Sunflowers Sway to Summer's Rhythms

Mature sunflowers are remarkable for their uniform eastward orientation.

Young sunflowers do what looks like a slow dance each day, turning and swaying of their own apparent volition, and now new research finds that these moves are driven by the sun, plant hormones and the sunflowers' internal clock.

This behavior of sunflowers was noticed way back in 1898, but the new study -- published in the journal Science -- is the first to explain in detail why it happens. The study is also the first to show that internal clock regulation of growth promotes overall plant yield, which in this case can lead to hefty, leafy tall sunflowers.

A field of sunflowers in Lopburi, Thailand.

The findings add to growing evidence that plants are more animal-like than most of us might think.

"Plants are exquisitely sensitive to the environment -- that is how they survive while stuck in one place -- and have senses very analogous to all the human senses," senior author Stacey Harmer, a professor in the University of California at Davis' Department of Plant Biology, told Discovery News. "For me, the big difference is the time scale of many of the responses."

"Plants actually have color vision," she continued. "They have several families of photoreceptors that allow them to see many different wavelengths of light: UV, blue, green, red, far-red … . Note that plants can see wavelengths of light that humans can't detect (UV and far-red). They use these photoreceptors, in particular, those that are sensitive to blue light, to track the sun."

Growing sunflowers "watch" the sun and move with it, beginning their days with their heads facing east, swinging west throughout the day, and turning back to the east at night.

For the study, funded by the National Science Foundation's Plant Genome Research Program, Harmer and postdocs Hagop Atamian and Nicky Creux joined forces with scientist Benjamin Blackman and his lab members Evan Brown and Austin Garner.

Atamian, collaborating with other members of the team, carried out a series of experiments on sunflowers in the field, in pots outdoors and in indoor growth chambers.

By staking plants so that they could not move, or turning potted plants around daily so that they were facing the wrong way, Atamian showed that he could disrupt their ability to track the sun. He also noticed that sunflowers prevented from moving were not as bulky and leafy as those that were free to move.

A sunflower just prior to bud opening.

When plants were moved into an indoor growth chamber with an immobile overhead light, they continued to swing back and forth for a few days, which Harmer said is what would be expected when behavior is driven by an internal clock.

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.

Next, Atamian put ink dots on some sunflower stems and filmed them. Using time-lapse video, he measured the changing distance between the dots and determined that when sunflowers track the sun, the east sides of their stems grew more rapidly than the west sides. At night, the west sides grew faster as the stem swung the other way.

Harmer explained that, as a result, "the back and forth rhythmic growth of sunflowers is due to asymmetrical growth on the opposite sides of the stem."

Read more at Discovery News