Agriculture (0600) | Germination

🌱 Principles of Plant Growth: Germination (Topic 3.3)

Hello future agriculturalists! Welcome to one of the most exciting topics: Germination. This is where life begins! Understanding how a tiny seed bursts into a powerful seedling is fundamental to farming. If we know what a seed needs, we can ensure we give our crops the best start possible.

Don't worry if the terminology seems new; we will break down the seed step-by-step using two classic examples: the Bean and the Maize plant.

1. What is Germination?

Definition: Germination is the process by which a plant grows from a seed. More formally, it is the resumption of growth of the embryo (the baby plant) inside the seed, leading to the emergence of the radicle (young root) and the plumule (young shoot).

Did you know?

Seeds are essentially tiny, self-contained survival packets. They are in a state of dormancy (a resting phase) until the conditions are just right for growth to begin.

2. Understanding Seed Structure

To germinate, the seed must use the food reserves it has stored. We focus on two types of seeds: the Dicot (Bean) and the Monocot (Maize).

2.1 The Bean Seed (Dicot / Legume)

Dicot means 'two cotyledons'. The bean seed is a great example of a dicot, and it’s relatively easy to see its parts.

• Testa: The tough outer layer that protects the internal embryo from damage and disease. Think of it as the seed's armor.
• Hilum: The scar where the seed was attached to the pod (the parent plant).
• Micropyle: A small pore near the hilum. This is the main point where water enters the seed during imbibition (the soaking process).
• Cotyledons: These are the two large halves of the bean. Their primary function is to store large amounts of digested food (starches, proteins, and fats) that the embryo will use for energy before it can photosynthesize.
• Embryo: The tiny, complete baby plant tucked inside, consisting of:
  • Radicle: The embryonic root (will grow downwards first).
  • Plumule: The embryonic shoot (will grow upwards, becoming the stem and leaves).

2.2 The Maize Seed (Monocot / Cereal)

Monocot means 'one cotyledon'. Maize (corn) seeds are a common example of a monocot. Their structure is slightly different, especially how they store food.

• Seed Coat (Testa) and Fruit Wall (Pericarp): These are fused together, providing protection.
• Endosperm: This is the largest part of the seed and the primary storage unit for food (mostly starch). Unlike the bean, where the cotyledons store the food, the maize uses a separate endosperm tissue.
• Cotyledon (Scutellum): This is a thin layer positioned between the embryo and the endosperm. It doesn't store much food itself; instead, it absorbs and transfers the nutrients from the endosperm to the growing embryo.
• Embryo: The baby plant contains the radicle and plumule, protected by sheaths:
  • Coleorhiza: The sheath protecting the radicle (root).
  • Coleoptile: The sheath protecting the plumule (shoot).

3. The Process of Germination

The process requires energy, and that energy comes from breaking down the stored food in the cotyledons or endosperm using respiration.

Step-by-Step Process (General)

1. Imbibition: The seed rapidly absorbs water through the micropyle and testa. The seed swells dramatically.
2. Enzyme Activation: Water activates digestive enzymes within the seed.
3. Digestion and Translocation: Enzymes break down the stored, complex food (starch, protein) into simple, soluble forms (sugars, amino acids). These soluble nutrients are moved to the embryo.
4. Respiration: The embryo uses the soluble nutrients and oxygen to respire, releasing energy for growth.
5. Growth: The radicle emerges first (downwards) to secure water and anchor the plant. Then, the plumule emerges (upwards) to reach the light.

3.1 Germination Types: Bean vs. Maize

The way the shoot emerges determines the type of germination.

a) Bean Germination (Epigeal)

In epigeal germination (Epi = above, geal = ground), the cotyledons are pushed above the soil surface.

• The part of the stem below the cotyledons (the hypocotyl) elongates rapidly and forms a hook shape, pulling the cotyledons and plumule out of the soil.
• Once above ground, the cotyledons open up, sometimes turning green and acting as temporary leaves until the true leaves of the plumule unfurl.
• Analogy: Imagine a gardener pulling a small, heavy flower pot (the cotyledons) out of the soil by the hook of the stem.

b) Maize Germination (Hypogeal)

In hypogeal germination (Hypo = below, geal = ground), the cotyledon and the food reserve (endosperm) remain below the soil surface.

• The part of the stem above the cotyledons (the epicotyl) elongates, pushing the plumule (protected by the coleoptile) straight up through the soil.
• The food store remains safely underground, meaning the seedling is less reliant on its food reserve being protected from pests or frost at the surface.

4. Conditions Required for Germination

For a farmer, knowing the conditions required is essential for successful planting. If any one of these factors is missing, the seed will not germinate, regardless of how good the seed quality is.

We can use a simple mnemonic to remember the key factors: W.O.T. (Water, Oxygen, Temperature).

4.1 Water (Moisture)

Water is the most crucial requirement to break dormancy.

• Water softens the hard testa, allowing the embryo to break out.
• Water provides the necessary medium to activate the enzymes that digest the stored food.
• Water helps transport the digested, soluble food from the storage organs (cotyledons/endosperm) to the growing parts of the embryo.
• Too little water = No enzyme activation, no growth. Too much water (waterlogging) = Lack of oxygen (see below).

4.2 Oxygen (Air)

The growing embryo needs energy, which it gets through aerobic respiration.

• Oxygen is vital for respiration, which releases energy from the stored food for rapid cell division and growth.
• If the soil is waterlogged (poor drainage), the air spaces fill up with water, cutting off the oxygen supply. The seed will fail to germinate or die.

4.3 Suitable Temperature

The rate of germination is directly controlled by temperature.

• Temperature influences the speed at which enzymes work.
• Every seed has an optimum temperature range for germination. Temperatures that are too low or too high can denature (destroy) the enzymes, halting the process.
• Planting crops too early in the cold season, for instance, means the enzymes will work too slowly, resulting in poor or delayed germination.