East-facing sunflowers are happier, healthier, and more productive than those that face other directions – and it's all down to the warmth of the morning Sun.
That's the conclusion drawn by a new study that sought to find why the cheerful yellow blooms tend to mature facing east, in spite of a more flexible youth in which their orientation changes to face the moving Sun.
The greater warmth and light from the rising Sun attracts more bees in the morning, which results in better growth, earlier pollen production, more plentiful seeds, and higher reproductive success.
"It's quite striking that they face east," said plant biologist Stacey Harmer of the University of California Davis. "It's better for them to face east, as they produce more offspring."
As they grow, young sunflowers (Helianthus annuus) exhibit a particular behavior: The flower head, or capitulum, moves to track the Sun across the sky, as seen in the video below.
As the sunflower matures, however, its stem stiffens, significantly limiting movement. As this occurs, the capitulum settles into facing east.
Exactly why sunflowers did this remained a mystery to researchers in 2016, so another team led by UC Davis biologist Nicky Creux designed an experiment to find out.
One of the ways to figure out why something does the things it does is to change its situation so that it's no longer doing that thing. With east-facing sunflowers, this is incredibly simple: All you have to do is turn them around. That's exactly what the researchers did, taking measurements and comparing normal, east-facing controls with the turned, west-facing sunflowers.
The first thing the researchers noticed was that the flowers facing east attracted a significantly higher number of bees in the morning. During the rest of the day, the bees showed no preference, which suggests that that morning window could make a big difference.
The east-facing sunflowers also started releasing their pollen earlier in the morning, by about 30 minutes – timing which pretty neatly matched the time delay between peak pollinator visit times.
Follow-up analysis suggests that this all has to do with the temperature of the capitulum. The Sun warms the flower head, which triggers it to release pollen; when warmed artificially, the west-facing flowers showed similar pollen release behavior.
Interestingly, this artificial heating made no difference to pollinator visits, although it's not entirely clear why. The researchers believe that markings on the flowers visible to pollinators under UV light from the Sun may have something to do with it: On the east-facing sunflowers, the flowers were bright, and these markings were distinctly more visible.
The effect on the sunflowers was also remarkable. East-facing sunflowers produced more plentiful and heavier seeds than those facing west. And the pollen from east-facing sunflowers seemed more successful in producing offspring.
The researchers found this out by surrounding sunflowers that produced only seeds and no pollen, with both east- and west-facing sunflowers. Using genotyping, they then determined whether the seed plants were pollinated by east or west pollen. The east-facing sunflowers produced significantly more offspring than the west-facing ones.
All this suggests that, while facing east may not be absolutely key to a sunflower's success, it does provide enough of a boost to make it worthwhile.
"Our results demonstrate that the easterly orientation of mature sunflower capitula plays an important role in managing the floret microclimate and ensuring the correct conditions for anthesis, pollination, and seed development," the researchers wrote in their paper.
The research has been published in New Phytologist.