Germination (Leaving Cert Biology): Revision Notes

Germination

There are three essential conditions required for germination:

1. Water – activates enzymes and softens the testa (seed coat).
2. Oxygen – required for aerobic respiration to release energy.
3. Suitable temperature – allows enzyme-controlled reactions to occur at an appropriate rate.

Events in Germination

• Seeds contain stored food reserves in the form of oils, starch, and proteins.
• Germination begins when the seed absorbs water (imbibition) through the testa.
• Water activates enzymes that digest the stored food:
  • Oils → fatty acids and glycerol
  • Starch → glucose
  • Proteins → amino acids
• The soluble products of digestion are transported to the growing embryo.
• Glucose and amino acids are used to:
  • Build new structures such as cell walls and enzymes
• Fats and some glucose are used in respiration to release energy for growth.
• The dry mass of the seed decreases as stored food is used up.
• The mass of the embryo increases as new cells and tissues are formed.
• The radicle (future root) emerges first, growing downward and absorbing water and minerals.
• The plumule (future shoot) grows upward and emerges above the soil.
• When the first leaves develop and begin photosynthesis, the seedling becomes independent and its dry mass increases again.

Types of Germination

There are two types of germination:

1. Epigeal germination
2. Hypogeal germination

• After imbibition, the radicle grows downward due to gravity (geotropism).
• Roots develop and begin absorbing water and minerals from the soil.
• Growth of the hypocotyl (region below the cotyledons) determines the type of germination.

Epigeal Germination

• The cotyledons are pushed above the soil surface.
• Cotyledons may become green and photosynthesise.
• Example: Sunflower.

Epigeal = above ground
E for exposed above the soil_

Hypogeal Germination

• The cotyledons remain below the soil.
• They do not photosynthesise and act only as food stores.
• Example: Bean.

Hypogeal = below ground
H for hidden underground_