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Why do sunflowers turn to face the morning sun?

Bees that pollinate the flowers are up early looking for food.

Sunflowers in California were part of an experiment at the University of California at Davis on why the flowers follow the sun until they mature, then they face only east. (Kathy Keatley Garvey)

Plant a field of sunflowers, and watch them grow. All summer you will see something amazing. In the morning, all the sunflowers will face east, the direction of the rising sun. As the sun moves, so will the sunflower heads to follow it. But as summer turns to autumn and the sunflowers get big and ripe and heavy with seeds, they will stop moving. Until they are harvested, they will face only east.

This has to do with the changing needs of the sunflowers as they grow, said Stacey Harmer. She’s a plant biologist at the University of California at Davis.

“When they’re holding their seed heads, the stems have to become super-rigid and reinforced to support their weight, to stop them from falling over,” she says. To do that, “They have to make lignin, which is the material in tree trunks” that makes them stiff.

Harmer and members of her lab had already discovered that bees prefer flowers that face east. The researchers wondered whether there was any other advantage to the flowers facing in the same direction.

To find out, they planted genetically identical sunflowers and let some face east when they were mature. They turned the pots of other sunflowers 180 degrees, so they were forced to face west. Bees swarmed to the east-facing flowers and ignored the west-facing ones. The researchers found that the east-facing flowers also had heavier seeds than the west-facing ones.

“That was an important finding, because it means the mother plants put more resources into the east-facing flowers,” Harmer says.

Another experiment showed that the east-facing flowers made more seeds than the west-facing flowers. “We thought that was a pretty super result — a real-world consequence of the [physical] differences we saw,” Harmer said.

She points out that sunflowers release pollen in the morning, which is also appealing to the bees that are foraging at that time.

“They collect it like crazy, and I think . . . they want to be the first bee on the spot to collect that pollen.”

You can witness sunflowers “working” to attract bees if you watch them for a little while. Harmer says that over weeks you can see their heads enlarge. When they are mature, you can see their pollen tubes pop out in the early morning and the pollen coming out of the tubes.

In the afternoon, you can see the stigmas come out, ready for the next morning’s visiting bees. Stigmas are the parts of a flower that collect pollen from bees as they move from flower to flower. This process, called cross-pollination, fertilizes sunflowers and helps them reproduce.

Harmer says her study isn’t just a cool look at bees and flowers. It gives important information to people who breed plants.

Breeders work to make plants “have resistance to [disease-causing organisms] or the ability to tolerate drought,” Harmer says. In selecting for those traits “they may not look at other traits, because it may not seem so important.

“But this study highlights the need to consider things like timing — you wouldn’t want to select for drought tolerance [and have] pollen released later in [the] morning,” she says. That’s because fewer bees might stop by at that time, and without them you might end up with fewer sunflowers.

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