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Shining a light on the young sunflower’s east to west tracking of the sun

Fields of sunflowers bloom at Power Line Road and Garden Highway. UC Davis researchers recently reported the underlying biology of how sunflowers track the sun throughout the day, tilting from east to west.
Fields of sunflowers bloom at Power Line Road and Garden Highway. UC Davis researchers recently reported the underlying biology of how sunflowers track the sun throughout the day, tilting from east to west. aseng@sacbee.com

Summer drivers cruising along Interstate 5 through Yolo County are treated to breathtaking views of sunflowers growing by the side of the road. Many pull over to take an obligatory selfie, but these snapshots don’t capture the fascinating ability of young sunflowers to track the sun’s movement.

“It is a pretty sight when they’re all blooming,” said Steve Soares, a Woodland farmer who rents out land for growing sunflowers and volunteers occasionally as a photographer when travelers stop by his fields to admire the flowers.

Young sunflowers have been known to tilt their flower heads from east to west over the course of each day. Stacey Harmer, a UC Davis professor of plant biology, and her colleagues now have a better understanding of how sunflowers exhibit the solar tracking behavior known as heliotropism.

The research findings published this month in the journal Science identify the critical role of coordinated stem growth for heliotropism. During the day, the east side of a sunflower stem grows more quickly than the west side, causing flower heads to tilt from east to west. At night, the opposite happens as the stem’s west side grows more quickly than the east, reorienting flower heads to face east at dawn. After sunflowers bloom and reach maturity, their stem growth slows, which restricts flower head movement. Older sunflowers adopt a more permanent position facing east.

Studying dwarf sunflowers revealed the important role of a particular plant growth hormone, called gibberellin. Dwarf plants that lack gibberellin have short stems and no noticeable heliotropism. Administering gibberellin supplements to the dwarf plants restored stem growth as well as the more familiar east-to-west daily tilt.

Harmer’s team concluded flower movements are guided in part by internal circadian clocks. After moving sunflower pots from natural outdoor lighting to growth chambers with constant overhead lighting, they noticed sunflowers continued their east-to-west tilt for several days. The persistence of sunflower movement in lieu of light signals suggests sunflowers don’t simply respond to sunlight.

In another experiment, changing the light-dark cycle from a 24-hour to a 30-hour cycle caused erratic behavior and uncoordinated growth. These results suggest internal clocks work together with environmental sensing to regulate heliotropism.

Advantages of facing east are warmer flower head temperatures, which attracted five times more pollinators than sunflowers facing west. Portable heaters warming the west-facing sunflowers restored some of the pollinator visits, but the plants were still visited less than east-facing flowers.

The U.S. Department of Agriculture reports California farmers grew about 45,000 acres of sunflowers this year, an increase of more than 25 percent from 2015. Farmers growing sunflowers in the Sacramento Valley collect the seeds, the bulk of which are replanted in other parts of the United States and around the world, often for sunflower oil production.

Sean Doherty, 45, a farmer in the Dunnigan and Arbuckle area, has been growing sunflowers for nearly 20 years. His largest crops are rice and tomatoes, but he enjoys having sunflowers to diversify his portfolio. Sunflowers don’t require as much water to grow and are a good option for Sacramento Valley farmers looking to rotate their crops.

“It’s a really pretty crop to grow,” said Doherty. “It’s always nice to be able to drive by and take home the first flowers to the missus.”

Having a successful sunflower crop is not always easy. Bob Dettling, owner of Dettling Farms in Woodland, says timing is critical during the planting phase to ensure male and female flowers open at the same time for pollination.

The Sacramento Valley is the world’s primary supplier of hybrid sunflower seeds, made by crossing different strains of sunflowers. Planting different male seeds must be done over a mile’s distance apart to limit cross-pollination that can lead to off-types and reduce the yield of the hybrid of interest.

“There’s a lot of science behind it,” said Dettling, “and luck.”

The east-to-west tilt spans about 10 degrees of curvature throughout the day. This subtle shift may explain the conflicting reports among farmers concerning sunflower motion.

“When they’re bright and yellow, they move,” said Dettling. “I say they move a decent amount.”

But Doherty was less certain.

“I don’t know if it does,” said Doherty. “I’m not going to stick my neck out on it.”

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