Sun's Effect on Plants

Tropism

• Plants always turn toward light and away from shady areas. This phenomenon is called phototropism, and is a pronounced and noticeable effect of the sun on plants. Phototropism is the result of a hormone within the plant called auxin that makes the plant grow more quickly. Auxin is found on the shady side of a plant, not the sunny side, making the plant grow unevenly. The shady side grows faster than the sunny side, which causes the stem to bend toward the sunlight. This chemical is also found in roots, causing them to grow downward.

Plant Timetables

• The length of time a plant is exposed to sunlight on a given day determines its schedule. The amount of daylight can vary greatly, depending on the time of the year. In the Northern Hemisphere, summer brings long days with lots of sunlight, while in winter some areas start to get dark at 4 o'clock in the afternoon. The number of hours a plant receives sunlight triggers the plant to flower when there is more sunlight, and drop leaves to prepare for winter when it receives less sunlight. This response is called photoperiodism. Plants have a substance called photochrome that enables them to react to changes in the amount of light.

Changing Color

• When the sunlight hours decrease in autumn, photochrome triggers the plant to prepare for winter. Plants form winter-resistant buds to seal off the flow of water into the leaves. The leaf stems form abscission zones at their base, and the leaf dies. As chlorophyll in the leaf fades, leaves turn the familiar shades of fall foliage -- red, orange and yellow.

Provides Food for the Plant

• A well-known effect of sunlight on plants is photosynthesis, whereby the plant uses sunlight to produce its own food. Photosynthesis requires water, carbon dioxide, chlorophyll found in the plant and sunlight. Chlorophyll is what makes plants appear green.