5.1 - Types of Responses

• Plants normally respond to environmental stimuli through growth responses.
• The two types of plant responses are tropism responses and nastic responses.

Tropism

• Definition: Response of certain parts of a plant, such as roots and shoots, towards or away from a stimulus.
• Characteristics:
  • Permanent and influenced by plant hormones.
  • Slow and not apparent.
  • Direction depends on the direction of the stimulus.
  • Positive tropism: Growth towards the stimulus.
  • Negative tropism: Growth away from the stimulus.

Types of Tropisms

• Thigmotropism: Plant response to touch.
  • Tendrils show positive thigmotropism by coiling or wrapping around an object for support.
• Geotropism: Plant response to gravity.
  • Roots show positive geotropism.
  • Shoots show negative geotropism.
• Hydrotropism: Plant response to water.
  • Roots show positive hydrotropism by growing towards water.
• Phototropism: Plant response to light.
  • Shoots show positive phototropism by growing towards light.
• Chemotropism: Plant response to chemicals.
  • Roots show positive chemotropism by growing towards mineral salts.
  • Roots show negative chemotropism by growing away from chemicals such as poison.

Nastic Response

• Definition: Response of certain parts of a plant that is independent of the direction of the stimulus.
• Characteristics:
  • Not all nastic responses are growth responses.
  • Quicker and more apparent than tropism responses.
  • Some nastic responses are intended for survival.

Types of Nastic Responses

• Photonasty: Response to light.
  • Japanese roses open when they receive maximum sunlight and close at night.
• Seismonasty: Response to mechanical stimuli such as shock, touch, wind and raindrops.
  • Mimosa leaves fold when touched.
• Nyctinasty: Circadian rhythm that responds towards the onset of darkness.
  • Leaves of legume plants such as river tamarind (Leucaena leucocephala) close at night and unfurl during daytime.
• Thermonasty: Response to changes in surrounding temperatures.
  • Tulips open when the temperature increases and close when the temperature drops.
• Thigmonasty: Response to vibration.
  • The Venus flytrap clamps its leaves shut and traps an insect when stimulated.