Heliotropism
Heliotropism is the phenomenon whereby certain plants orient their leaves and other organs toward the sun to maximize sunlight absorption. This behavior is primarily divided into two types: diaheliotropism, where plants track the sun to increase direct solar radiation for enhanced photosynthesis and growth, and paraheliotropism, where plants adjust their orientation to minimize sunlight exposure, helping to conserve water and reduce overheating. Plants such as alfalfa, cotton, and some species in the mallow family exhibit this solar tracking ability, adjusting the position of their leaves throughout the day to optimize light intake.
The adjustments of heliotropic plants are often controlled by a specialized structure known as the pulvinus, which manages leaf movement through changes in turgor pressure. These movements can occur rapidly, allowing plants to adapt continuously to shifting light conditions. However, heliotropic behavior typically ceases when clouds obstruct sunlight. The specialized adaptations of these plants reflect their evolution to thrive in varying environmental conditions, balancing the need for sunlight with the risks of dehydration and damage from excessive heat.
On this Page
Heliotropism
Categories: Movement; physiology
Plants that orient their leaves to receive maximum sunlight are called diaheliotropic. Diaheliotropism is the tendency of leaves or other organs of a plant to track the sun by turning their surfaces toward it. Tracking the sun maximizes the amount of direct solar radiation received. Diaheliotropic movements can increase radiation interception, enhance photosynthesis, and increase growth rates of plants.
Plants that move their leaves to avoid sunlight are called paraheliotropic. Paraheliotropism is a plant response to minimize surface exposure to the sun. By orienting leaves and other plant organs parallel to the sun’s rays, light absorption is minimized. It is a process that some plants use to reduce dehydration by reducing leaf temperatures and water loss during times of drought.
Solar Tracking
Alfalfa, cotton, soybean, bean, and some wild species of the mallow family Malvaceae are plant species that exhibit solar tracking. Heliotropic plants change the orientation of their leaves toward the sun. This solar tracking mechanism can occur as a continuous adjustment throughout the day so that the leaf blades are always oriented perpendicular to the sun’s rays. The leaves are in a nearly vertical position, facing the eastern horizon as the sun rises. During the morning and later afternoon, when solar radiation is not at its most intense, the leaves move to a horizontal orientation. When there is increased solar radiation near midday, the leaves move to become more vertical so that they are not damaged by overheating. At sunset, the leaves are nearly vertical, facing the west when the sun sets. During the night, the leaves assume a horizontal position and reorient just before dawn, to start the solar tracking cycle over again. Leaves only track the sun on clear days. They stop when clouds block the sun.
Mechanisms
In many cases, the leaves of solar tracking plants are controlled by a specialized organ called the pulvinus. This organ is a swollen part of the petiole that may occur where it joins the stem, the leaf blade, or both. It contains motor cells that generate mechanical forces that control the orientation of the petiole and thus the leaf blade. The forces are produced by changes in the turgor in the pulvinus. The cells of this organ have highly elastic cell walls that allow them readily to change size and shape. The cells of the upper pulvinus have the capability of increasing their turgidity with water uptake, while the lower pulvinus can lose water very easily. The net effect is a force that moves the petiole.
Another mechanism producing heliotropism is produced by small mechanical changes along the length of the petiole and by movements of the younger parts of the stem. Heliotropic plants are continuously moving their leaves, leaflets, and pinnules to readjust to prevailing light conditions. Movements occur rather rapidly, every fifteen to sixty seconds, which is just slow enough to be imperceptible to most humans.
Bibliography
Hart, James Watnell. Plant Tropisms and Other Growth Movements. Boston: Unwin Hyman, 1990. This text focuses specifically on the adaptations of plants to effect movement in response to various environmental stimuli. Bibliography, indexes, illustrations.
Haupt, W., and M. E. Feinleib, eds. The Physiology of Movements. New York: Springer-Verlag, 1979. This college-level textbook provides much detailed information about tropisms. Includes an index, illustrations, and extensive bibliography.
Nobel, Park S. Physicochemical and Environmental Plant Physiology. 2d ed. San Diego: Academic Press, 1999. Discusses plant chemistry, physiology, cells and tissues, and tropisms. Includes bibliographical references, appendices, and index.
Satter, R. L., and A. W. Galston. “Mechanisms of Control of Leaf Movements.” Annual Review of Plant Physiology 32 (1981): 83-103. An excellent description of how leaves move. Summarizes much research and includes an extensive bibliography. Includes few illustrations.