quote:

from:
http://www.madsci.org/posts/archives/2001-02/982689431.Bt.r.html


Re: Why and how the sunflower can follow the movement of the sun?

This type of response is known as solar tracking and occurs in sunflower as well as many other types of plants. It does not have to involve the flower; in many types of plants the leaves are involved. Solar tracking is a type of tropism. Tropisms are responses plants have to an environmental signal.

The response depends upon the direction of the stimulus. The orientation of the sunflower to the sun is a direct result of the direction of light. The actual movement of the flower occurs as a result of motor cells in a structure called the pulvinus. The pulvinus is a region that is located where the flower head is connected to the stem. The motor cells get bigger
or smaller depending on how much water they absorb or lose. Potassium is involved in all this. If the motor cell has a lot of potassium more water will go into the cell and the cell will inflate. With less potassium, the cell loses water and deflates. Which cells inflate and deflate causes the flower head to move as the sun moves. At night when the sun sets the flower head will move and face the direction the sun should rise in the next morning. This happens because of the amount of water contained in the motor cells of the pulvinus.

Source
Salisbury and Ross. 1992. Plant Physiology, 4th edition. Wadsworth

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