Why Do Sunflowers Face The Sun?

Blooming sunflowers always track the sun, but they stop once they reach maturity. The flower exhibits heliotropism, the ability of the flower to follow the sun while it arcs from east to west all across the sky.

Sunflowers make optimum use of the sunlight during their blooming phase, to catalyze the growth rate and the process of photosynthesis. The floral parts of the sunflower exhibit heliotropism which is essentially controlled by its motor cells placed just underneath the head of the flower. These cells are responsible for moving ions of potassium to the adjacent cells - pushing potassium out and retracting in the sun, and drawing potassium in and expanding in the shade. The solar tracking phenomenon in sunflowers can be witnessed only in apical buds and flowers in their developing stage, and it ceases after the flower has fully bloomed.

It is necessary for sunflowers to absorb direct sunlight for six hours each day. The budding flowers may become spindly if required to recline or stretch to absorb uninterrupted sunlight. Even on days with cloud-cover, sunflowers display solar tracking movement, suggesting that the flowers follow a learned diurnal rhythm even when the intense directional light is absent. Several researchers over the years have studied heliotropism in sunflowers and have suggested that the solar tracking ability of the budding flowers follows circadian rhythms, implying that the flowers face the east at the time of dawn to greet the sun and slowly turns west following the sun’s trail. After sundown, the sunflowers slowly turn to the east to follow the same cycle again.

The growth rate of sunflowers on the eastern side is very low at night and high during the day.  On the other hand, the growth rate in the western side is higher after sundown and low during the daytime. Heliotropism makes the sunflower plants warm and it attracts more insects, thereby enabling increased pollination. The sunflowers do not just constitute pollen grains in their flowerheads, but also offer a source of food, sunflower seeds. Heliotropism ensures that the flowers face the sun during the daytime, enabling the flowers to dry out soon from any moisture that gathers at night, mitigating the chances of fungal infection. The overall growth of sunflowers slows down after the flower has bloomed completely and the plants stop their motion during the daytime and settle down facing the east.

Wouldn’t it be fascinating to observe heliotropism in a labyrinth of sunflowers?