In-depth Notes on Germination and Seedling Growth

Germination Process

  • Radicle Emergence
  • Seed must overcome barriers like endosperm, seed coat, or pericarp.
  • Two processes of radicle emergence:
    • One-step process:
    • Surrounding tissues weaken during imbibition, allowing radicle to emerge without obstacles.
    • Radicle expands during imbibition, rupturing tissues.
    • Two-step process:
    • Surrounding tissues undergo metabolic weakening prior to radicle emergence.

Water Uptake Phases

  • Phase I:
  • Rapid water uptake by dry seeds during imbibition.
  • Water flows from higher to lower potential until equilibrium is reached.
  • Phase II:
  • Cells begin to expand, leading to radicle emergence.
  • Increased metabolic activity and cell wall loosening promote growth.
  • Phase III:
  • Resumption of water uptake supports seedling establishment.
  • Increased cell wall relaxation and solute accumulation maintain water potential gradient.

Mobilization of Stored Reserves

  • Food Reserves:

  • Major food reserves are in cotyledons and endosperm of angiosperm seeds.

  • Types of reserves:

    • Proteins: Stored in protein storage vacuoles (PSVs).
    • Lipids: Stored in oil bodies or lipid droplets.
    • Carbohydrates (Starch): Stored in amyloplasts, significant in cereals.

  • Starch Breakdown:

  • Initiated by enzymes α-amylase (hydrolyzes starch chains) and β-amylase (produces maltose from oligosaccharides).

  • Maltose converted to glucose by maltase.

  • Industrial Importance:

  • Mobilization of food reserves is critical for producing malted grains used in bread, beverages, etc.

Protein Hydrolysis

  • Stored proteins serve as primary nitrogen and amino acid source for seedling growth.
  • Hydrolyzed by proteases to provide nutrients for new protein synthesis.

Summary of the Germination Process in Phases

  • Phase I: Water uptake; seeds imbibe.
  • Phase II: Seedling growth; metabolic activity increases, radicle emerges.
  • Phase III: Continued growth; established seedling undergoes further water uptake and metabolic activity.

Visualization

  • Refer to Figure 17.8 for a graphical representation of water uptake phases in seeds showing transition from dormant, dry state to actively growing seedlings.