Mature sunflowers face east, while younger blooms are more flexible, changing their position to face the sun as it moves across the sky.
Now, a new study provides insight into why.
It turns out bees are attracted to the warmth and light of the morning sun, and more visits from bees result in healthier and happier blooms.
For their study, a research team led by UC Davis biologist Nicky Creux turned some sunflowers westward and kept the rest facing east to see what would happen. Bees favoured the east-facing sunflowers in the morning, but during the rest of the day, showed no preference.
The eastward flowers produced more seeds and had more successful offspring. They also started releasing their pollen about 30 minutes earlier than their westerly counterparts, suggesting the morning visits provide a significant competitive advantage.
Created by Cheryl Santa Maria using public domain vectors and images. Sunflower photo courtesy: FIR0002/Wikipedia GFDL 1.2
Previous research by the same team found the 'tracking' exhibited by younger flowers helps with growth and attracting pollinators. But during that 2016 study, scientists couldn't figure out why blooms eventually stiffened to face east, kickstarting the most recent paper.
Further analysis hints it's associated with the temperature of the capitulum - the sunflower's 'head,' which is composed of tiny florets. As the sun warms this part of the flower, it triggers the release of pollen, something that was also found to be true when west-facing blooms were warmed artificially.
The full paper can be found in the journal New Phytologist.