Tropic Responses

Section: Understanding Tropic Responses

• Tropic responses are the directional growth movements that occur in plants in response to external stimuli.
• These responses can be towards the stimuli, named positive tropism, or away from the stimuli, known as negative tropism.
• Major types of tropic responses are phototropism, geotropism (or gravitropism), hydrotropism, and thigmotropism.
• The growth movement in tropic responses is due to differential cell growth, where certain cells grow faster than others, resulting in bending towards or away from the stimuli.

Section: Key Tropic Responses in Plants

• Phototropism is the growth movement in response to light. Roots usually show negative phototropism (growth away from light) while shoots show positive phototropism (growth towards light).
• Geotropism, also known as gravitropism, is the growth response to gravity. Roots display positive geotropism (growth towards gravity) while shoots exhibit negative geotropism (growth against gravity).
• Hydrotropism is the growth response to water. Roots generally exhibit positive hydrotropism as they grow towards moist areas.
• Thigmotropism is the response to touch or physical contact. An example is a climbing plant tendril curling around a support, exhibiting positive thigmotropism.

Section: The Role of Plant Hormones

• Plant hormones known as auxins play a vital role in controlling tropic responses.
• Auxins are produced at the tips of shoots and roots and move from the tips downwards.
• In phototropism and geotropism, auxins move to the shaded or lower side of the plant, causing these cells to elongate more and resulting in bending towards the light or gravity.
• Cytokinins, another group of plant hormones, also help regulate growth responses by promoting cell division and delaying leaf aging.

Section: Tropic Responses and Plant Adaptations

• Tropic responses enable plants to adapt to their environment and optimise their growth.
• For example, phototropism allows plants to maximise exposure to light for photosynthesis.
• Geotropism ensures that roots grow into the soil for water and mineral nutrients, and shoots grow upwards for light.
• Thigmotropism enables climbing plants to find support and reach towards light.

Section: Key Takeaways on Tropic Responses

• Tropic responses in plants demonstrate their ability to respond and adapt to environmental conditions.
• The difference in cell growth rates under the influence of plant hormones such as auxins results in these characteristic responses.
• Understanding tropic responses also helps in horticultural practices, such as training plants to grow in certain directions, and is relevant to understanding wider plant behaviour and adaptations.