Plant Movements: Unraveling Tropic and Nastic Responses
Paratonic Movements:
The movements exhibited by plant organs in response to external stimuli or external causes are broadly categorized into two types.
i. Tropic Movements:
- Definition: Tropic movements refer to directional growth responses of plant organs in response to external stimuli such as light, gravity, or water. The direction of growth is influenced by the direction of the stimulus.The term "tropic" is derived from the Greek language. It has its roots in the Greek word "tropos," which means "turn" or "direction." In the context of plant biology, "tropic movements" refer to directional growth responses of plant organs in response to external stimuli such as light, gravity, or moisture. The term reflects the turning or bending nature of these movements in response to specific environmental cues.
- Types:
- Phototropism
- Geotropism
- Hydrotropism
- Thigmotropism
- Chemotropism
Phototropism:
- Definition: Phototropism is a tropic movement in which plants orient their growth towards or away from a light source.
- Types:
- Positive phototropism: growth towards light.
- Negative phototropism: growth away from light.
- Example: Sunflowers (Helianthus annuus) exhibit positive phototropism as their flower heads follow the movement of the sun during the day.
Geotropism (Gravitropism):
- Definition: Geotropism is a tropic movement in response to gravity, influencing the direction of growth.
- Types:
- Positive Geotropism: Growth towards gravity (e.g., roots).
- Negative Geotropism: Growth against gravity (e.g., shoots).
- Example: The roots of most plants exhibit positive geotropism by growing towards the gravitational pull, anchoring the plant in the soil.
Hydrotropism:
- Definition: Hydrotropism is a tropic movement in response to water, with roots growing towards or away from moisture.
- Example: The roots of Arabidopsis thaliana exhibit hydrotropism, growing towards areas with higher water concentrations.
Thigmotropism:
- Definition: Thigmotropism is a tropic movement in response to touch or physical contact with solid objects.
- Example: The tendrils of climbing plants like Passiflora exhibit thigmotropism, coiling around structures for support when touched.
Chemotropism:
- Definition: Chemotropism is a tropic movement in response to chemical stimuli, such as the growth of pollen tubes towards ovules.
- Example: The pollen tube of flowering plants growing towards ovules during fertilization is an example of positive chemotropism.
These tropic movements showcase the adaptability of plants to their environment, allowing them to optimize their growth and development based on external cues.
Nastic Movements:
Definition: non-directional responses in plants triggered by external stimuli, rapid and reversible. It has two types.
I.Nyctynasty
II.Haptonastic
Nyctinasty:
- Definition: Nyctinasty is a form of nastic movement in plants, specifically associated with the opening and closing of certain plant organs, such as flowers or leaves, in response to changes in light and darkness over a daily cycle. There are two types:.
Types of nyctinasty:
Movements associated with the daily light-dark cycle
i. Thermonasty:
- Definition: Nyctinastic movement in response to temperature changes.
- Examples: Petals of Chestnut Rose (Rosa × centifolia) open during warm days and close during cooler nights.
ii. Photonasty:
- Definition: Nyctinastic movement in response to light.
- Examples: Flowers of Daisy (Bellis perennis), opening during the day and closing at night.
iii. Haptonastic Movement:
- Definition: Nastic movement in response to touch.
- Examples: Leaves of the Sensitive Plant (Mimosa pudica) rapidly fold in response to touch.
Role of Plant Growth Substances in Plant Movements:
Tropic Movements:
-
Phototropism:
- Hormone: Auxin (particularly Indole-3-Acetic Acid, or IAA)
- Explanation: Auxin accumulates on the shaded side of the stem, causing elongation and bending towards the light source.
-
Geotropism:
- Hormone: Auxin
- Explanation: In roots, auxin moves away from gravity (negative geotropism), promoting downward growth. In shoots, it moves towards gravity (positive geotropism), promoting upward growth.
-
Hydrotropism:
- Hormone: involvement of hormones like abscisic acid (ABA) and cytokinins.
- Explanation: Hormones influence root growth in areas with higher water concentrations.
-
Thigmotropism:
- Hormones: jasmonic acid and ethylene.
- Explanation: These hormones are involved in the response to mechanical stimuli, leading to changes in plant structure and growth.
Nastic Movements:
-
Nyctinasty:
- Hormone: Circadian rhythms linked with light and dark cycles, involving phytochromes.
- Explanation: Light-responsive hormones trigger the opening and closing of flowers or leaves during the day and night.
-
Thigmonasty:
- Hormone: rapid changes mediated by electrical and calcium signals, possibly involving jasmonic acid and ethylene.
- Explanation: Mechanical stimuli lead to rapid movements, such as the closure of Venus flytrap leaves.
Coleoptile and Hormonal Growth:
- Definition: A coleoptile is a protective sheath covering the emerging shoot tip in grass seedlings.
- Hormone: Gibberellins (GA)
- Explanation: Gibberellins stimulate cell elongation in the coleoptile, allowing it to push through the soil and reach sunlight.
In summary, plant growth substances, especially hormones like auxin, abscisic acid, cytokinins, jasmonic acid, ethylene, phytochromes, and gibberellins, play crucial roles in regulating tropic and nastic movements, as well as in the growth and development of specific plant structures like the coleoptile. These substances coordinate complex signaling pathways, allowing plants to adapt and respond to their environment.
Why are tropic movements called "movements of curvature"?
Tropic movements, such as phototropism and geotropism, are labeled "movements of curvature" because they involve differential growth rates on opposing sides of plant organs, resulting in visible bending or curvature in response to external stimuli like light or gravity.
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Differences Between Tropic and Nastic Movements:
| Feature | Tropic Movements | Nastic Movements |
|---|---|---|
| Direction of Movement | Towards or away from a directional stimulus. | Not specifically directed; response is non-directional. |
| Stimulus Dependency | Direction of growth is influenced by the stimulus direction. | Not dependent on the direction of the stimulus. |
| Example | Phototropism (response to light), Geotropism (response to gravity). | Nyctinasty (opening and closing of flowers in response to light-dark cycles). |
| Types of Movements | Phototropism, Geotropism, Hydrotropism, Thigmotropism, Chemotropism. | Nyctinasty, Thigmonasty, Seismonasty, Thermonasty. |
| Controlled by Hormones | Yes, hormones like auxin play a significant role. | Hormones may be involved, but the response is often rapid and non-hormonal. |
Example of Nastic Movement:
- Nyctinasty: The opening and closing of the flowers of the "Morning Glory" (Ipomoea purpurea) in response to daily light-dark cycles.
Tropic Movement in Plants:
- Example: Phototropism in Sunflowers: The bending of sunflower stems toward the sun during the day.
Difference Between Tropic and Nastic Movements (Class 9):
Tropic Movements:
- Directional response to an external stimulus.
- Examples: Phototropism, Geotropism.
- Hormone involvement, especially auxin.
Nastic Movements:
- Non-directional response.
- Examples: Nyctinasty, Thigmonasty.
- Rapid, reversible responses, not always hormone-controlled.
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