From Seed to Seed:
Plant Science for K-8 Educators

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 of the flowers are facing the "wrong" way! In both of these cases, the plants are reacting to the location and intensity of the light source.

The phenomenon of plants bending toward the light is called phototropism (Gr. 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 plant growth regulator.

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 the plant's young, actively growing tissues-so young growth is the quickest to respond to changes in light.

Grow a light-tracking plant-like sunflower-on the windowsill in your classroom or in the garden. Students can monitor the position of the plant throughout the day as the sun moves across the sky. They can record the position of the plant through drawings in their journals or by taking photographs. Before discussing phototropism, allow your students to make observations for at least one sunny day and hypothesize why the plant changes position. Students can continue to make observations during the course of the year to investigate seasonal trends.

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