Do sunflowers really follow the Sun?
Sunflowers (Helianthus annuus) are so named not just for their sunny flower heads. Young sunflowers exhibit heliotropism, meaning that they track the Sun’s movement across the sky. At dawn the plants face east. During the day their stems gradually bend so that their leaves and developing flower buds follow the Sun westward. Overnight, they turn back toward the east, ready for sunrise.
This daily movement is driven partly by the sunflower’s internal circadian clock, which coordinates growth on different sides of the stem. During the day cells on the east side of the stem grow faster, causing the plant to bend westward. At night growth is greater on the west side, bending the plant back toward the east.
Following the Sun appears to benefit young plants by increasing their exposure to light for photosynthesis and promoting growth. In experiments plants that are rigidly staked to prevent their movement or that are grown in pots and turned out of sync with the Sun grow less vigorously and have fewer or smaller leaves. But this sun-tracking phase does not last forever. As the plant matures and begins to flower, its daily movement gradually decreases and eventually stops. Mature flower heads are generally fixed facing east, toward the morning Sun.
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This stationary orientation also has benefits. Researchers have found that east-facing flowers warm more quickly in the morning and thus attract many more pollinating insects than west-facing flowers. They also tend to release pollen earlier in the day, when more pollinators are active. Eastward orientation may even improve reproductive success: East-facing flowers have been found to produce larger, heavier seeds.
