Heliotropic Leaf Movements in Plants
Many plant species, especially those in the Leguminosae and Malvaceae have the ability to orient their leaves in relation to the sun's direct rays. These movements can be classified as tracking the sun (diaheliotropism), or avoiding the sun (paraheliotropism). Tracking the sun maximizes the amount of direct solar radiation a leaf recieves, while paraheliotropism reduces the amount a leaf receives. These leaf movements are not growth movements, rather they are rapid, reversible movements in response to turgor changes in cells in an organ at the base of the leaf petiole called a pulvinus. Many desert annuals, plants that grow in response to unpredictable rains show heliotropic movements. These tracking movements can increase radiation interception, photosynthesis and growth rates - advantageous properties in an ephemeral environment. Some of these annuals also show paraheliotropism in response to water stress, thus, extending life by reducing leaf temperatures and water loss as soils dry out.
This is an animation constructed frompictures taken every 5 minutes over a 45 minute span. A soybean (Glycine max) leaf was held horizontally under a bright light. The leaf rapidly moves into a vertical position (paraheliotropism). The pictures were put in reverse order at the end of the animation to show the complete cycle of movement.
These are pictures of two desert annuals in Death Valley, CA. On the left is Eremalche rotundifolium, a solar tracker in the Malvaceae family. On the right is Lupinus arizonicus, a leguminous plant that shows a mixture of tracking and paraheliotropism, depending upon its water status.
Here is a series of pictures taken at midday every day during a drought cycle. The lupine starts with a fully saturated pot, and over a 9 day period dries out. Each day, the leaf cups more, reducing midday irradiance. The pulvinus can be seen as a swelling of a lighter color at the base of the leaflets. The same 9 pictures are used in reverse order in the animation, to show the full cycle of movement.