PRELIMS

Plant Physiology and Hormones

Plant processes involve chemical biosynthesis and hormonal regulation for growth, development, and environmental response.

I. Photosynthesis

Equation: .

• Requirements: (from atmosphere/water), (from roots), Light, and Chlorophyll.
• Chlorophyll: Photoreceptor molecule; contains Magnesium (Mg) at its center. Absorbs Violet, Blue, and Red light.
  • Rate: Maximum in Red light; Minimum in Violet light.
• Reaction Stages:
  1. Light Reaction (Hill Reaction): Completed in the Grana. Photolysis of water occurs, releasing and forming ATP.
  2. Dark Reaction (Calvin Cycle): Completed in the Stroma. Reduction of occurs, forming Glucose (Sugar/Starch).

II. Major Plant Hormones

• Auxins: Discoverer: Darwin (1880). Primary Function(s): Controls growth; prevents leaf fall; formation at apex.
• Gibberellins: Discoverer: Kurosawa (1926). Primary Function(s): Stretches dwarf plants; breaks seed dormancy; increases fruit size.
• Cytokinins: Discoverer: Miller (1955). Primary Function(s): Promotes cell division/development; makes RNA/Protein.
• Abscisic Acid (ABA): Discoverer: Carnes/Adicote. Primary Function(s): Growth inhibitor; keeps seeds dormant; promotes leaf fall.
• Ethylene: Discoverer: —. Primary Function(s): Only gaseous hormone; helps in fruit ripening; increases female flowers.
• Florigens: Discoverer: —. Primary Function(s): Flowering hormone; formed in leaves.
• Traumatic: Discoverer: —. Primary Function(s): Dicarboxylic acid; heals injuries in plant cells.

UPSC Relevance

Plant physiology explains energy production and growth regulation in nature.

• Photosynthesis: Knowing that Oxygen is released from Water (not ) is a standard MCQ fact.
• Hormones: Identifying the "Inhibitor" (ABA) versus the "Growth Promoter" (Auxin/Gibberellin).
• Industrial Applications: The use of Ethylene for artificial fruit ripening and Gibberellins for increasing grape/fruit size.