Growing in the direction of the light source is called PHOTOTROPISM. The mechanism has been worked out: Light receptor molecules (other than the ones for photosynthesis) affect certain hormones that regulate enzymes that control the rigidity of cellulose, causing stems to bend toward light. Totally cool!
Actually phototropism involves direction of GROWTH, but the ability of sunflowers to TRACK the sun during the day is actually known as HELIOTROPISM, which is different.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...The motion is performed by motor cells in a flexible segment just below the flower, called a pulvinus."
In Sunflower due to presence of auxin, a growth harmone in the stem. When the stem tip receives unilateral light, the concentration of auxin increases in the shaded side.The higher concentration of auxin in the shaded side causes that side to grow faster, resulting in a positive phototropic curvature.
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