Sunflowers use 'internal clock' to track the sun: study

Plant biologists have long documented the remarkable ability of sunflowers and other plants to follow the sun.

This behaviour, called heliotropism, has been observed for centuries, but a breakthrough study released Friday out of the University of California, Davis, is the first explain how and why sunflowers follow this east-to-west routine.

The research, led by UC Davis plant biology professor Stacey Harmer, found that sunflowers combine their ability to detect light with an “internal clock” to act on hormones in way that allows them to achieve maximum growth.

"Just like people, plants rely on the daily rhythms of day and night to function," said the director of the National Science Foundation's Plant Genome Research Program, Anne Sylvester. "Sunflowers, like solar panel arrays, follow the sun from east to west. These researchers tap into information in the sunflower genome to understand how and why sunflowers track the sun."

Why follow the sun?

Growing sunflowers start the days facing east then swing west, following the sun’s trajectory through the sky.

To find out more about why the flowers do this, the researchers used staked plants in pots to prevent them from following the sun.

The study, published in Friday’s Science journal, found that those flowers grew smaller and weaker than those that were able to follow the sun.

Also, by turning the potted plants around daily so that they were facing the wrong way, the researchers were able to interfere with their ability to track the sun.

That means a sunflower’s health relies heavily on its ability to maximize the amount of sunlight it receives.

The internal clock

But heliotropism, Harmer said, involves more than the sunflowers’ ability to detect where the sun is.

Part of the behaviour involves a 24-hour “internal clock,” also called a circadian cycle, which also explains the plants’ ability to “reset” back towards the east overnight in the absence of any sunlight.

"The plant anticipates the timing and the direction of dawn, and to me that looks like a reason to have a connection between the clock and the growth pathway," Harmer wrote in the study. "It's the first example of a plant's clock modulating growth in a natural environment, and having real repercussions for the plant.”

The study, funded by the National Science Foundation's Plant Genome Research Program, found that when flowers were placed indoors under a steady source of light, they continued to sway back and forth for several days -- further proof that the plants were following an internal, 24-hour clock, Harmer said.

When this artificial light was moved to one end of the room, the flowers resumed their back-and-forth routine. But when the 24-hour cycle of light was artificially extended into a 30-hour cycle, researchers observed that the plants were confused about which way to bend.

How sunflowers move

Next, the researchers had to explain how, in the absence of muscles, the plants actually achieving this swaying motion.

Plant biologist and co-author of the study Hagop Atamian put ink dots on the stems and filmed a time-lapse video of them to measure the changing distance between the dots.

He found that when plants were tracking the sun, the east side of the stem grew more rapidly than the west side. At night, the opposite happened, allowing the flower to swing the other way.

That means there are two growth mechanisms at work in the flower’s stem, the researchers said. One of them determines the plant’s growth rate, simply based on available sunlight. The other uses the flower’s circadian clock, causing the stem to grow more on one side than the other in order to achieve the plant’s east-west motion.

When the clock stops

Mature sunflowers eventually stop following this routine, the study found, spending the end of their lives dedicated to reproduction.

By always facing east, the flowers get a head start on heating up in the morning, thus attracting more pollinating species than surrounding plants, the researchers said.

By measuring the flowers’ temperature with an infrared camera, the study found east-facing sunflowers heated up more quickly in the morning, and attracted five times as many pollinating insects as the cooler west-facing flower.

Researchers were able to confirm the link between warmer flowers and pollinating insects because the west-facing flowers began to draw more pollinators when heated up artificially.

"Bees like warm flowers," Harmer said.