heliotropism

n. tendency to turn or grow in the direction of sunlight

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.
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Noun
1. an orienting response to the sun
(hypernym) tropism

(n.)
The phenomenon of turning toward the light, seen in many leaves and flowers.
  

The tendency of plants to turn towards the sun