Phototropism: Definition, Discovery, Mechanism & Examples - Testbook

Defining Phototropism

"Phototropism signifies a plant's capacity to grow towards the light."

What Exactly is Phototropism?

The term "phototropism" combines "photo," which means light, and "tropism," which means turning. Essentially, phototropism is a plant's ability to bend towards a light source. Plants need light to carry out the process of photosynthesis , which is vital for their survival.

Auxins, a type of hormone found in plant cells, play a crucial role in photosynthesis, helping to produce more protein and generate energy for the plant.

Almost every plant species responds to photosynthesis to absorb nutrients and energy efficiently. Stems and shoots generally exhibit positive phototropism by bending towards sunlight, whereas roots display negative phototropism by growing away from light. This behavior is influenced by gravitropism or geotropism.

Further Reading: Understanding Photosynthesis

 

Phototropic Response

Historical Experiments on Phototropism

Several scientists have conducted experiments to understand the mechanism of phototropism. Here are some notable ones:

The Experiment by Charles Darwin

In 1880, Charles Darwin, along with his son, conducted research on phototropism using canary grass and oat coleoptiles. They observed that the seedlings bent towards sunlight. They concluded that the tip of the grass, or coleoptiles, was sensitive to light and bent towards it, while the middle section activated protons, reducing the pH in the cells. This action acidified the cell wall, activating an enzyme known as expansins, which broke down the cell wall , making it less rigid.

Boysen Jensen’s Experiment

Inspired by Darwin's work, Boysen Jensen conducted his own experiment in 1913. He cut off the tip of seedlings and placed a thin layer of gelatin between the tip and the cut stem. This did not inhibit the stem from curving towards light. His conclusion was that a substance (later identified as auxins) that could pass through gelatin and mica sheet was responsible for this curvature.

The Mechanism of Phototropism

The process of phototropism involves the following steps:

1. The plant is illuminated by light with a wavelength of 450 nm.
2. A photoreceptor receives this light, reacts to it, and triggers a response.
3. Proteins called phototropins receive blue light during phototropism.
4. When exposed to light, auxin moves to the darker side of the stem.
5. In the shaded area of the stem, auxin releases hydrogen ions, which lowers the pH. This pH decrease activates expansin, causing the cells to swell and forcing the stem to bend towards the light.

Examples of Phototropism

Morning glories are an excellent example of phototropic plants. They open their flowers towards the sun and follow its path throughout the day. Similarly, ivy plants also exhibit phototropism by growing towards any available light source, ensuring their survival even in low-light environments.

Further Reading: Photosynthesis Concept Map

Keep exploring to learn more about the fascinating world of Phototropism, its definition, mechanism, and examples.