What is solar tracking in plants?

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.
photo tropism
Alfalfa,Sunflower, cotton, legumes including soybean and bean, and some wild species
of the Malvaceae are examples of the numerous species capable of solar
tracking. In many cases, leaf orientation is controlled by a specialized
organ called the pulvinus, found at the junction between the blade and the
petiole. The pulvinus has motor cells that change their osmotic potential
and generate mechanical forces that cause the blade dispacement. In other
plants, leaf orientation is controlled by small mechanical changes along the
length of the petiole and by movements of the younger parts of the stem.
Both phytochrome and a blue light sensitive pigment have been implicated as
photoreceptors in solar tracking.The mechanism for solar tracking invloves changes in hydrostatic pressure
induced by the absoption of blue light by photoreceptors in the leaf veins
or the pulvinus, perhaps via steps similar to those for stomatal opening
discussed in the next champter, leading to changes in leaf orientation.Heliotropism is the diurnal motion of plant parts (flowers or leaves) in response to the direction of the sun. Heliotropic flowers track the sun's motion across the sky from East to West. During the night, the flowers may assume a random orientation, while at dawn they turn again towards the East where the sun rises. This behavior is exhibited, for example, by the snow buttercup (Ranunculus adoneus), an alpine plant. The motion is performed by motor cells in a flexible segment just below the flower, called a pulvinus. The motor cells are specialized in pumping potassium ions into nearby tissues, changing the turgor pressure. The segment flexes because the motor cells at the shadow side elongate due to a "turgor rise". Heliotropism is a response to blue light. If at night a heliotropic species is covered with a red transparent cover that blocks blue light, the plant does not turn towards the sun next morning. In contrast, if it is covered with a blue transparent cover, the plant does track the sun.

In the past the term heliotropism was used for what is now called phototropism of the stem tip, which makes a plant grow towards the light. An important difference is that heliotropism in the current sense is reversible, elongated motor cells can resume their original size at night.

Leaf heliotropism is solar tracking behavior of plant leaves. Some plant species have leaves that orient themselves perpendicular to the sun's rays in the morning (diaheliotropism), and that orient themselves parallel to those rays at midday (paraheliotropism). Floral heliotropism is not necessarily exhibited by the same plants as leaf heliotropism.
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