PICATINNY ARSENAL, N.J. (Nov. 27, 2013) -- Most of us are familiar with photosynthesis, but photosynthesis would not be possible without another, lesser known, process called phototropism. Phototropism is the process that plants utilize to get to a light source for photosynthesis, or to optimize the amount of light intake. Phototropism could not happen without a hormone named indolacetic acid: or more commonly known as auxin.

Auxin is a major hormone in plants. It is the hormone that is responsible for most of the plant's body development and growth. Auxin helps and regulates cell growth, cell division, organ growth, and root extension. Auxin is also a vital piece in another process called gravitropism, which helps the plant's roots reach water. This is why auxin is so important for plants!

Phototropism begins when plants discover light with photoreceptors, which are simply proteins that are sensitive to light. These little light sensors recognize the sunlight and cause the auxin to start moving across the side of the plant that is not receiving sunlight or less sunlight. This will cause the plant to start growing on the side with the auxin, and bend towards the light.

Teaching points

Phototropism is extremely important for plants.

Auxin is the primary chemical compound responsible for phototropism.

Plants can adapt to seemingly bleak situations with phototropism.

Phototropism is an important concept to teach to our youth because plants are all around us, and plants are a vital part of everyday life.


Bending Plants

Here is a simple project to help you visualize the bending action in the plant. Grab a sheet of paper and cut a 2 inch x 11.5 inch strip. Make a vertical cut through the middle, but leave about an inch on the top and bottom.

Fold the paper in half horizontally then take one of the halves and fold it upwards so that there is about a ½ inch fold. If you look from the side the folds will look like a 'Z'.

Then tape one of the folds together so that it cannot bend. Fold the paper in half again, but this time vertically.

As you unfold the un-tapped fold, you should discover how making one side of a plant stem grow will bend the plant towards the light. Thus, the side that is not taped represents the side of the plant in which the auxin moves across and the bending of the paper is analogous to the bending of the plant.

Phototropism in Action

A great experiment is to place a small plant near a window on a sunny day. Make sure the plant is not already in the sunlight, but simply close to the light. Check the plant regularly to observe what you see. To make easy observations take a photo of the plant from the same spot every hour!

You can change this experiment by placing the plant in the sunlight and it will start bending in different directions when the sun changes positions in the sky. This shows how plants respond to the sun's position to absorb the most light.

The Plant Will Find a Way

Since plants can detect sunlight with photoreceptors, the plant does not necessarily have to be in direct sunlight for phototropism to work. In fact, phototropism is best exemplified when it is farthest from the light source.

One way to show this is to first find a small potted plant that can be used for this experiment. Get a box with a cover, preferably a shoebox, and cut a small hole in the box on the side.

Remember to have an adult help you when handling scissors or sharp objects.

Make the hole about two inches wide, or two inches in diameter. Place the plant in the box, away from the opening. Make sure you water the plant regularly.

While the plant and its roots are stationary at the other end of the box, the plant will extend its branches towards the hole. This shows that the plant will find a way to find light even when originating from darkness. You can even set up walls for the plant to maneuver around to see the full power of phototropism!

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