Why Sunflowers Face East | UC Davis


Sunflowers face the rising sun because the increased morning heat attracts more bees and also helps plants reproduce more efficiently, according to a study by researchers at the University of California at Davis. The results were published on August 9 in New Phytologist.

“It’s quite striking that they’re facing east,” said Stacey Harmer, professor of plant biology at UC Davis’s College of Biological Sciences and lead author of the article. “It’s better for them to face east, as they produce more offspring.”

As the sunflowers grow, their heads turn back and forth to follow the sun during the day. Previous work from Harmer’s lab has shown that this tracking is controlled by the plant’s internal circadian clock.

But as the flower heads, or flower heads, mature and their stems become stiff and woody, this movement decreases until the flower heads are all facing the morning sun.

When postdoctoral researcher Nicky Creux changed the orientation of sunflowers by turning their pots upside down, she noticed that east-facing flower heads attracted many more bees, especially in the morning, than west-facing plants.

In a series of experiments, Creux, Harmer and their colleagues found that east-facing flower heads were significantly warmer in the morning than west-facing flower heads. This heat provides an energy benefit to foraging bees early in the morning, Harmer said. Direct sunlight also illuminates ultraviolet markings on flower petals which are visible to bees but not to human eyes.

Orientation affects pollen release and flower development

A sunflower is actually made up of hundreds, sometimes thousands, of individual flowers. These individual florets first develop on the outer edge of the flower head, forming characteristic spiral patterns.

The orientation of the plants also affected flower development and reproductive success. East-facing plants tended to produce larger, heavier seeds. They also released pollen earlier in the morning, coinciding with the bees’ visiting hours.

These effects appeared to be controlled by temperature at the flower head. When the researchers used a portable heater to warm the west-facing buds, they were able to achieve similar results to the east-facing flower heads.

Finally, Evan Brown, an undergraduate student supervised by Ben Blackman at the University of Virginia, took sterile male plants, which could produce seeds but not pollen, and surrounded them with normal east- or west-facing plants. Using genotyping, they were able to distinguish whether sterile male plants were pollinated by east- or west-facing plants. The team found that pollen from east-facing plants was responsible for more offspring than that from west-facing plants.

The work was supported by grants from the NSF and the US Department of Agriculture-National Institute of Food and Agriculture.

Additional authors on the paper are: Sana Saeed and Julin Maloof at UC Davis; Srinidhi Holalu and Daniel Yang, UC Berkeley; Evan Brown, Austin Garner, and C. Lane Scher, University of Virginia. Ben Blackman is now at UC Berkeley and Nicky Creux is now at the University of Pretoria, South Africa.

Bees visiting west-facing sunflowers (left) vs. east-facing sunflowers in the morning. Facing east allows sunflowers to warm up faster and affects floret development, promoting pollination and reproductive success.

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