12 April 2017

Phototropism - updated

This past week I had the distinct pleasure of visiting Pope Farm Conservancy in Middleton, Wisconsin.
Pope Farm Conservancy is 105 acres that sits on top of three recessional moraines in the Town of Middleton, Wisconsin, where three different watersheds come together.

Six different Prairie Restoration projects and seven different crops including a field of sunflowers provide tremendous synergy that attracts wildlife to the conservancy. Forty interpretive signs follow the historical aspects of the land. They start with the Glaciers and land formation, followed by the Native Americans, settlers, the CCC project in the 1930’s, to today’s methods of erosion control.

All of this, combined with eight miles of walking trails and picnic areas provide the visitor with an unforgettable experience.
I chose to visit now because of the spectacular bloom a half a million sunflowers (and I wasn't the only one...).

I'll use this occasion to raise a question about phototropism.   These sunflowers were all facing east when I photographed them in mid-morning.  During the day they will track the sun as it moves across the sky.  But what happens during the night?  Are they programmed to turn back toward the east in anticipation of the next sunrise, or does it require the first beams of morning light to trigger them to swivel back toward the east again?

I could probably look this up, but I need to do something else now, and I suspect some reader with knowledge of plant biology will be able to supply the answer for us.

Addendum:  best answers so far are in the link offered by Kyle Michelli and in the video link provided by Zhoen in the comments.

Addendum #2:  Two tips of the blogging cap to readers Rob from Amersfoort and to Patty, both of whom back in August (I'm a bit behind in my blogging), provided links to the research of Stacy Harmer at UC Davis, who has studied phototropism in sunflowers.

As the plants grew from young seedlings into mature, yellow-headed adults, the researchers found that the sun-tracking movements of the plant became less and less noticeable, until they stopped altogether.

“A really common misconception is that mature sunflowers follow the sun. Actually, they do not,” Harmer said. “Mature sunflowers always face east.”...

Using a time-lapse camera, they were able to see that the east side of the stem grew longer during the day, turning the plant’s head to the west. At night, the reverse was true — the west side elongated, causing the plant to face the east...

The scientists report that even when the plants were grown under constant, fixed overhead lighting, they maintained the same head-turning rhythms they displayed in the field for several days... these results suggest that the sunflowers’ movements are regulated by something other than simple growth toward the sun. Some kind of circadian clock was also controlling the plants’ twists and turns...

The authors found that east-facing sunflowers attract up to five times the number of pollinators compared with those that were rotated in their pots so that they were facing west. Yet another experiment showed that this is almost certainly because east-facing sunflowers are more effectively warmed by the morning sun than sunflowers that are facing west.
More at the Los Angeles Times.  Fascinating research.


  1. I find the sunflower-following-the-sun story to be too simplistic. Look at your last picture, with the sunflowers facing you (in the direction of where you took the photo) and where the sun must be in order to create shadows leaning to the right of the photo (best observed with the umbrella man @ 2:00). I've driven around Sacramento here in CA around sunset and have observed the massive fields of sunflowers are facing almost the exact opposite direction of the setting sun.

    1. At our latitude, the morning sun is in the southeast, not the true east. These sunflowers are facing mostly east. In the topmost photo the two that are turned slightly to the left (southeast) have full sun on their "faces." The ones facing the camera directly get some shadowing of their faces from their southmost petals.

  2. There might be a prefered hibernation position. I worked on a solar panel tracker and we had to park the solar panels at extreme angles (as far from zeneth as possible), to avoid excessive condensation on the mirrors (as pointing up at the sky is the coldest) and to let any condensation that did form roll off the mirrors due to gravity. Flowers may not want to face up at night to avoid frost.

  3. "The flower head of a young sunflower tracks the arc of the sun as it moves across the sky, but the flower head of a mature sunflower typically faces east."
    "The sunflower doesn't stop tracking the sun after the sun dips below the horizon. The sunflower's bloom continues to move until it faces east, put the flower head in position to catch the sun's morning rays. Sometimes, the flower faces east before the sun fades completely in evening."

  4. The heads don't move once they've bloomed, way too heavy for a start. Ours tend to face east, but not all. Seems like it depends on where the sun is when they open.


  5. There is a plant hormone called auxin that causes phototropism. There are some neat simple experiments around how it works, starting with ones Darwin did.



  6. Here north of 60, where the sun hardly sets in midsummer, a favorite tall tale is to say that sunflowers track the sun around & around themselves until their blooms twist off. (in reality they do just fine)

  7. sorry for the topic drift, but... what do they do with all those sunflowers?


    1. I think sunflower seeds are used in oil and in bird seed.

      They're also a health snack with vitamin E and selenium.


  8. The state DOT here in the NC mountains is planting huge beds along highways. Rte-74 in Rutherford County has some large beds and they are really great. Alas most Millennial dopes will be too busy talking, texting, sipping a Starbuck's Latte or preening while going 90mph in a 65mph zone to even notice them.

    Happily, I'm 74 with severe Obsessive Compulsive Disorder so try to count every single flower in the 5,000 flower beds.

    1. Please tell me you're a passenger, not the driver.

  9. It was in the news yesterday: Using a time-lapse camera, they were able to see that the east side of the stem grew longer during the day, turning the plant’s head to the west. At night, the reverse was true — the west side elongated, causing the plant to face the east. http://www.latimes.com/science/sciencenow/la-sci-sn-sunflowers-direction-20160804-snap-story.html

    When I was young I was very impressed by the extremely large sunflower fields in Southern France, very impressive! Where I live you can also buy them in a small format for indoor use, or you could plant a few seeds in area 51!

    1. Thank you, Rob. I've bookmarked your link for an addendum after I get caught up on other things. It seems to answer the questions I raised.

  10. Due to insomnia, I am often listening to the BBC overnight (I am in NC). They had an extended interview with Stacy Harmer, of the University of California Davis, on this very subject that I heard two nights ago. Some of it I knew from my botany courses long ago, some of it was new to me.

    Here is the link to a small excerpt:

    And here is the entire 30 min. of Science in Action, which contains the longer interview:

    1. Thank you, Patty, for the link. I've subscribed to the podcasts. :.)

  11. If a sunflower is potted and you rotate it 180 degrees, will it resync with the sun?


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