plant growth

• Tropism

  • Tropism is when a part of a plant grows in response to a stimulus from a specific direction.
  • Responses can be positive (towards the stimulus) or negative (away from the stimulus).
  • Tropisms help plants adapt to their surroundings.
  • Shoots grow towards light (positive phototropism) for photosynthesis.
  • Roots grow away from light but towards gravity (negative phototropism and positive geotropism) to penetrate soil for water and minerals.
  • Roots also grow towards water (positive hydrotropism) for hydration and nutrient absorption.
• Indoleacetic Acid (IAA):

  • IAA, also called auxin, is a plant growth hormone that promotes plant growth.
  • It's produced in growing areas like shoot and root tips, then moves to other parts of the plant.
• IAA and phototropism?

  • Cells in shoot tip release IAA which spreads down the shoot promoting upward growth
  • When I shoot is exposed to light from one direction IAA moves from the light side to the shaded side
  • IAA accumalates on the shaded side causing cells there to elongate more
  • This uneven growth makes the shoot bent towards the light
  • Even though light is detected from the shoot tip, removing or covering the tip stops a response to directional light
• synthesis and distribution of auxins

  • Auxins made in shoot apex and young leaves.
  • Move downward from tip to base only
  • Movement by Diffusion and active transport.
  • eventually degraded by enzymes
  • Transported long-distance via phloem.
  • Some produced in roots.
• What is geotropism?

  Geotropism is the growth or movement of an organism in response to gravity.

  most roots = positively geotropic, grow downwards

  shoots = often negatively geotropic
• Use of clinostat to show effect of gravity on root and shoot growth

  Roots + shoots placed on rotating platforms, eliminates one sided effect of gravity

  roots will grow horizontally due to lack of consistent gravitational cue

  none rotating control = normal response to gravity, one sided, roots grow down and shoots up