Seed Germination

1. Introduction

• Seed germination is one of the most fundamental biological processes in the life cycle of higher plants.
• It marks the transition from a dormant embryo to an active, growing seedling, capable of independent life.
• Understanding seed germination is crucial in agriculture, horticulture, forestry, and seed technology, as it directly affects crop establishment, yield, and seed quality.

2. Definition

• Seed germination is the process by which a viable seed, under favorable environmental conditions, resumes metabolic activity, leading to the emergence of the embryonic root (radicle) and the shoot (plumule).
• It is the beginning of active growth of the embryo after a period of dormancy or quiescence.

3. Importance of Seed Germination

• Ensures successful establishment of the next generation.
• Determines seed viability and vigor in seed testing.
• Influences crop population and yield.
• Essential for nursery management and propagation.
• Basis for seed technology and quality control.
• Helps in ecological adaptation and species continuity.

4. Conditions Required for Seed Germination

Seed germination requires a combination of internal and external factors.

• External (Environmental) Factors

• Internal Factors

5. Phases (Stages) of Germination

Seed germination is a physiological and biochemical process that can be divided into three main phases.

Phase I: Imbibition (Water Uptake)

• The first phase of germination.
• The dry seed absorbs water through the seed coat by imbibition.
• Causes swelling of the seed, softening of tissues, and rupture of the seed coat.
• Metabolic reactivation begins — enzymes become active.

Phase II: Lag Phase (Activation or Metabolic Phase)

• Little or no increase in seed size.
• Intense biochemical activity inside the embryo:
  • Activation of enzymes: amylase, protease, lipase.
  • Hydrolysis of stored food (starch → sugar; protein → amino acids).
  • Increased respiration and ATP production.
• Synthesis of new cell materials begins.

Phase III: Radicle Emergence (Growth Phase)

• The radicle (embryonic root) emerges through the micropyle — this marks the completion of germination.
• The plumule (shoot) later emerges and grows upward toward light.
• The young seedling starts autotrophic life through photosynthesis.

6. Types of Seed Germination

Based on the position of cotyledons during germination, there are two main types:

Epigeal Germination

• Hypocotyl elongates and pushes the cotyledons above the soil surface.
• Cotyledons become green and photosynthetic before they shrivel.
• Common in dicotyledonous plants.

Examples: Bean (Phaseolus vulgaris), Castor, Cotton, Papaya.

Hypogeal Germination

• Epicotyl elongates; cotyledons remain below the soil surface.
• Cotyledons act as food storage organs.
• Common in monocots and some dicots.

Examples: Pea (Pisum sativum), Maize (Zea mays), Mango, Coconut.

Vivipary (Special Type)

• In some coastal and mangrove species, seeds germinate while still attached to the parent plant.
• Example: Rhizophora (Mangrove), Avicennia.
• An adaptive mechanism to saline and tidal environments.

7. Types Based on Duration

8. Physiology and Biochemistry of Germination; Water Absorption: Water uptake initiates cell hydration, enzyme activation, and membrane reorganization.

Enzyme Activation

• Amylase: Converts starch → maltose → glucose.
• Protease: Hydrolyzes proteins → amino acids.
• Lipase: Converts lipids → fatty acids and glycerol.

• Energy Production: Stored food oxidized in mitochondria through aerobic respiration to produce ATP, needed for cell division and elongation.

 Hormonal Regulation

• Gibberellins (GA₃): Promote enzyme synthesis and growth of embryo.
• Abscisic Acid (ABA): Maintains dormancy and inhibits premature germination.
• Cytokinins: Promote cell division in emerging shoots.

• Growth and Development: Cell elongation of radicle and plumule occurs. New tissues form as the embryo transforms into a seedling.

9. Factors Affecting Seed Germination

• External (Environmental) Factors

• Internal (Seed-Related) Factors

10. Measurement of Germination

• Germination Percentage (GP): Germination % =Number of seeds germinated / Total number of seeds tested ×100   
• Germination Rate; Time taken for germination; faster germination indicates higher vigor.
• Germination Energy; Percentage of seeds that germinate within a specified early period (indicates seed vigor).

11. Abnormal Seed Germination

Occurs due to poor viability, damage, or unfavorable conditions.
Examples:

• Decayed embryos
• Deformed seedlings
• Incomplete radicle or plumule development

Abnormal seedlings are excluded in seed germination tests (as per ISTA standards).

13. Difference Between Seed Germination and Seed Dormancy