Response to Light

Response to Light



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Response to Light  

We’ve all witnessed our windowsill plants leaning toward the light. And some of us have experienced the frustration of a perennial bed where all the flowers are facing the "wrong" way! In both these cases, the plants are reacting to the location and intensity of the light source.

The phenomenon of plants bending toward light is called phototropism (photo=light, tropos=turn). For centuries, scientists struggled to explain how seemingly motionless plants were able to turn and bend toward a light source. Eventually, scientists found that the concentration and location of a certain chemical seemed to be associated with this phenomenon. This chemical was named auxin (Gr. auxein = to increase), and through various ingenious experiments it was determined that auxin is a growth-promoting substance.

Plants respond to light by transporting auxin from one part of the plant to another. When light shines on the side of a stem, auxin accumulates on the dark side of the stem. The growth-promoting auxin causes these cells to elongate more rapidly than the cells on the sunny side. As a result of this rapid growth, the stem curves toward the light. As you might expect, the largest concentrations of auxin are in a plant’s young, actively growing tissues—thus young growth is the quickest to respond to changes in light.


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The term auxin was initially used to describe what were thought to be a group of chemicals responsible for promoting growth in response to stimuli. It is now known that one chemical is responsible for many of these responses; this chemical is called indole-3-acetic acid, or IAA. However, the term auxin is still frequently used to refer to this chemical.

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