Phototropism Explained

You placed your favorite plant on a sunny windowsill. Soon, you notice the plant bending toward the window instead of growing straight upward. What in the world is this plant doing and why is it doing this?

What Is Phototropism?

The phenomenon you are witnessing is called phototropism. For a hint on what this word means, note that the prefix "photo" means "light," and the suffix "tropism" means "turning." So, phototropism is when plants turn or bend toward light.

Why Do Plants Experience Phototropism?

Plants need light to stimulate the production of energy; this process is called photosynthesis. The light generated from the sun or from other sources is needed, along with water and carbon dioxide, to produce sugars for the plant to use as energy. Oxygen is also produced, and many life-forms require this for respiration.

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Understanding Plant Tropisms

By Regina Bailey

Phototropism is likely a survival mechanism adopted by plants so that they can get as much light as possible. When plant leaves open toward light, more photosynthesis can take place, allowing for more energy to be generated.

How Did Early Scientists Explain Phototropism?

Early opinions on the cause of phototropism varied among scientists. Theophrastus (371 B.C.-287 B.C.) believed that phototropism was caused by the removal of fluid from the illuminated side of the plant's stem, and Francis Bacon (1561-1626) later postulated that phototropism was due to wilting. Robert Sharrock (1630-1684) believed plants curved in response to "fresh air," and John Ray (1628-1705) thought plants leaned toward the cooler temperatures nearer to the window.

It was up to Charles Darwin (1809-1882) to conduct the first relevant experiments regarding phototropism. He hypothesized that a substance produced in the tip induced the curvature of the plant. Using test plants, Darwin experimented by covering the tips of some plants and leaving others uncovered. The plants with covered tips did not bend toward light. When he covered a lower part of the plant stems but left the tips exposed to the light, those plants moved toward the light.

Darwin did not know what the "substance" produced in the tip was or how it caused the plant stem to bend. However, Nikolai Cholodny and Frits Went found in 1926 that when high levels of this substance moved to the shaded side of a plant stem, that stem would bend and curve so that the tip would move toward the light. The exact chemical composition of the substance, found to be the first identified plant hormone, was not elucidated until Kenneth Thimann (1904-1977) isolated and identified it as indole-3-acetic acid, or auxin.

How Does Phototropism Work?

The current thought on the mechanism behind phototropism is as follows.

Light, at a wavelength of around 450 nanometers (blue/violet light), illuminates a plant. A protein called a photoreceptor catches the light, reacts to it and triggers a response. The group of blue-light photoreceptor proteins responsible for phototrophism are called phototropins. It is not clear exactly how phototropins signal the movement of auxin, but it is known that auxin moves to the darker, shaded side of the stem in response to the light exposure. Auxin stimulates the release of hydrogen ions in the cells in the shaded side of the stem, which cause the pH of the cells to decrease. The decrease in pH activates enzymes (called expansins), which cause the cells to swell and lead the stem to bend toward the light.

Fun Facts About Phototropism

• If you have a plant experiencing phototropism in a window, try turning the plant in the opposite direction, so that the plant is bending away from the light. It takes only about eight hours for the plant to turn back toward the light.
• Some plants grow away from light, a phenomenon called negative phototropism. (Actually, plant roots experience this; roots certainly don't grow toward light. Another word for what they are experiencing is gravitropism---bending toward a gravitational pull.)
• Photonasty might sound like a picture of something yucky, but it is not. It is similar to phototropism in that it involves the movement of a plant due to light stimulus, but in photonasty, the movement is not toward the light stimulus, but in a predetermined direction. The movement is determined by the plant itself, not by the light. An example of photonasty is the opening and closing of leaves or flowers, due to the presence or absence of light.