86.7 F

Davis, California

Tuesday, May 21, 2024

Why do sunflowers face east?

A UC Davis study determined a close relationship between sunlight and the reproductive health of these plants


By ARYAMAN BHATIA — science@theaggie.org 


Earlier in the year, researchers at UC Davis published a paper in New Phytologist explaining why sunflowers face east. They have found that sunflowers face the rising sun because increased morning warmth attracts more bees and also helps the plants reproduce more efficiently. 

Effective insect pollination requires appropriate responses to internal and external environmental cues in both the plant and the pollinator,” the study reads.

As a sunflower is in its growing stages, it moves its head in the direction of the sunlight. This happens due to the plant’s internal circadian clock, as was found in a previous study by Stacey Harmer, a professor of plant biology and senior author of the paper. As a sunflower matures, its neck becomes stiffer, causing it to face only one direction. 

“It’s better for [sunflowers] to face east, as they produce more offspring,” Harmer said in an interview with UC Davis.

Postdoctoral researcher Nicky Creux found that when she placed some pots of plants facing east and some facing west, the east-facing plants attracted more bees than the west-facing plants. The reason for this, after conducting numerous experiments, was found to be that the east-facing plants were warmer than the west-facing plants. The higher temperature gave an energy boost to foraging bees in the morning, and direct sunlight also lit up ultraviolet markings on the flower petals that were visible to bees but not to human eyes.

The direction in which these sunflowers face, it turns out, plays a pivotal role in their growth and reproductive success. Sunflowers exhibit a fascinating tendency to orient themselves, much like the dynamic behavior of a sunflower’s head as it tracks the sun’s journey across the sky. East-facing sunflowers, as discovered through meticulous research, tend to produce seeds that are not only larger but also heavier. 

Furthermore, they release pollen earlier in the morning, perfectly aligning with the times when bees embark on their foraging missions. The driving force behind this directional preference lies in the temperature at the flower head. 

During an experiment, researchers employed a portable heater to warm up the west-facing sunflower heads. Remarkably, they achieved results akin to those observed in their east-facing counterparts.

Evan Brown, an undergraduate student at the University of Virginia, conducted experiments involving sterile male sunflower plants. These plants possessed the ability to produce seeds but were incapable of producing pollen. Brown placed these sterile male plants amidst normal sunflowers, some facing east and others facing west. 

According to an article from UC Davis, through genotyping, researchers discerned the origin of the pollen responsible for pollinating the sterile male plants. 

The team found that pollen from the east-facing plants was responsible for more offspring than that from west-facing plants,” the article reads.


Written by: Aryaman Bhatia — science@theaggie.org