Sunflowers Use Internal Circadian Clock to Follow Sun

A team of scientists led by Prof. Stacey Harmer of the University of California, Davis, has discovered how sunflowers use their circadian clock, acting on growth hormones, to follow the Sun during the day as they grow.

Growing sunflowers begin the day with their heads facing east, swing west through the day, and turn back to the east at night.

“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,” Prof. Harmer said.

This behavior of sunflowers had been described by biologists as far back as 1898, but no one had previously thought to associate it with circadian rhythms.

“Before our experiments, few studies – the latest more than 50 years ago – had assessed how sunflowers returned at night, and they had suggested some internal ‘habit’ was involved but did not directly implicate the clock,” said co-author Dr. Benjamin Blackman, from the University of California, Berkeley.

The scientists carried out a series of experiments with sunflowers in the field, in pots outdoors and in indoor growth chambers.

They staked plants so that they couldn’t move or turned potted plants daily so that they were facing the wrong way.

They found that they could disrupt the ability to track the Sun, and documented that plants that follow the Sun get a growth boost.

Sunflowers staked so they can’t move have decreased biomass and less leaf area than those that do, the scientists found.

When plants were moved into an indoor growth chamber with immobile overhead light, they continued to swing back and forth for a few days.

Finally, the indoor plants did start tracking the Sun again when the apparent source of lighting was moved across the growth chamber by turning adjacent lights on and off during the day.

The plants could reliably track the movement and return at night when the artificial day was close to a 24-hour cycle, but not when it was closer to 30 hours.

The team identified a number of genes that were expressed at higher levels on the sunward side of the plant during the day, or on the other side at night.

Apparently, there are two growth mechanisms at work in the sunflower stem.

The first sets a basic rate of growth for the plant, based on available light. The second, controlled by the circadian clock and influenced by the direction of light, causes the stem to grow more on one side than another, and therefore sway east to west during the day.

“Sunflowers have linked their internal clock genes to stem growth, so that the eastern side of the stem elongates more than the western side during the day, turning the stem and flower westward to track the transiting Sun,” the scientists explained.

“At night, the western side grows faster, turning the flower head back east in time to capture the rays of the rising Sun.”

The team also investigated why mature sunflowers always face east, and found that east-facing sunflowers heated up more quickly in the morning – and also attracted five times as many pollinating insects.

“Bees like warm flowers,” Prof. Harmer said.

“Again, the circadian clock helps lead to the eastward orientation of sunflower heads when they stop tracking as they bloom,” Dr. Blackman added.

“I expect that future work in those systems will find parallel results to ours if the movement is mediated by differential growth.”

“The more general point, that one of the circadian clock’s adaptive functions is to regulate the timing and strength of growth responses to environmental signals, is one that I think will apply to a broad range of traits and species.”

The team’s findings were published August 5 in the journal Science.