What Is Phototropism Definition Process and Types
What is Phototropism?
The orientation of organisms in the direction of light or source of light is called phototropism. It is a phenomenon shown by plants, as they can only prepare food in the presence of light. Photo in the word phototropism means light and tropism stands for turning. Thus, phototropism is bending of the plant body towards the source of light. Light stimulates energy production in plants by the process of photosynthesis. Plants growing towards light are called positive phototropism while growing away from the source of light is termed as the phenomenon- negative phototropism.
Plant cells contain water-soluble hormones known as auxins. These hormones respond to photosynthesis and stimulate the production of proteins and generate energy by photosynthesis for the plant. Almost all plants show photosynthesis for nutrition and energy. The stem and shoots of the plant body show positive phototropism by turning towards the sunlight, while roots show negative phototropism and turn away from the source of light. Roots show a phenomenon called geotropism which means growing towards the ground. Hence, roots are negatively phototropic and positively geotropic.
Discovery of Phototropism – Early Experiments
1. Charles Darwin’s Experiment
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Before the discovery of phototropism, Botanists had various other explanations for the bending of plants. Starting from wilting to bending of plants to reach fresh air, various theories were put forward by early scientists. However, Darwin conducted the first relevant experiment to prove the phenomenon of phototropism.
In the year 1880, Charles Darwin along with his son experimented on canary grass and oat coleoptiles to research phototropism. Darwin recorded his observations in the book ‘The Power of Movement in Plants’. He was the first to observe the bending of seedlings towards sunlight.
At first, he covered the tips of the test plants that prevented it from photosynthesis. Simultaneously, when he covered the lower portion of these test plants, they turned phototropic. Darwin concluded from this and several other experiments that the tip of the taste plants have a strong sense of light, due to which it bends towards its source, while the middle section activates protons. This decreases the pH in the cells. This entire act acidifies the cell wall, thus, activating an enzyme called expansions. These break down the cell wall making it less rigid.
2. Boysen Jensen’s Experiment
Following Darwin’s experiment with test plants- oat coleoptiles, Boysen Jensen in 1913 experimented on seedlings by cutting the tip-off and replacing it with a thin layer of gelatin between the tip and the cut stem. But this did not prevent the stem to curve towards the source of light. For the next step, he placed a small mica sheet below the tip of coleoptiles on the shaded side. This new addition of the mica sheet also did not prevent developing a curvature. When Boysen Jensen placed the same mica sheet on the illuminated side however there was no curvature. From his experiment, he concluded that a flow of material substance which was later named auxins passes through gelatin and mica sheet. And that this substance was responsible for the curvature.
Phototropism Diagram
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Mechanism of Phototropism
The mechanism behind the phenomenon phototropism is as follows-
1. Light at a wavelength of nearly 450nm may be blue or violet light illuminates the plant.
2. The photoreceptor which is a protein found in the plant receives the light. The photoreceptors react to it and initiate a response.
3. The group of blue light photoreceptor proteins is known as- Phototropins. They are the proteins that receive blue light during phototropism.
4. Auxin moves to a darker, shade side in the stem in response to the exposure to light.
5. Auxins stimulate the release of hydrogen ions in the shaded region of the stem. This causes a decrease in the pH level. This decrease in pH activates the enzymes' expansins.
6. Activated expansins cause the cells to swell and forces the stem to bend towards the light.
Phototropism Examples
The best example of a highly phototropic plant is Sunflower. Sunflower plants always grow towards the sun and are also observed tracking the movement of the sun throughout the day. This means that the flowers of the plant keep changing its direction with the movement of the sun. Sunflower plant requires more light for its growth and survival. Apart from this shoot and stem of all green plants show phototropism.
FAQs on Phototropism in Plants and Its Mechanism
1. What is phototropism in plants?
Phototropism is the directional growth of a plant in response to light. It allows plants to grow toward or away from a light source to optimize photosynthesis.
- Positive phototropism: growth toward light (e.g., shoots).
- Negative phototropism: growth away from light (e.g., some roots).
- It is a type of tropism, meaning a growth response to an external stimulus.
2. How does phototropism work in plants?
Phototropism works through the unequal distribution of the plant hormone auxin, causing one side of the plant to grow faster than the other. The process occurs in steps:
- Light is detected by photoreceptors in the shoot tip.
- Auxin moves to the shaded side of the stem.
- Cells on the shaded side elongate more.
- The stem bends toward the light source.
3. What is the role of auxin in phototropism?
The role of auxin in phototropism is to promote cell elongation on the shaded side of a plant shoot, causing it to bend toward light. Auxin functions by:
- Redistributing from the illuminated side to the shaded side.
- Stimulating cell wall loosening and elongation.
- Creating unequal growth rates across the stem.
4. What is the difference between positive and negative phototropism?
The difference between positive and negative phototropism is the direction of plant growth relative to light.
- Positive phototropism: growth toward light (commonly seen in shoots and stems).
- Negative phototropism: growth away from light (seen in some roots).
5. Why is phototropism important for plants?
Phototropism is important because it helps plants orient their shoots toward light to increase photosynthetic efficiency. Its significance includes:
- Maximizing sunlight capture.
- Enhancing food production through photosynthesis.
- Improving survival and growth in competitive environments.
6. Which part of the plant detects light in phototropism?
The shoot tip detects light in phototropism using specialized photoreceptors. These include:
- Phototropins, which sense blue light.
- Cells in the apical meristem, where growth is active.
7. What is an example of phototropism?
A common example of phototropism is a plant stem bending toward a window where light enters. For example:
- A sunflower seedling growing toward sunlight.
- Indoor plants leaning toward a light source.
8. How is phototropism different from phototaxis?
Phototropism is directional growth in response to light, whereas phototaxis is the movement of a whole organism toward or away from light. Key differences include:
- Phototropism: growth-based response in plants.
- Phototaxis: locomotion-based response in motile organisms like bacteria or algae.
- Phototropism is slow and permanent; phototaxis is often rapid and reversible.
9. Does phototropism occur in roots?
Yes, phototropism can occur in roots, and it is usually negative phototropism. In roots:
- Growth occurs away from light.
- Higher concentrations of auxin inhibit cell elongation.
- This response helps roots remain underground.
10. What type of stimulus causes phototropism?
Phototropism is caused by a light stimulus, particularly blue light. The stimulus characteristics include:
- Detection by phototropins in plant cells.
- Directional light exposure.
- Triggering redistribution of auxin.
