Sunflowers are known for their distinctive habit of turning to follow the sun. This behavior, called heliotropism, is observed in young sunflowers as they track the sun’s path across the sky. Mature sunflowers, however, do not exhibit this daily movement; instead, they typically remain fixed, facing east.
Young Sunflowers and Their Daily Dance
Young sunflowers track the sun from east to west throughout the day. This movement is regulated by the plant’s internal circadian clock, allowing it to anticipate the sun’s position. At night, the sunflower reorients itself, returning to an eastward facing position for the next sunrise.
The mechanism behind this tracking involves differential growth within the sunflower’s stem. During the day, the eastern side of the stem grows faster, causing the plant to bend westward and follow the sun. As evening approaches, growth on the western side increases, prompting the head to swing back towards the east overnight. This uneven growth is controlled by plant hormones called auxins. These hormones accumulate on the shaded side of the stem, promoting cell elongation and bending the stem towards the light source.
The Purpose Behind the Movement
The heliotropic movement of young sunflowers provides several benefits for their growth. By continuously facing the sun, sunflowers maximize sunlight absorption throughout the day. This increased exposure is important for photosynthesis, the process by which plants convert light energy into chemical energy. Studies indicate solar tracking can increase light interception by 10%, contributing to improved growth rates.
Tracking the sun also allows the sunflower’s head to warm more quickly in the morning. This warmth enhances metabolic activity and attracts pollinators like bees.
The Eastward Stand of Mature Sunflowers
As sunflowers mature and begin to flower, they cease daily sun-tracking movements and settle into an eastward orientation. This change occurs because the growth patterns facilitating heliotropism slow, and the stem becomes more rigid. The mature flower head remains fixed in one direction.
This eastward facing position offers distinct advantages for the mature plant. Facing east allows the flower head to warm rapidly in the morning sun, attracting pollinators like bees earlier in the day. Research shows east-facing sunflowers attract a significantly higher number of pollinators, up to five times more, compared to those facing other directions. The warmth also helps in quicker drying of morning dew, which may prevent fungal diseases on the flower head. This eastward orientation contributes to the plant’s reproductive success, leading to more plentiful and heavier seeds.