Plant Growth Responses and Movements

Tropisms

• Definition: Growth responses of plants to external stimuli.
• Types of Tropisms:
  • Phototropism: Growth in response to light.
    • Allows plants to absorb more sunlight for photosynthesis.
  • Gravitropism (Geotropism): Growth in response to gravity.
    • Roots grow downwards; shoots grow upwards.
  • Hydrotropism: Growth in response to water.
    • Enables plants to absorb water from their environment.
  • Thigmotropism: Growth in response to touch.
    • Important for climbing plants like vines to grow around surfaces.

Plant Hormones and Auxin

• Auxin: A plant hormone that regulates growth by causing cell elongation.
  • Cells on the shaded side produce more auxin, causing them to elongate and bend the plant towards light.

Positive and Negative Tropism

• Positive Tropism: Growth towards stimuli.
• Negative Tropism: Growth away from stimuli.

Nastic Movements

• Definition: Rapid plant movements not caused by growth, but by osmosis.
• Mechanism:
  • Involve changes in turgor pressure by releasing ions like potassium and calcium.
  • Causes water to rapidly move in or out of cells, stiffening or drooping them.
• Examples:
  • Photonasty: Rapid response to light (e.g., purple shamrock).
    • Turgor pressure drops in darkness, causing leaves to sag.
  • Thigmonasty: Rapid response to touch (e.g., touch-me-nots).
    • Turgor pressure drops when touched, causing leaves to sag.
• Figma-Nastic Movement:
  • Defense mechanism to startle herbivores or remove insects.
  • Used by carnivorous plants like the Venus flytrap to trap insects.

Plant Response to Water Loss

• Transpiration: Evaporation of water from plant leaves through stomata.
• Water Conservation in Arid Climates:
  • Hormones signal guard cells to keep stomata closed during the hottest parts of the day to conserve water.