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.
The phenomenon that is heliotropism was first described by Leonardo da Vinci (along with geotropism) in his botanical studies. The actual term "heliotropism," though, was introduced in the early 1800s by A. P. de Candolle, for the growth of the stem tip towards the light, which is now called phototropism. Now, however, the term heliotropism is used only for solar tracking, which is distinctly different from phototropism, or helio-directional growth: it is merely a temporary change of orientation, reverting in the dark of night.
Leaf heliotropism is the 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 others have those that orient themselves parallel to these rays at midday (paraheliotropism). Floral heliotropism is not necessarily exhibited by the same plants as leaf heliotropism.
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.
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