Heliotropism

Floral heliotropism is the behavior of some plant species that their flowers track the sun's motion from east to west across the sky on sunny days. 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 the pulvinus, a flexible segment of the stem just below the flower. 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 snow buttercup 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 phototropia 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 leafs. Floral heliotropism is not necessarily exhibited by the same plants as leaf heliotropism.