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Sunflower’s Circadian Clock

Thursday, March 30, 2023
filed under: Optimizing Plant Development/Yields

sf heads
Three views of a head undergoing normal development; you can clearly see the ring-like patterns on the outside of the disk and the spiral patterns in the center. You can also see that the rings appear because all the florets in each ring develop at the same time. (See how much longer the white ovaries are in the florets labeled ‘PW2’ than those in ‘PW3’ in the photo on the right.) Photos Courtesy of Stacey Harmer

If you ask Stacey Harmer, time really does fly when you’re doing work about which you are passionate.
        For the past 25 years, the professor of plant biology at the University of California-Davis College of Biological Sciences has been studying plants and their clocks.
        “Pretty much every aspect of plant growth and development is somehow affected by their internal clocks,” Harmer says.  “I think it’s really crucial for them. If you think about it, it’s obvious.  Plants are stuck in one place.  They can’t run away from the cold or put on a jacket. They have to ramp up their own metabolism to deal with it.”
        Her team’s latest research focuses on sunflower – specifically on the circadian clock that controls sunflower blooms.
        “We got interested in sunflower because [it] had this amazing solar tracking behavior.  And what a lot of people don't realize is that before they start blooming, they bend from east to west during the day, and then at night they bend back again,” Hamer explains.
        She and her team wondered how sunflower knew to follow the sun, and how do the plants know where and when the sun will rise?  Their research quickly showed the sunflower plants use their circadian clock to guide them and to help them reorient at night.
        From there, their curiosity about sunflower in general was sparked.  Before they knew it, they were studying the plants from start to finish.
        “I was fascinated by the beautiful spiral patterns on the sunflower’s disk,” Harmer says.  “If you look at a young head before it’s gone through maturation, you see that beautiful spiral.”
2 sf heads
This photo shows a normal head (left) and one grown in constant light (right).  The constant light condition messes up the circadian clock, and now development occurs gradually across the disk, not in rings.  Also, this condition prevents anther filament elongation and makes the heads male sterile.

        A sunflower head is made up of hundreds of tiny florets, the youngest in the center and the oldest on the outside. In young plants, florets are specified in a spiral pattern, from the rim of the inflorescence disk into its center.
        “That happens really early in development: the pattern is set when the plant is as young as four weeks old,” Harmer observes.
        Harmer also noticed that just before pollination, in the last stage of development, the pattern on the head changed from spirals to rings.  She wondered about that: what caused it, where it came from, and why it mattered.
        What she learned is that in the last stage of maturation, the male and female parts of the flower take turns opening. One day, the male parts of the flower open and present pollen.  Bees pick up that pollen and spread it to other flowers. Then the next day, the female stigma unfold to receive pollen.
        “Sunflower [plants] are really good crossers,” Harmer says.  “We got really interested in the timing of all of this because this proved that timing matters in a couple of ways.  The first is in separating male development and female development so that you’re promoting outcrossing, which is a really good idea for wild plants.  The second thing we learned is that timing matters when flowers release pollen, and that will influence how good they are at spreading their pollen around.”
stacy harmer
Stacy Harmer
Harmer found that if she manipulated the plants, they released their pollen a couple of hours later, which led to fewer offspring. She and her team found that the plant’s circadian clock controls the opening of florets.  When the clock is disrupted, the florets don’t open in concentric rings, but rather in the spiral patterns seen early in development, starting at the outside of the disk and working inward.  Those plants then attract fewer pollinators.
        Now, Harmer is working to understand the genetic pathways that control those processes.  She says if they can figure out what the key themes are that control the timing, they might be on to new ways to either make male sterile plants or make flowers better targets for bees.  But she adds, just being more aware of plants’ timing and matching the timing of these events with the environment can help maximize pollination efficiency.
        Harmer hopes to finish her research in the next year and publish her results on the genetic pathways that control the timing events. — Jody Kerzman              
        More details on the UC-Davis plant biology team’s work with sunflower and its circadian clock appear in the January 13 edition of eLife.  To view it, visit
        This site also includes a YouTube video showing the process of a sunflower plant head blooming and optimizing its usefulness for pollinators.
          “Understanding how the circadian clock and the environment affect flowering will help breeders develop cultivars that flower at the optimal times of day to promote pollination,” says UC-Davis’ Stacey Harmer.
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