Sunflowers actually face the sun as it rises in the east and follows it across the sky until it sets in the west. The flower head of a young sunflower tracks the arc of the sun as it moves across the sky, but the flower head of a mature sunflower typically faces east.
The researchers compared mature flowers facing east with those they turned to face west and found that the east-facing blooms attracted five times as many helpful pollinators.
A sunflower contains a growth hormone called auxin that is sensitive to sunlight. Therefore, when the sun shines, the auxin moves towards the shaded portion of the plant. Thus, making the flower head face the sun.
To maximize photosynthesis, sunflowers like many flowers follow a repetitive natural solar tracking behaviour, called heliotropism.
Therefore, a sunflower does follow the sun when it is young because of a growth hormone called auxin. The hormone-sensitive to sunlight moves to a shaded area of the flower when sunshine. Sunflowers follow a natural repetitive solar tracking movement.
However, as a sunflower plant matures, it behaves differently. According to the co-author of the study published in Science. “As the overall growth of the plant slows down gradually, the circadian rhythm ensures that the plant reacts more strongly to sunlight early in the morning than in the afternoon or evening. This is why a mature sunflower doesn’t move with the sun throughout the day; rather, it just faces east.”