Phototropism Discovery – Early Experiments

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.