5.1 - Types of Responses

Responses of the plants towards environmental stimuli helps to ensure survival of the species. Normally, response in plants is a growth response. There are 2 types of responses in plants.

• Tropism responses
• Nastic responses

Types of Plant Responses

Tropism responses

• Responses from certain parts of the plants such as the roots and shoots.
• Aim to get closer or get away from a certain stimulus.
• 5 characteristics of tropism responses:
  • Permanent and affected by plant hormones
  • Slow and not evident
  • The direction of responses depends on the direction of the stimuli
  • Positive tropism means growth towards the direction of a stimulus
  • Negative tropism means growth away from the direction of a stimulus
• 5 types of tropism responses which are thigmotropism, geotropism, hydrotropism, phototropism and chemotropism.

Thigmotropism

• A growth response of plants towards touch.
• For example, the tendrils of the plant coil around an object for support.

Geotropism

• A growth response of the plant towards gravitational pull.

Hydrotropism

• A growth response of the plant towards water.
• For instance, the roots show positive hydrotropism because they grow towards the direction of water.

Phototropism

• A growth response of the plant towards light.
• For example, the shoot of the plant shows positive phototropism because it grows towards the direction of sunlight.

Chemotropism

• A growth response of the plant towards chemical substances.
• For instance, the roots can also show negative chemotropism by growing away from chemical substances such as toxins.

Nastic responses

• Include photonasty, seismonasty, nyctinasty, thermonasty and thigmonasty.
• 3 characteristics of nastic responses:
  • Not all nastic responses are growth responses
  • Faster and more apparent compared to tropism responses
  • Aim for survival of the species

Photonasty

• Responses towards light.
• For example, Japanese roses open when there is maximum sunlight and close during the night.

Seismonasty

• Responses towards mechanical stimuli such as shock, touch, wind and raindrops.
• Can be seen at the leaves, stigma and stamens of the plants.

Thigmonasty

• Response of plants towards vibrations.
• Can be seen in carnivorous plants such as Venus fly traps.
• The stimuli cause the leaves to close and trap the insects.

Thermonasty

• Response of plants towards changes to surrounding temperature.
• For instance, tulip opens when the surrounding temperature increases and closes when the temperature drops.

Nyctinasty

• Circadian rhythm that responds towards darkness.
• For example, the leaves of legume plants like river tamarind close at night and open again in the morning.