The Effects of Light on a Sunflower

Seedling Behavior

• Sunflower seedlings will track a light source, changing their orientation throughout the day to face the sun. Most plants display this behavior -- called phototropism -- to an extent, though rarely as dramatically as the sunflower seedling.

Phototropism

• Phototropism describes the tendency of plants to grow or lean toward a light source. Phototropism causes sunflower seedlings to swivel to follow the sun, just as it causes houseplants to press against a sunny window. Since plants need light to make energy, phototropism helps them position themselves where they'll receive maximum light, allowing them to maximize energy production as well.

  A hormone called auxin allows sunflower seedlings to bend to follow the sun's light. When light hits a plant stem, it slows production of auxin, causing the shaded side of the stem to grow faster. This pushes the plant toward the light source. As the sun shifts throughout the day, auxin production shifts as well, and different parts of the stem grow faster, pushing the seedling in an arc-shaped motion.

Adult Behavior

• Once sunflowers mature and begin producing flowers, they no longer tilt to follow the sun. However, they do continue to exhibit a curious light-related behavior. Mature sunflowers always face east, seeming to herald the rising sun each morning. Unlike phototropism, plant scientists haven't teased out the reason for this behavior yet, although the National Sunflower Association speculates that it may be a defensive tactic for the plant. Since sunflowers are generally grown in regions with hot summers, keeping their flowers facing away from the harsh light produced as the sun sets in the west keeps the seeds from suffering from sun scald.

Moving Flowers

• Some flowers throughout the world do tilt their faces to follow the sun, as sunflowers are rumored to do. Unlike the sunflower, though, they typically grow in arctic and alpine environments where they benefit from the warming effect of the sun on their flowers. Warm flowers -- which can be 14 degrees Fahrenheit warmer than the air -- entice pollinating insects to come for a visit. Phototropism and auxin likewise account for sun-tracking flowers. Because they grow in warm climates, there is no evolutionary advantage for other flowers to develop the same behavior.