Light from the sun provides the solar energy used by plants for photosynthesis. Heliotropism, or solar tracking, is when a plant follows the movement of the sun during the day. Rooted in ancient Greek, “helio” refers to the sun and “tropism” means a turning or movement of a living organism toward or away from an external stimulus, such as light, heat or gravity.
The sunflower (Helianthus annuus) is the best example of a plant that displays this phenomenon. Young sunflower plants follow the sun from east to west during the day and then, reorient themselves during the night to face east in anticipation of the sunrise.
Heliotropism optimizes light interception of young sunflower plants, increasing it by 10% or more. Increased light capture improves plant performance with more leaf area and increased biomass.
Sunflowers perform their daily dance from east to west by the coordinate action of two mechanisms. Light-signaling pathways set a basic rate of growth for the plant, based on available light. The apex of the plant is the most sensitive to light. The circadian or internal clock of the plant is influenced by the direction of light and causes the stem to grow more on one side than the other.
At the final stage of flower development, called anthesis, sunflowers conclude their solar tracking ways and turn their flowerheads eastward. These east-facing sunflowers heat up more quickly in the morning, making them more attractive to pollinating insects, such as honeybees.