Ask Mr. Smarty Plants

Heliotropism is the term used to describe the tracking of the sun by the flower buds of sunflowers. Other plants also have flowers and/or leaves that track the sun. First, let's discuss how the plants are able to track the sun. There is a collection of specialized cells at the base of the flower bud or leaf called a pulvinus that carry the "motor" cells that enable the plant leaf or flower to track the sun. These cells enlarge or shrink according the turgor pressure from the water inside them. In response to blue wavelength light, potassium ion concentration increases in the "motor" cells on the shadow side of the pulvinus. With the increase of potassium ions the osmotic potential in the cells becomes more negative and the cells absorb more water and elongate, turning the face of the flower to the sun.

It is easy to understand why heliotropism is advantageous for leaves. Turning the leaf perpendicular to the sun provides maximum sunlight to power photosynthesis. At mid-day, to avoid overheating, these same leaves may change their orientation to parallel rather than perpendicular so that the edge of the leaves, rather than the surfaces, are facing the sun. It is thought that one reason the buds of the sunflower (and other flowers) track the sun is that insects are attracted to the warmth from the sun and their presence is necessary for pollination to occur. The warming from the sun is especially important for flowers in cold environments such as alpine and arctic regions. It is interesting to note that mature sunflowers that have been pollinated no longer track the sun. A study done on the snow, or Alpine, buttercup, which also exhibits floral heliotropism, suggests that sun tracking also increases pollen development and germination. You can learn more about heliotropism in an article "Sun Stalker—flowers and sun" by Candace Galen in Natural History, May 1999.