Sunflowers Sway to Summer's Rhythms
New findings add to evidence that plants are more animal-like than many might think.
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.
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.
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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.
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."
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. A plant growth-regulating hormone, called auxin, appears to be a key driver.
The "dance" to the sun cycle dramatically slows when the sunflower matures and its flowers open up. At that point, the plants stop moving during the day and settle down facing the sun in the east.
"Bees like warm flowers," Harmer said, adding that the buzzing pollinators are cold-blooded, so landing on a warm flower saves them energy and perhaps feels really good.
"Our other idea is that the morning warmth changes the flowers in a way to make them more appealing to insects, perhaps causing them to release more attractive scents earlier in the day," she said. "We're currently testing this idea."
Sunflowers actually are composite flowers since the head consists of hundreds of small florets. This makes them very showy and appealing to pollinators. As for the sunflower's bright yellow color – bees love the hue.
Steve Kay is the director of Convergent Biosciences at the University of Southern California. He told Discovery News that Harmer and her colleagues "have done an amazing job at bringing what was once a curiosity observed by Darwin and others -- the heliotropic (sun tracking) movements of plants -- into the modern world of molecular cell biology."
"Their studies clearly show that the sun tracking activity of juvenile phase sunflowers is controlled by their internal circadian clock, interacting with light perception mechanisms," he continued. "These movements not only follow the sun, but anticipate its rise in the east in the morning, and confer an adaptive advantage in two ways."
Anne Sylvester, director of the NSF's Plant Genome Research Program, points out that people also have internal circadian clocks.
"Just like people, plants rely on the daily rhythms of day and night to function," Sylvester said.
Harmer added that "plants are far 'smarter' than most people understand. We and others have found that they anticipate changes in the environment, remember the seasons that they've experienced, and are very skilled at integrating different types of information to control important life events like when to flower, etc."
While she does not credit sunflowers and other plants with thoughts, feelings or consciousness, she said, "I think of them as a different type of life that processes information in a very different way, and usually on a very different time scale, than animals."