heliotropism

Heliotropism is the diurnal motion of plant parts (flowers or leaves) in response to the direction of the sun.

Heliotropism was first described by Leonardo da Vinci (along with gravitropism) in his botanical studies. The term "heliotropism," though, was introduced in the early 1800s by A. P. de Candolle, for the growth of the stem tip towards light, which is now called phototropism. The term heliotropism is now used only for solar tracking.

Floral heliotropism

Heliotropic flowers (like the Alpine Buttercup, Ranunculus adoneus, an alpine plant) 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. 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 their 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.

Some solar tracking plants are not purely heliotropic: in those plants the change of orientation is an innate circadian motion triggered by light, which continues for one or more periods if the light cycle is interrupted.

Leaf heliotropism

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

External links

• Natural History article on heliotropic flowers
• Animation of Heliotropic Leaf Movements in Plants

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