissolved in the water. Effect of Light on Seed Germination: Non-photoelastic plants germinate irrespective of the presence or absence of light. Positive photoelastic seeds require exposure to light and cannot germinate in the absence of it. Negatively photoelastic seeds can germinate only in the dark. Process of Germination Mobilisation of Reserved Food During Seed Germination: The cotyledons of seed store food for the embryo. During germination, starch, fats and proteins stored in the endosperm are digested to produce the energy required for metabolic activities and multiple cell divisions in the embryo by aerobic respiration. Enzymes are released during this to promote the breakdown of complex food material into a form that can be used by the embryo. Water helps in the translocation of these soluble food materials to growing parts of the seed, mainly radicle and plumule. Development of Embryo Axis into Seedling: After the translocation of food materials embryo multiplies rapidly, resulting in the rapid growth of the seed into a seedling. The radicle of the seed grows first to form the root that grows down into the soil and begins to absorb water and minerals from the soil. After this, the plumule grows in an upwards direction to form the shoot. When the shoot grows above the ground, it develops green leaves. Green leaves of shoots begin to synthesise their food in the presence of sunlight (photosynthesis). Internal factors • Maturity of Embryo: The seeds of some plants contain immature embryos even if they are shed out from the plant. Such seeds germinate only after the maturation of the embryo. • Viability: Seeds germinate only within the period of viability. The viability of seeds ranges from a few days (E.g., Oxalis) to more than a hundred years. Maximum viability (1000 years) has been recorded in lotus seeds. • Dormancy: The condition of a seed when it fails to germinate even in the suitable environmental condition is called seed dormancy. Seeds of many plants are dormant at the time of shedding. External Factors • Water: Water activates enzymes that help digest the complex reserve foods of the seed to be used by the embryo. Seeds fail to germinate when the water content of the seed goes below a critical level. • Temperature: The optimum temperature is 25 to 35°C for most species. Seeds fail to germinate at very low and high temperatures. • Oxygen: Aerobic respiration is a physiological requirement for germination. Most of the plants germinate well in air containing 20% oxygen. • Light: Many seeds respond to light and these seeds are called photoblastic. Some seeds do not require light to initiate germination, but as they develop further, light serves as an essential requirement for the growth of the plant. • Soil condition: Soil conditions such as water holding capacity, mineral composition and aeration play an important role in seed germination. Soil with sufficient moisture content and important nutrients enhances the rate of germination as well as the further growth of a plant. Seedling establishment The appearance of the radicle marks the end of germination and the beginning of "establishment", a period that utilizes the food reserves stored in the seed. Germination and establishment as an independent organism are critical phases in the life of a plant when they are the most vulnerable to injury, disease, and water stress. Emergence Seed Viability • Sometimes seeds aren’t mature enough to become viable because they were harvested too early, or they were harvested from sterile plants, or the plant was never pollinated. • Sowing seed that is old is a bit of a gamble. It might germinate well, but it might not. Over time, seed viability decreases, and different seeds have different storage times. Poor storage conditions can also lead to loss of seed viability. • To take the guesswork out of sowing old seed, a simple 14-day viability test can be done. It will tell you if the seed is worth sowing at all, and how much to sow. If only a small percentage of seeds germinate, you’ll need to sow a greater number to ensure a decent crop. Germination rate • The germination rate describes how many seeds of a particular plant species, variety or seedlot are likely to germinate over a given period. It is a measure of germination and is usually expressed as a percentage, e.g., an 85% germination rate indicates that about 85 out of 100 seeds will probably germinate under proper conditions over the germination period given. Seed germination rate is determined by the seed genetic composition, morphological features and environmental factors. The germination rate is useful for calculating the number of seeds needed for a given area or desired number of plants. Seed Germination Questions Q.1: What is seed germination? Q.1: What is seed germination? Ans: Seed Germination and growth of an embryo from seed into seedling during favourable conditions is called seed germination. Seed germination can also be defined as a process in which a dormant seed activates and grows into a new plant. Q.2: What are the conditions necessary for Seed Germination? Q.2: What are the conditions necessary for Seed Germination? Ans: Water, temperature, oxygen, and light are important conditions necessary for food germination. Water: For metabolic activities, breakdown, translocation of food material, etc. Temperature: Seeds cannot begin to germinate under very low or high temperatures. Temperature is an important factor in the activation of various important enzymes. Oxygen: It is required to produce the energy required for the growth of the embryo with the help of anaerobic respiration. Light: Once the shoot system develops new leaves, light becomes an essential requirement for the further development of the seedling. Q.3: Explain seed dormancy. Q.3: Explain seed dormancy. Ans: The condition of a seed when it fails to germinate even in the suitable environmental condition is called seed dormancy. This can be caused by various reasons like rudimentary embryos, the presence of inhibitors, lack of light, very high or low temperature, etc. Q.4: What are Epigeal and Hypogeal germination? Q.4: What are Epigeal and Hypogeal germination? Ans: There are two methods of seed germination: epigeal germination and hypogeal germination. During epigeal, the cotyledon is pushed out of the soil. This happens because of the elongation of the hypocotyl. E.g., castor and bean. During hypogeal germination, cotyledons remain below the soil due to the rapid elongation of epicotyl. It mostly occurs in monocotyledonous seeds. E.g. Maize. Q.5: How can you test germination rate? Q.5: How can you test germination rate? Ans: A 14-day seed viability test can be done to calculate the % of germination.
