Scientists from the University of California have determined that sunflower plants always track the sun‘s movement during the day, as they reorient towards the east as the night passes.

The study, published in Science, reveals that sunflowers possess a circadian rhythm, also known as sunflower heliotropism, which allows them to undergo biological processes according to the perceived stimuli from the environment. On daytime, sunflowers’ circadian rhythm tells its stems to allow the cells located on their eastern section to grow longer, which points the whole flower to the west. The same occurs at night for the cells on the western section of the stem, allowing the plant to point towards the east and face the sun as it rises on the horizon.

Sunflowers have the ability to reorient themselves towards the sunlight at day and night. Image credit: Black Thumb Gardener.

The internal clock of sunflowers

Although many plants have genes that mimic the sleep cycles of animals, this is the first time that changes in growth have been linked to a plant’s circadian rhythm. The sunflower’s sun-seeking behavior was compared to solar panel arrays, which follow the sun from east to west and then relocate in order to receive the day’s first rays of sunlight.

Scientists performed experiments to assess the sunflower’s unique features. They turned potted plants in different directions, thus forcing the flowers to face the wrong direction, and also fixated some of the plants so it was impossible for them to move. The research team saw that altered sunflowers had smaller leaves and thinner stems when compared to control specimens.

Another phase of experimentation had the flowers isolated in an indoor growth environment with a single overhead light, directly above the plant. The sunflowers swung on different directions as days passed. According to lead researcher Stacey Harmer, this evidences that the flower’s patterns are dictated by some sort of internal clock.

When the overhead light source was moved several times throughout the day, simulating the day-night cycle, the plants started to aim towards it. The researchers tried to elongate the artificial day, but the flowers’ tracking patterns were not reliable after the 30th hour.

It was also determined that sunflowers facing the sun become heated more quickly, which in turn attracts a larger amount of insects, promoting pollination. Harmer suggested that pollinating insects, such as bees, are more likely to go after warm flowers.

Humans also have a circadian rhythm. On a regular schedule, humans tend to have a high alertness level from 10 a.m. to 3 p.m., while the best physical performance has been determined to occur around 5 p.m.

At 7 p.m. the body perceives its highest temperature throughout the day and at 9 p.m. the secretion of melatonin starts, which in turn promotes sleep. A new day starts at around 7 a.m. when the secretion of melatonin stops.

Our circadian rhythm can be modified according to the body’s exposure to external stimuli. One of the most notable ways of altering the circadian rhythm is by traveling across time zones, which produces a sensation known as jet lag. Sunflowers, on the other hand, rely on a more primitive version of the same mechanism. But the fact that a plant is able to alter its growth direction to always face its primary energy source is something to admire.

Source: Science