Seed Germination: Have you ever wondered how plants grow? Where do they come from? Many times, we would have seen small plants in nurseries. Where do these small plants come from? How are they formed? Here, you will learn and understand how a seed germinates and grows into a plant, the process and different developmental stages of the seed, and the factors that affect or influence the growth of the seed, which helps the seed to grow into a new plant.

Different plants require different conditions for successful seed germination. In this article, we will not only get to know about seed germination but will also get to know about its types, experiment to understand the process, seed germination diagram, five stages of seed germination, the temperature required for germination, necessary conditions for the seed germination process to take place, and lastly, the external and internal factors that affect or influence seed germination.

Seed Germination Definition

Seed germination is a process where a seed germinates, i.e. develops and grows into new plants. The growth of an embryo from seed into seedling under favourable conditions is called seed germination. Seed germination can also be defined as a process in which different plant species grow from a single seed into a plant.

An Experiment on Seed Germination

We can understand the process of seed germination with a simple but fun experiment. You may think soil is the most important requirement for germination. Well, that is not a complete truth; seeds can also germinate without soil. Yes! Without wasting much time, let us jump into the experiment to know the process that is often hidden in dark soil.

Materials Required: Green gram seeds, small plastic bag with zipper, paper towel, etc.

Procedure:

Step 1: Soak the seeds a day before conducting the experiment. This plays an important role in bringing about faster results.

Step 2: Drain the water from the seeds before placing them in the bag. Damp a paper towel and fold it into a bag, seal it tightly and hang the bag in a window.

Step 3: Approximately after 24 hours, you will be able to observe the seeds opening up and sprouting. Within 2-3 days, these seeds will be fully sprouted and after a few more days, small leaves start to develop.

Conclusion: Seeds have the ability to germinate in the presence of moisture, air, and light under the right temperature, even in the absence of soil.

If you are interested in knowing more about seed germination, you can repeat the same experiment but keep the bag in a dark place instead of placing it in light.

Types of Seed Germination

Seed germination can be classified into two types based on the fate of the cotyledons:

1. Epigeal Germination
2. Hypogeal Germination

1. Epigeal 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.

2. Hypogeal 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.

Viviparous Germination

This is a special type of germination seen in some mangrove plants like Rhizophora and Sonneratia. In this type, the seed germinates inside the fruit while it is still attached to the parent plant. After germination, the plant drops the seedling into the soil, which develops roots and fixes itself. This method is called viviparous means producing live young ones.

Five Stages of Seed Germination

The process of germination can be understood in five different stages mentioned below:

1. 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.
2. 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.
3. Effect of Light on Seed Germination: Plants are classified as photoelastic and non-photoelastic based on their response to light for 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, whereas negatively photoelastic seeds can germinate only in the dark. 
4. 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 such as amylases, proteases, etc., 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.
5. 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). 

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 \(25\)-\({30^ \circ }{\rm{C}}\)
4. Light: Most of the seed requires sunlight to germinate into a seedling.

Factors Affecting Seed Germination

Seed germination can be affected by certain internal and external factors:

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 \({25^ \circ }{\rm{C}}\) to \({35^ \circ }{\rm{C}}\) 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.

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.

Wrapping-Up

Seeds are the parts of the plant that contain a baby plant (embryo) and the food material required for its growth. When these seeds are dispersed from the parent plant to various places, they have the ability to grow into new plants.

This process is called seed germination. Seeds are not only important for plants but hold a very important role in fulfilling the food requirements of a large population. Hence, seed germination or dormancy is of great significance in the field of agriculture.

Better quality seeds enhance the quality and quantity of crops. Water, temperature, oxygen, and light (only for some seeds) are the essential conditions for the germination process. The absence of one or more of these affects the sprouting of seeds. Similarly, the maturity of an embryo, seed viability, and dormancy are internal factors that affect this process.

Frequently Asked Questions (FAQs) on Seed Germination

Q.1: 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.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: 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.4: Define epigeal 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.

Q.5: What is Hypogeal Germination?
Ans: During hypogeal germination, cotyledons remain below the soil due to the rapid elongation of epicotyl. It mostly occurs in monocotyledonous seeds. E.g. Maize.