Question

The movement of a plant organ in response to the unilateral effect of light is known as
A. Geotroprism
B. Hydrotroprism
C. Thigmotroprism
D. Phototropism

Answer 1

Hint: Movement of plant parts in response to a unidirectional light stimulus is called phototropism. Some parts of the plant e.g., stem moves towards the source of light. These organs are called positively phototropic.

Complete answer: Some other organs e.g., roots move away from light and they are called negatively phototropic. In a plant, leaves are placed at right angles, to the direction of light source, and are called transversely phototrophic or plageo-phototropic.
1. The curvature of the stem tip is explained by the unequal distribution of Auxin on either side of the light-induced stem. Plants respond to light stimuli by growing, differentiating, tracking the time of day and seasons, and moving toward or away from the light.
2. Plants have a number of sophisticated uses for the light that goes far beyond their ability to perform photosynthesis. Plants can differentiate and develop in response to light which allows plants to optimize their use of light and space.
3. Plants use light to track time, which is known as photoperiodism.
4. Light can also elicit a directional response in plants that allows them to grow toward, or even away from, light; this is known as phototropism. The sensing of light in the environment is important to plants; it can be crucial for competition and survival.
5. The response of plants to light is mediated by different photoreceptors: a protein covalently-bonded to a light-absorbing pigment called a chromophore; together, called a chromoprotein.
6. The chromophore of the photoreceptor absorbs light of specific wavelengths, causing structural changes in the photoreceptor protein. The structural changes then elicit a cascade of signaling throughout the plant.
7. The phytochrome system acts as a biological light switch. It monitors the level, intensity, duration, and color of the environmental light. The shoot of a pea plant wraps around a trellis while a tree grows on an angle in response to strong prevailing winds. These are examples of how plants respond to touch or wind.

So, the correct answer is “Option D”.

Note: Thigmomorphogenesis is a slow developmental change in the shape of a plant subjected to continuous mechanical stress. When trees bend in the wind, for example, growth is usually stunted and the trunk thickens. Strengthening tissue, especially xylem, is produced to add stiffness to resist the wind’s force.