Phototropism

Phototropism is the term used to describe the orientation of organisms in the direction of light or the source of light. A good example is the behaviour demonstrated by plants, which can only prepare food in the presence of light. The word phototropism comes from the Greek word meaning light, and the word tropism indicates turning. As a result, phototropism is the bending of the plant’s body in the direction of the light source. The process of photosynthesis, which occurs in plants, drives the generation of energy. Plants that grow towards the direction of light are referred to as positive phototropism, whereas plants that grow away from the source of light are referred to as negative phototropism.

Auxins are hormones that are found in plant cells that are water-soluble. These hormones react to photosynthesis and encourage the production of proteins, as well as the generation of energy for the plant through photosynthesis. Photosynthesis is used by almost all plants for nutrition and energy production. The stem and shoots of the plant body exhibit positive phototropism, which means they bend towards the source of light, but the roots exhibit negative phototropism, which means they turn away from the source of light. Geotropism is a process that occurs in roots, which means that they grow towards the ground. As a result, roots have a negative phototropic and a positive geotropic polarity.

Discovery of Phototropism – Early Experiments

1. The Experiment of Charles Darwin

Prior to the discovery of phototropism, botanists proposed a variety of other theories for the bending of plants in response to light. Various theories were advanced by early scientists, ranging from the wilting of plants to the bending of plants to reach for new air. Darwin, on the other hand, carried out the first significant experiment to demonstrate the concept of phototropism.

Phototropism was discovered by Charles Darwin and his son in 1880 when they conducted experiments on canary grass and oat coleoptiles to learn more about it. Darwin documented his discoveries in the book ‘The Power of Movement in Plants,’ which was published in 1859. He was the first to notice the bending of seedlings in the direction of the sunlight.

He started by covering the tops of the test plants, which prevented them from producing photosynthesis. At the same time, when he covered the lower portion of these test plants with plastic, they began to turn phototropic. According to Darwin’s findings from these and other studies, the tip of the taste plants has a strong sense of light, as evidenced by the fact that it bends towards the source of light, and the central region stimulates the production of protons. The pH of the cells is lowered as a result of this. This entire process acidifies the cell wall, resulting in the activation of an enzyme known as expansions. These cause the cell wall to break down, making it less stiff.

2. The Experiment of Boysen Jensen

Boysen Jensen conducted an experiment on seedlings in 1913 that was inspired by Darwin’s work with test plants, oat coleoptiles, in which he cut the tip of the plant and replaced it with a thin layer of gelatine between the tip and the cut stem. However, this did not prevent the stem from curving in the direction of the light source. He then placed a little mica sheet below the tip of the coleoptiles on the shadowed side of the coleoptiles for the following phase. The inclusion of the mica layer did not prevent the development of a curvature, which was previously seen. Boysen Jensen discovered that there was no curvature when he placed the identical mica sheet on the lit side of the glass. In the course of his experiment, he came to the conclusion that a flow of a material substance, which was later named auxins, travels between gelatine and mica sheet. And that it was this substance that was responsible for the curvature of the earth.

Mechanism of Phototropism

The following is the mechanism that underlies the occurrence of phototropism:

1. The plant is illuminated by light with a wavelength of roughly 450nm, which may be blue or violet in colour.
2. The light is received by the photoreceptor, which is a protein contained in the plant’s cells. The photoreceptors detect it and respond by triggering a reaction.
3. Phototropins are a category of blue light photoreceptor proteins that are found in large numbers in the retina. The proteins that accept blue light during phototropism are known as phototropins.
4. In reaction to exposure to light, auxin shifts to a darker, shade-side position in the stem.
5. In the darkened part of the stem, auxins encourage the release of hydrogen ions. There is a resultant reduction in the pH level. The enzymes’ expansins are activated as a result of the reduction in ph.
6. When the expansins are activated, the cells enlarge, which causes the stem to bend towards the light.

Illustrations of Phototropism

Sunflower is the best example of a phototropic plant that is highly photosensitive. Sunflower plants are always growing towards the direction of the sun, and it has been noticed that they track the sun’s movement throughout the day. This indicates that the blossoms of the plant are constantly altering their direction in response to the movement of the sun. The sunflower plant demands more light for its growth and survival than other plants. Aside from this, phototropism can be seen in the shoot and stem of all green plants.

Conclusion

Phototropism is defined as the process of a thing expanding in response to a light stimulus (or stimulus from light). Even while this process is most usually observed in plants, it can also be detected in other organisms, including fungi. A molecule known as auxin is present in plant cells that are the furthest away from light, and it is active and reacts when phototropism occurs. Auxin is found in the tips of leaves and stems, which causes them to grow in a positive direction toward the light.