Sunflower, belonging to the family Asteraceae, is a herbaceous plant widely admired as a symbol of brightness and hope.

Sunflower is an annual plant. Its stem is erect, reaching a height of 1-3 meters, stout, covered with coarse white hairs, unbranched, or sometimes with branches in the upper part.

The leaves are alternate, heart-shaped ovate or ovate, abruptly pointed or gradually pointed at the tip, with three primary veins, serrated margins, and covered with short, rough hairs on both sides, with long petioles.

The head inflorescence is enormous, about 10-30 centimeters in diameter, solitary at the apex or tip of the branch, often nodding. The involucre consists of many layers of leaflets, membranous, arranged in overlapping tiles, ovate to lanceolate, gradually pointed at the tip, covered with long stiff hairs or bristles.

In the morning, it smiles to welcome the rising sun in the east; at noon, it faces the midday sun; in the evening, it silently watches the setting sun in the west.

So, why does the sunflower turn towards the sun?

Let's analyze it.

Plants produce a miraculous hormone called auxin, which mainly accumulates in the growing parts, promoting growth but disliking sunlight. Therefore, auxin at the tip of the sunflower stem moves from the sunny side to the shady side, accumulating on the shaded side, causing it to grow faster, while the sunny side grows slower. The unequal growth rate on both sides of the stem causes it to bend toward the slower-growing side; hence, the plant turns towards the light source.

With the advancement of scientific technology, scientists have supplemented the explanation of the sunflower's heliotropism. Besides the difference in auxin concentration on both sides of the sunflower's stem apex, there is also a difference in the concentration of a growth-inhibiting substance called abscisic acid.

Unlike auxin, abscisic acid inhibits cell elongation and accumulates on the sunny side, causing the two sides of the stem to grow at different rates. Therefore, the uneven distribution of abscisic acid also contributes to the plant's phototropic rotation.

In addition to the combined action of auxin and abscisic acid, the sun-following behavior of sunflowers is also related to their sun-tracking characteristics. The apices of sunflowers and other plants rotate with the direction of sunlight, known as heliotropism.

Their leaves always remain perpendicular to the light, controlled by solutes in the motor cells of the pulvinus, a lateral movement of phototropism in plants. This lateral phototropism of the leaves causes them to strive to remain perpendicular to the sunlight, thereby driving the stem and inflorescence to rotate with the sun.

Thus, the sun-loving nature of sunflowers is the result of multiple factors combined. Still, their leaves and inflorescences do not immediately follow the sun during the day. Botanists have measured that the orientation of their inflorescences lags behind the sun by about 12 degrees or 48 minutes. After sunset, the sunflower's inflorescence gradually swings back, and around 3 a.m., it faces east again, awaiting the sunrise.

However, once the sunflower's inflorescence blooms, it no longer follows the sun but remains fixed facing east. Factors inhibiting the rotation include the increasing weight of the inflorescence and the approaching maturity, as the sunflower's stem tends to age, with less auxin content, and the growth rate on both sides of the stem is no longer significantly different. So why does the sunflower fix its orientation to the east?

This may result from natural selection, meeting the reproductive needs of sunflowers. Facing the sun in the early morning helps dry the dew collected overnight, reducing the risk of fungal infections.

In addition, when the temperature is low in the morning, direct sunlight can raise the temperature of the sunflower's inflorescence, creating a warm space that attracts insects for pollination. At the same time, facing eastward avoids direct sunlight at noon, preventing heat damage to the pollen.