Autumn has arrived, and the sunflowers in the park are about to enter their prime viewing season.

Most of us have heard since childhood that sunflower heads turn towards the sun—it's as unquestionable as their tendency to produce seeds. However, in everyday life or some photographic works, some people may notice that not all sunflowers constantly face the sun.

Occasionally, individual "rebellious" sunflowers may even turn their backs to the sun, or an entire patch of sunflowers may seem to "ignore" the sun altogether.

So, why do these "rebellious" sunflowers turn away from the sun?

To understand why these "rebellious" sunflowers deviate from facing the sun, we first need to understand why sunflower heads turn with the sun.

As early as the late 18th century, scientists began studying this phenomenon. Botanists found through experiments that the movement of sunflower heads is primarily due to two substances within them—auxin and auxin oxidase.

Auxin is distributed in the stem beneath the sunflower head and stimulates rapid cell division and reproduction, promoting the growth and development of sunflowers. However, auxin itself exhibits negative phototropism—it is inhibited by light.

As a result, cells on the side of the stem beneath the sunflower head that is away from the light elongate faster. This disparity in growth rates between the light and shaded sides causes the stem of the sunflower to bend towards the light, making the flower head lean towards the sun.

On the other hand, auxin oxidase functions oppositely—it inhibits plant cell growth and exhibits positive phototropism. Therefore, auxin oxidase accumulates on the side of the sunflower head facing the sun, causing the stem to bend towards the sun.

The sunflower head continuously changes direction in response to the shifting light, driven by the changing activity levels of auxin and auxin oxidase as the sun moves. By noon, the sunflower is upright, but by afternoon, its stem bends westward.

However, measurements by botanists revealed that sunflower heads do not follow the sun in real-time; they lag by approximately 12 degrees, equivalent to a difference in movement time of 48 minutes.

After the sun sets and darkness falls, the influence of light gradually diminishes, causing auxin and auxin oxidase within the sunflower to redistribute. Consequently, the sunflower head gradually turns back, and the bent stem straightens once again.

According to observations by researchers, typically around 3 a.m., sunflowers turn back towards the east, ready to receive sunlight for the next day's photosynthesis.

So, why do some sunflowers stop following the sun's movement? Is it because they are no longer influenced by these two substances?

Some may have already guessed it. For a sunflower, from the budding stage to full bloom, its head does indeed track the sun. However, as the sunflower begins pollination and seed formation, its stem gradually ages, and the influence of auxin and auxin oxidase diminishes. Consequently, the degree to which the sunflower head moves with the sun decreases until it almost stops rotating.

At this stage, the sunflower head tends to mature, and as the weight of the fruit increases, its orientation is influenced more by gravity. Therefore, we often see mature sunflowers drooping.

Mature sunflowers typically face southeast because this orientation allows them to receive morning sunlight, aiding in drying the dew accumulated on the flower head overnight, reducing the risk of fungal infection, and slightly increasing the temperature of the flower head to attract pollinating insects.

Additionally, sunflower pollen is sensitive to high temperatures; exposure to temperatures above 30 degrees Celsius can cause damage. Facing southeast effectively shields the pollen from direct intense sunlight at noon.

In summary, sunflowers turn towards the sun out of growth necessity, while their refusal to do so signifies their maturity.