With their signature yellow petals that ignite gardens and their skillful ability to attract pollinators, sunflowers are an integral part of any habitat for plants. However, sunflowers are not always easily understood. After nearly six years of research, a group of scientists, including UC Berkeley faculty members, revealed the answer to a long-standing question: Why do sunflowers face east?
Working with researchers from UC Davis’ Harmer Lab and the University of Virginia, campus scientists were able to learn more about how pollinators and sunflowers work together. Published July 27, the study found that sunflowers face east because the sun’s heat attracts pollinator visitation in the morning.
Among those who contributed to the report were campus associate professor Benjamin Blackman and two members of his lab: Daniel Yang, a former campus undergraduate researcher, and Srinidhi Holalu, postdoctoral researcher and assistant project scientist.
To discover these results, the study involved work with timing, heat, seeds and pollen, as well as a multitude of tests, such as paternity tests. Different experiments were also conducted with wild sunflowers, which yielded similar results.
The study found that temperature is a key factor in determining the growth of sunflowers.
“East-facing plants warm up the (sunflower) heads more quickly than west-facing plants,” Blackman said. “That raises two questions for us. The first is, what is temperature doing that is attracting those pollinators? Is it just that pollinators are getting some heat reward?”
To answer this, Blackman noted that higher heat leads to higher pollination. Because the sun provides heat, the sunflower seeds and plants face east to attract greater pollination in the morning and optimize repopulating effects for the plants, according to Blackman.
He added that an undergraduate student in his lab, Evan Brown, sparked the discovery of sunflowers’ directionality.
Blackman also discussed the importance of timing for the flowers as they develop.
“Small changes in the timing of flower development that impact how soon the morning plant can get its pollen to another plant has a measurable effect on what genes get transferred to the next generation,” Blackman said.
The scientists will continue to study sunflowers to uncover more information about them, according to Blackman, as they are interested in the “daily rhythms” of the sunflower heads. Even in the dark, these rhythms persist, which raises questions about the coordination between temperature and internal rhythms that control pollen presentation for the plants, Blackman added.
“One of the things that I’m proud of about this study is that it benefited greatly from collaboration between our group and the Harmer group, and that collaboration led itself to replication across multiple years,” Blackman said. “It convinced us that what we were seeing was really valid.”
