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Heliotropism

What is Heliotropism

Heliotropism is a growth movement in plants that is induced by sunlight. It is sometimes called solar tracking, a directional response to the sun. Because plants react in a similar way toward artificial sources of light, heliotropism is sometimes termed phototropism, a growth movement induced by any light stimulus. We can trace heliotropism back to the Greek roots helios, “sun,” and tropos, “a turn or change.”

Heliotropism is an example of tropism. It is the orienting response to the source of sunlight (or the sun). It may be positive or negative. A positive heliotropism is a response towards the direction of the sun whereas a negative heliotropism is a response away from the direction of the sun. The term heliotropism was first used in 1832 by A. P. de Candolle, a Swiss botanist, to refer to the phenomenon in any plant.

It enhances the photosynthesis process and increases growth rates.

Examples

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.

How It Works

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.

The pulvinus is an amazing, fluid-controlled joint found at the base of a plant leaf stem (petiole) or just below a flower.

The pulvinus causes movement by altering fluid pressure in the surrounding plant tissue. These changes in fluid pressure start when sucrose is moved from the phloem into the apoplast. The apoplast is the conjoined spaces between plant cells. As sugar is pumped into the apoplast, potassium ions are pushed out, followed by water molecules. This changes the pressure within the affected cells, causing movement. This is called turgor-mediated heliotropism. But not all heliotropic flowers have a pulvinus. Those that do not are still able to move by permanently expanding individual cells. This is called growth-mediated heliotropism. Pulvini are also used in response to nyctinastic and thigmonastic movements.

In Flowers

Heliotropic flowers face the sun from dawn to dusk. Slowly tracking the sun’s path across the sky, these flowers are believed to use heliotropism as a way to improve pollination, fertilization, and seed development. Heliotropic flowers often have five times as many beneficial insects present, due to the added warmth. [Many tropical flowers exhibit a modified form of heliotropism in which flowers maintain an indirect tracking of the the sun. This is believed to reduce the chance of potential overheating.] Beans, alfalfa, sunflowers, and many other species turn their blooms to follow the sun’s path across the sky each day. But sunflowers only use heliotropism in their early development, in the bud stage. Once a sunflower head emerges, it may track the sun for a short time, as an expression of phototropism, until the flower head reaches full size. The majority of sunflowers found in the northern hemisphere nearly always end up facing east.

In Leaves

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.

Like floral heliotropism, leaf heliotropism is the method by which plants focus their leaves perpendicularly to the sun’s morning rays (diaheliotropism), or parallel to midday sun (paraheliotropism). Diaheliotropism allows leaves to capture the maximum amount of energy from the sun, while paraheliotropism protects plants from overheating and dehydrating.

Heliotropism vs Phototropism

The difference between phototropism and heliotropism is that phototropism is (biology) the movement of a plant towards or away from light while heliotropism is the property of some plants of turning under the influence of light; either positively (towards the light) or negatively (away from the light).

Article was last reviewed on Wednesday, May 11, 2022

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