Phototropism

Phototropism is a plant's response to light. Learn more!

Tropisms are a plant's response to it's environment. Because plants can respond to everything from chemicals in soil to gravity, there are different kinds of tropisms. This article will address the affect that light has on growing plants.

Phototropism is generally described as a plant's response to light. There are two kinds of phototropism: positive photropism, or the growth of a plant stem towards light, and negative phototropism, or the growth of a plant's roots away from light. Phototropic responses are caused by auxins, which modify cell walls within a plant to acheive phototropic effect. Auxins concentrate on the side of a plant stem away from light, in order to stretch rigid plant cell walls into growth toward light. A good way to measure the effect of a phototropism in plants is to measure the length and curvature of a stem before and after phototropic changes.

Phototropism was originally termed "heliotropism," or a plant's response to the sun. This definition changed when it was found that plant responded the same way to artificial light. Some of the first experiments on this subject were done by Darwin. He noticed that a plant with foil on its tip did not grow toward light, and concluded that something was blocking the normal reaction of the tip toward the sun. His experiments were continued by Boyson-Jenson, who later realized that the water-soluble nature of plants contributed to their phototropisms. With the contribution of more scientists, it was gradually concluded that auxins were responsible for phototropism, auxins elongated cells on the shady side of the plant, and this elongation made a curvature in the stem.



The first theory in the process of cell elongation is the "acid growth hypothesis." According to this theory, auxins pump H+ into cell walls, and cell walls decrease the PH of the cell. Then the walls soften under contact with acid and turgor pressure stretches them, producing phototropism. It is interesting to note that the color of light used also has an effect. Blue, ultraviolet and white light have effects. Green, yellow and red lights usually do not have effects. One theory, the Bunsen-Ruscoe Law, states that the amount of light and the phototropic reaction are linked. While this theory is still being tested, it infers that the greater the intensity of light, the greater the phototropic reaction.

To create you own phototropism, you will need:

Two single-stemmed plants

Two completely dark rooms

A desk lamp

A ruler

An angle measure

One or these plants will serve as a control, or stable component, and one will serve as the variable component. Before beginning, measure the main stems of the two plants and record. Then chart the angle of their stems with an angle measure and record. Take one room and rig the desk lamp so that it is in a far-away corner. Place one plant on the floor. Turn on the lamp and leave the plant in the room for several hours. Place the other plant in a totally dark room for the same length of time. Finally, measure the stems of the plant in the totally dark room and the plant in the room with the desk lamp. Is there a recorded change in the length and curve of the stem? This change is your phototropic effect.

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