8.3.4 Plant Responses To Stimuli (tropisms and Nastic Movements)

Introduction

Plants are fixed in one place, but they can still respond to changes in their environment. These responses help them survive, grow, and reproduce. Plant responses to stimuli are called tropisms (directional growth responses) and nastic movements (non-directional responses). Understanding these responses helps us understand how plants adapt to their surroundings.

Definition

Tropism: A tropism is a directional growth response of a plant towards or away from a stimulus.

Nastic Movement: A nastic movement is a non-directional response of a plant to a stimulus.

• Tropism = directional growth response (towards or away from stimulus).
• Nastic movement = non-directional response (not dependent on the direction of stimulus).

Core Concept

• Plants respond to environmental stimuli.
• These responses help them survive, grow, and reproduce.
• Plant responses are classified into:

Detailed Explanation

1. Tropisms

Definition: A tropism is a directional growth response of a plant towards or away from a stimulus.

• Phototropism – response to light.
• Gravitropism (Geotropism) – response to gravity.
• Thigmotropism – response to touch.
• Hydrotropism – response to water.
• Chemotropism – response to chemicals.

a. Phototropism:

• Definition: Phototropism is the growth response of a plant towards or away from light.
• Positive phototropism: Growth towards light (e.g., shoots).
• Negative phototropism: Growth away from light (e.g., roots).
• Mechanism: Auxins accumulate on the shaded side of the shoot, causing cell elongation and bending towards light.

b. Gravitropism (Geotropism):

• Definition: Gravitropism is the growth response of a plant towards or away from gravity.
• Positive gravitropism: Growth towards gravity (e.g., roots).
• Negative gravitropism: Growth away from gravity (e.g., shoots).
• Mechanism: Auxins accumulate on the lower side of the root and shoot. In roots, high auxin concentration inhibits growth; in shoots, it promotes growth.

c. Thigmotropism:

• Definition: Thigmotropism is the growth response of a plant to touch.
• Example: Tendrils of climbing plants coil around a support.
• Mechanism: Auxins accumulate on the side of the tendril opposite to the touch, causing it to coil.

d. Hydrotropism:

• Definition: Hydrotropism is the growth response of a plant towards water.
• Example: Roots grow towards moisture.
• Mechanism: Roots detect moisture and grow towards it.

e. Chemotropism:

• Definition: Chemotropism is the growth response of a plant towards or away from chemicals.
• Example: Pollen tube grows towards the ovule due to chemical signals.
• Mechanism: Chemical gradients guide the growth.

2. Nastic Movements

Definition: Nastic movements are non-directional responses of a plant to a stimulus.

• Seismonastic movement – response to touch or vibration.
• Nyctinastic movement – response to light and dark (sleep movements).
• Thermonastic movement – response to temperature.
• Photonastic movement – response to light.

a. Seismonastic Movement:

• Definition: Seismonastic movement is a response to touch or vibration.
• Example: Leaves of Mimosa pudica (touch-me-not) fold up when touched.
• Mechanism: Changes in turgor pressure in the pulvini (swollen bases of leaves).

b. Nyctinastic Movement:

• Definition: Nyctinastic movement is a response to light and dark (sleep movements).
• Example: Leaves of some plants fold up at night and open during the day.
• Mechanism: Changes in turgor pressure in the pulvini.

c. Thermonastic Movement:

• Definition: Thermonastic movement is a response to temperature.
• Example: Petals of some flowers open or close in response to temperature.

d. Photonastic Movement:

• Definition: Photonastic movement is a response to light.
• Example: Petals of some flowers open in response to light.

Comparison of Tropisms and Nastic Movements

Summary Table of Plant Responses

Intermediate Level

Tropisms in Detail

Nastic Movements in Detail

Comparison of Tropisms and Nastic Movements

Summary Table of Plant Responses

Important Points

• 1. Plants are fixed in one place, but they can still respond to changes in their environment.
• 2. These responses help them survive, grow, and reproduce.
• 3. Plant responses to stimuli are called tropisms (directional growth responses) and nastic movements (non-directional responses).
• 4. Tropisms:
• - A tropism is a directional growth response of a plant towards or away from a stimulus.
• - Tropisms are directional (towards or away from the stimulus).
• - They are caused by differential growth (one side of the plant grows faster than the other).
• - They are mediated by plant hormones (especially auxins).
• - Types of tropisms:
• - Phototropism: Response to light.
• - Gravitropism (Geotropism): Response to gravity.
• - Thigmotropism: Response to touch.
• - Hydrotropism: Response to water.
• - Chemotropism: Response to chemicals.
• 5. Nastic Movements:
• - A nastic movement is a non-directional response of a plant to a stimulus.
• - Nastic movements are non-directional (not dependent on the direction of the stimulus).
• - They are caused by changes in turgor pressure.
• - Types of nastic movements:
• - Seismonastic movement: Response to touch or vibration.
• - Nyctinastic movement: Response to light and dark (sleep movements).
• - Thermonastic movement: Response to temperature.
• - Photonastic movement: Response to light.
• 6. Comparison of Tropisms and Nastic Movements:
• - Feature | Tropisms | Nastic Movements:
• - Direction: Directional | Non-directional.
• - Cause: Differential growth | Changes in turgor pressure.
• - Stimulus: Light, gravity, touch, water, chemicals | Touch, light, temperature.
• - Examples: Phototropism, gravitropism, thigmotropism, hydrotropism, chemotropism | Seismonastic, nyctinastic, thermonastic, photonastic.
• 7. Summary Table of Plant Responses:
• - Response | Type | Stimulus | Example:
• - Phototropism | Tropism | Light | Shoots bend towards light.
• - Gravitropism | Tropism | Gravity | Roots grow downwards; shoots grow upwards.
• - Thigmotropism | Tropism | Touch | Tendrils coil around support.
• - Hydrotropism | Tropism | Water | Roots grow towards moisture.
• - Chemotropism | Tropism | Chemicals | Pollen tube grows towards ovule.
• - Seismonastic | Nastic | Touch/vibration | Mimosa pudica leaves fold.
• - Nyctinastic | Nastic | Light/dark | Leaves fold at night.
• - Thermonastic | Nastic | Temperature | Petals open/close.
• - Photonastic | Nastic | Light | Petals open in light.

Remember

• - Tropisms = directional growth responses.
• - Nastic movements = non-directional responses.
• - Phototropism = response to light.
• - Gravitropism = response to gravity.
• - Thigmotropism = response to touch.
• - Hydrotropism = response to water.
• - Chemotropism = response to chemicals.
• - Seismonastic = response to touch/vibration.
• - Nyctinastic = response to light/dark.
• - Thermonastic = response to temperature.
• - Photonastic = response to light.

Common Mistakes

• 1. Confusing tropisms and nastic movements – Tropisms = directional; Nastic movements = non-directional.
• 2. Confusing phototropism and gravitropism – Phototropism = response to light; Gravitropism = response to gravity.
• 3. Thinking all tropisms are positive – Tropisms can be positive or negative.
• 4. Confusing thigmotropism and seismonastic movement – Thigmotropism = directional growth response to touch; Seismonastic movement = non-directional response to touch.
• 5. Thinking nastic movements are directional – Nastic movements are non-directional.
• 6. Confusing the causes of tropisms and nastic movements – Tropisms = differential growth; Nastic movements = changes in turgor pressure.
• 7. Forgetting the examples – Mimosa pudica (seismonastic), tendrils (thigmotropism), pollen tube (chemotropism).
• 8. Thinking plant responses are not important – Plant responses are essential for plant survival.
• 9. Confusing the types of tropisms – Phototropism, gravitropism, thigmotropism, hydrotropism, chemotropism.
• 10. Confusing the types of nastic movements – Seismonastic, nyctinastic, thermonastic, photonastic.
• 11. Thinking all plant responses are growth responses – Nastic movements are not growth responses.
• 12. Confusing the role of auxins – Auxins mediate tropisms, not nastic movements.

Quick Revision

• Plants are fixed in one place, but they can still respond to changes in their environment.
• These responses help them survive, grow, and reproduce.
• Plant responses to stimuli are called tropisms (directional growth responses) and nastic movements (non-directional responses).
• Tropisms:
• A tropism is a directional growth response of a plant towards or away from a stimulus.
• Tropisms are directional (towards or away from the stimulus).
• They are caused by differential growth (one side of the plant grows faster than the other).
• They are mediated by plant hormones (especially auxins).
• Types of tropisms:
• Phototropism: Response to light.
• Gravitropism (Geotropism): Response to gravity.
• Thigmotropism: Response to touch.
• Hydrotropism: Response to water.
• Chemotropism: Response to chemicals.
• Nastic Movements:
• A nastic movement is a non-directional response of a plant to a stimulus.
• Nastic movements are non-directional (not dependent on the direction of the stimulus).
• They are caused by changes in turgor pressure.
• Types of nastic movements:
• Seismonastic movement: Response to touch or vibration.
• Nyctinastic movement: Response to light and dark (sleep movements).
• Thermonastic movement: Response to temperature.
• Photonastic movement: Response to light.
• Comparison of Tropisms and Nastic Movements:
• Feature | Tropisms | Nastic Movements:
• Direction: Directional | Non-directional.
• Cause: Differential growth | Changes in turgor pressure.
• Stimulus: Light, gravity, touch, water, chemicals | Touch, light, temperature.
• Examples: Phototropism, gravitropism, thigmotropism, hydrotropism, chemotropism | Seismonastic, nyctinastic, thermonastic, photonastic.
• Summary Table of Plant Responses:
• Response | Type | Stimulus | Example:
• Phototropism | Tropism | Light | Shoots bend towards light.
• Gravitropism | Tropism | Gravity | Roots grow downwards; shoots grow upwards.
• Thigmotropism | Tropism | Touch | Tendrils coil around support.
• Hydrotropism | Tropism | Water | Roots grow towards moisture.
• Chemotropism | Tropism | Chemicals | Pollen tube grows towards ovule.
• Seismonastic | Nastic | Touch/vibration | Mimosa pudica leaves fold.
• Nyctinastic | Nastic | Light/dark | Leaves fold at night.
• Thermonastic | Nastic | Temperature | Petals open/close.
• Photonastic | Nastic | Light | Petals open in light.

60-Second Revision

• Tropisms = directional growth responses.
• Nastic movements = non-directional responses.
• Phototropism = response to light.
• Gravitropism = response to gravity.
• Thigmotropism = response to touch.
• Hydrotropism = response to water.
• Chemotropism = response to chemicals.
• Seismonastic = response to touch/vibration.
• Nyctinastic = response to light/dark.
• Thermonastic = response to temperature.
• Photonastic = response to light.
• Warning: Tropisms are directional; nastic movements are non-directional. Tropisms are caused by differential growth; nastic movements are caused by changes in turgor pressure. Phototropism = light; Gravitropism = gravity. Thigmotropism = touch; Seismonastic = touch/vibration. Nastic movements are not growth responses. Auxins mediate tropisms, not nastic movements. Plant responses are essential for plant survival.