The daily movement of a young sunflower head, which tracks the sun across the sky, is known as heliotropism. Derived from the Greek words for “sun” (helios) and “turning” (tropos), this phenomenon defines the plant’s relationship with its primary energy source. Young sunflowers begin the day facing east, follow the sun westward, and return to an easterly orientation overnight. This solar tracking allows the plant to maximize light capture throughout its growth phase, increasing its overall biomass and leaf area.
The Science Behind the Movement
The physical mechanism enabling the stem to bend is called differential growth, where cells on one side of the stem elongate faster than those on the other. This uneven growth is directed by the redistribution of the plant hormone auxin. Auxin molecules are sensitive to light and migrate away from the sun’s direct rays to the shaded side of the stem.
Once concentrated on the darker side, increased auxin stimulates those cells to expand more rapidly than the cells on the sunlit side. This disparity in cell elongation causes the stem to curve toward the light source, pulling the flower head westward during the day. As the sun sets, the plant reverses the process, with the west side of the stem growing longer overnight to reset the flower head toward the east.
The Sun-Tracking Schedule
The sunflower’s ability to track the sun is regulated by an internal biological timekeeper known as a circadian rhythm, not a simple reaction to light. This internal clock helps the plant anticipate the sun’s movement, allowing it to start its westward turn precisely at dawn. Researchers demonstrated this rhythm by moving plants indoors under constant light, where the sunflowers continued their east-to-west-to-east movement for several days.
The circadian clock regulates the timing of differential growth, ensuring the stem grows most rapidly on the east side in the morning and on the west side overnight. This internal programming is synchronized by the solar cycle, providing a consistent 24-hour schedule for solar tracking. The internal timing ensures the flower head is oriented for maximum light interception, which supports photosynthesis and overall growth.
Why Mature Sunflowers Stop Moving
Sun-tracking ceases once the sunflower reaches full maturity, a stage known as anthesis, when the flower has fully opened. At this point, the stem stiffens, and the differential growth that powered the daily movement halts entirely. The mature flower head becomes fixed in an easterly direction, which serves a reproductive advantage.
This final eastern orientation allows the flower head to warm up more quickly in the morning, a significant factor in attracting early-rising pollinators. Studies show that a warm, east-facing sunflower can attract up to five times as many pollinating insects, such as bees, compared to a flower facing west. By maximizing its morning temperature, the sunflower optimizes reproductive success, shifting its priority from maximizing growth to maximizing pollination.