When the sunflower plant, Helianthus annuus, is in the bud stage, the head and the leaves do indeed track the path of the Sun. The genus name Helianthus is from the Greek helios "sun" and anthos "flower."
Interestingly, however, and contrary to popular belief, once the massive topmost flower opens into the radiance of yellow petals, it slows and then stops moving, ending up permanently facing east.
This fixed orientation is thought to be an adaptive feature. Sunflower pollen gets damaged at temperatures greater than 30{+o}C, so by facing east all the time, the flower reduces the net radiation falling on its face at noon, keeping itself cool and promoting fertilisation and seed development.
It also increases the temperature of the flower head in the chilly early morning, attracting warmth-loving insect pollinators.
The heliotropic movements of the leaves and sunflower bud are the result of bending during rapid growth.
This is caused by build-up of the plant growth hormone auxin on the side of the stem opposite the Sun. The direction of the head lags behind the Sun's position by about 12 degrees, or a time of 48 minutes.
Source : The Hindu
