Free Introduction to Plant Responses revision notes for OCR A Level Biology – covering specification points 5.1.5 (ai), 5.1.5 (aii) and 5.1.5 (f).
Introduction to Plant Responses
Unlike animals, plants are immobile and unable to respond to both abiotic and biotic stimuli by moving towards (if it is beneficial) or away (if it is harmful) from them. Instead, plants must adapt to environmental changes by altering their physiology and growth.
Plant responses are coordinated by chemicals, such as hormones, which are transported slowly (compared to the nervous systems found in animals) by transport processes within a plant.
Plant responses can be directional, or non-directional, movements, or chemical responses.
Directional Responses: Tropisms
Tropisms are responses to a stimulus in the form of a directional response.
Tropic responses involve differential growth, where cells on one side of an organ elongate or divide more than those on the other side over time (making it a slow response). This usually results in the stimulus increasing (if desirable) or decreasing (if undesirable).
The table below outlines some of the main types of tropism:
| Tropism | Description | Example |
|---|---|---|
| Geotropism | Growth in response to gravity. |
|
| Phototropism | Growth in response to light. |
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| Thigmotropism | Growth in response to touch or contact. |
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Non-directional Responses: Nastic Movements
Nastic responses are non-directional movements, unrelated to the direction of the stimulus. The most commonly encountered example is thigmonasty.
The table below outlines thigmonasty, and its survival advantage:
| Nastic Response | Description | Advantage |
|---|---|---|
| Thigmonasty | Leaves or leaflets fold in response to touch or vibration. | Deters herbivores, limits water loss by shading leaves.* |
*It is useful to know that scientists do not definitely know why Mimosa pudica’s leaves fold, only that it has situational advantages and disadvantages. The OCR A Biology specification highlights its role in deterring herbivory.
Physiological Responses
Physiological responses alter how plant tissues function and result in the production of chemicals.
The table below outlines some of the major physiological responses and their survival advantage:
| Physiological Response | Description | Advantage |
|---|---|---|
| Stomatal closure | ABA-induced ion loss from guard cells reduces turgor and closes stomata. | Reduces water loss by transpiration. |
| Seed germination | Gibberellin (GA) stimulates enzyme production in the aleurone layer. | Provides glucose for respiration and embryo growth. |
| Leaf abscission | Decrease in auxin and subsequent increase in ethene levels weakens the abscission layer (connection point). | Reduces water loss. |
| Stress-induced defence chemistry | Increased production of chemical defences (e.g. tannins, alkaloids). | Reduces further herbivore feeding and tissue loss. |
Plant hormones are widely used in agriculture to control growth, ripening, development and storage.The most commonly used hormones in agriculture include auxin and ethene:
The most commonly used hormones in agriculture include auxin and ethene:
- Auxin is used as a herbicide (weed killer) and to encourage the development of roots in plant cuttings.
- Ethene is used to ripen fruit so as to make it more appealing to consumers.
