Phototropism in plants is a process where a plant bends in response to sunlight or any other light source for the production of energy to stimulate the growth of the plant. Phototropism is a mechanism that is necessary for the survival of the plant.
Therefore, phototropism is the “Survival mechanism” where the plant absorbs as much light. And, as much the plant leaves will open towards the light, will undergo more photosynthesis by allowing more energy to generate.
Content: Phototropism in Plants
- Definition of Phototropism
- History of Phototropism
- Mechanism of Phototropism in Plants
- Types of Phototropism
- Overview of Phototropism in Plants
- Role of Phototropins and Auxins in Phototropism
Definition of Phototropism
Photo means “Light” and tropism means “Turning”. Therefore phototropism merely refers as the bending of plants towards the light for its growth by absorbing solar energy.
Tropism can define as the growth of a plant towards any of the stimuli like heat, light, air, water, chemicals etc. Therefore, the growth of a plant towards a stimulus (light ) is the phenomena which will refer to as “Phototropism”.
Phototropism is of two type’s namely positive and negative phototropism. When the shoot system grows in response to light then it refers as “Positive phototropism”. And, when the root system grows in response to light then it refers as “Negative phototropism”.
History of Phototropism
To discover the theory of Phototropism in plants, there were many scientists who had contributed by introducing different experiments, approaches and opinions that have led to the discovery of the mechanism Phototropism.
| Year | Discoverer | Discovery |
|---|---|---|
| 371-287 BC | Theophrastus | Discovered that the phototropism was due to removal of fluid from the illuminated site of the plants stem |
| 1561-1626 | Franscis Bacon | He discovered that phototropism was due to wilting |
| 1630-1684 | Robert Sharrock | According to him, plants get bends, due to fresh air |
| 1628-1705 | John Ray | According to him, plants bends towards the cool temperature when placed near the window |
| 1809-1882 | Charles Darwin | Hypothesized that one substance is produced at the tip of the plant which is responsible for the curvature of plant |
| 1926 | Nikolai Cholodny and Frit’s Went | They found high level of that substance that moved to the shaded side of the plant stem |
| 1904-1977 | Kenneth Thimann | Isolated and identified substance like “Auxin” |
Mechanism of Phototropism in Plants
The phototropism works on the principle of the “Light Reaction”. In phototropism, a light of around 450nm is caught by a photoreceptor, present in the plant will trigger a response. Blue light photoreceptor protein forms a complex, which refers to as “Phototropins”. In the presence of light, auxin moves to the darker side of the stem.
Auxin releases hydrogen ions in the plant cell of the darker portion of the stem, which leads to a decrease in the pH of the cell. The decrease in pH activates the release of an enzyme known as “Expansins”. The enzyme expansins will cause swelling in the cell and finally results in the bending of the plant towards the light.
Types of Phototropism
There are two types of phototropism that occurs in the plant namely positive and negative phototropism.
Positive Phototropism: The movement or bending of plant part (Shoot system), in response to light, is the phenomena which refer as “Positive Phototropism”.
Negative Phototropism: The movement or bending of plant part (Root system), in response to light, is the phenomena which refer as “Negative Phototropism”.
Overview of Phototropism in Plants
If we place a pot near a window, the plant bends towards the light to get the energy that’s why it grows towards the light. The root system grows upward and once they break through the surface, the shoot system starts bending towards the light.
The whole phenomena of bending of the plant can be understood by the concept of the signalling pathway that is explained below. Plant senses light, by the specific molecules called “Photoreceptors”. The photoreceptors are the protein molecules which links with the chromophore pigment that absorbs light.
Chromophore when absorbs light, it changes the structural configuration of the protein and thus changes its activity and will produce a signal or response. The response produces towards the light will promote gene expression, growth or hormone production. The production of response will refer to as “Signalling pathway”.
When a signal is produced in plants with a response to the light is called “Phototropism”. Therefore, phototropism is a directional response where a plant bends towards the stimulus (Light).
Role of Phototropins and Auxins in Phototropism
The Phototropins are the photoreceptors which detect the light and absorbs the blue range of the spectrum. The plant’s tip is known as “Coleoptile”, where a plant growth hormone “Auxin” has a high concentration.
A coleoptile exposes to the sunlight or any other light source it will cause unequal distribution of phototropins. The Phototropins will be more active or absorbs more light. And the Phototropins will be less active on the darker side or absorbs less light.
There will be unequal transport of a plant hormone “Auxin” towards the darker side of the coleoptile due to the different level of Phototropins.
Auxin will shift more towards the darker side than the illuminated side. The auxin on the darker side of the stem will promote cell elongation and more growth which results in bending of the plant in response to a light source.
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