Plant Responses to Environmental Stimuli

Overview

• Plants, like animals, respond to environmental stimuli to survive.
• Unlike animals, plant responses are generally slow and are mediated by hormones, not a nervous system.

Hormones in Plants

• Auxins: A group of plant hormones controlling growth at the tips of shoots and roots.
  • Produced in the tips and dissolve in cell solution.
  • Diffuse backwards along shoots or roots.
  • Stimulate growth in shoots, inhibit growth in roots.

Plant Responses

• Phototropism: Response to light.
  • Shoots are positively phototropic (grow towards light).
  • Roots are negatively phototropic (grow away from light).
• Geotropism/Gravitropism: Response to gravity.
  • Shoots are negatively geotropic (grow against gravity).
  • Roots are positively geotropic (grow with gravity).

Mechanism of Auxin Action

• Auxins accumulate on the shaded or lower side of plant parts.
• In Shoots:
  • Auxins accumulate on the shaded side, stimulating growth.
  • Leads to faster growth on the shaded side, causing the shoot to curve towards the light.
  • Auxins on the lower side due to gravity cause the shoot to curve upwards.
• In Roots:
  • Auxins inhibit growth on the shaded side when exposed to light, causing the root to curve downwards.
  • On a horizontal root, auxins accumulate on the lower side due to gravity, inhibiting growth there and causing the root to curve downwards.

Key Points to Remember

• Auxins always accumulate on the shaded side in response to light and on the lower side in response to gravity.
• Growth stimulation occurs in shoots, while growth inhibition occurs in roots due to auxins.

This lecture provides an understanding of how plants coordinate growth and responses to environmental factors through hormonal regulation, especially focusing on auxins and their role in phototropism and geotropism.