Lecture Notes: Plant Response to Environment and Role of Auxins

Introduction

• Topic: How plants respond to their environment, focusing on the role of auxins.
• Key Point: Plants, like animals, respond to environmental stimuli but do so more slowly and without a nervous system.
• Reliance on hormones for responses, particularly auxins.

Plant Hormones and Auxins

• Function of Hormones: Essential for plant responses; act locally within plant tissues.
• Auxins:
  • Role in controlling growth at shoot and root tips.
  • Produced at tips and diffuse backward along the shoot/root.
  • Influence:
    • Shoots: Stimulate growth.
    • Roots: Inhibit growth.

Types of Tropisms

• Phototropism: Response to light.
• Geotropism (Gravitropism): Response to gravity.
• Key Characteristics:
  • Auxins accumulate on shaded/lower sides.
  • Different effects on shoots and roots.

Phototropism in Shoots

• Behavior: Shoots are positively phototropic (grow towards light).
• Mechanism:
  • Auxins accumulate on the shaded side.
  • Stimulate faster growth on the shaded side, causing curvature towards light.
• Example: If the sun is on the left, auxins gather on the right, leading to right side growth and leftward curvature.

Geotropism in Shoots

• Behavior: Shoots are negatively geotropic (grow away from gravity).
• Mechanism:
  • Auxins accumulate on the lower side.
  • Stimulate growth on lower side, causing upward curvature.

Phototropism in Roots

• Behavior: Roots are negatively phototropic (grow away from light).
• Mechanism:
  • Auxins accumulate on the shaded/lower side.
  • Inhibit growth on these sides, causing curvature away from light.

Geotropism in Roots

• Behavior: Roots are positively gravitropic (grow towards gravity).
• Mechanism:
  • Auxins inhibit growth on the lower side.
  • Causes upper side to grow faster, resulting in downward curvature.

Conclusion

• Summary of how auxins influence plant growth via phototropism and geotropism.
• Explanation of how the differential growth rates caused by auxin distribution result in plant curvature towards or away from stimuli.

This overview provides a comprehensive summary of how auxins control plant growth responses to environmental stimuli, such as light and gravity.