You may have noticed that sunflowers can move their 'heads' or capitula to track the sun as its position changes during the day. But as the capitula mature, their stems start to get stiffer and less flexible, taking on a woody characteristic that reduces their movement. Eventually, the sunflower heads will all face the morning sun in the East. Researchers have now found that the warmth of the morning Sun helps sunflowers attract more bees and reproduce more efficiently. The findings have been reported in New Phytologist.
"It's quite striking that they face east," said senior study author Stacey Harmer, a professor of plant biology in the University of California Davis College of Biological Sciences. "It's better for them to face east, as they produce more offspring."
Previous research by Harmer's team has suggested that the circadian rhythm of sunflowers enables the plants to move with the Sun.
The researchers experimented with the biological impact of the orientation of sunflowers by potting them and simply moving them around. When postdoctoral scientist Nicky Creux turned the sunflowers' pots around, changing their orientation, she saw that the flower heads that faced east attracted a lot more bees than plants facing west, particularly in the morning.
Follow up work indicated that the flowers' heads that face east are warmer. Warmer flower heads lend a hand to foraging bees early in the morning by giving them an energy boost, said Harmer. Bees can also see the ultraviolet marks on sunflower petals that are visible to them in direct sunlight.
Plant orientation also had an influence on the development and reproduction of the sunflowers. Plants that were facing east generated bigger, heavier seeds and tended to release pollen earlier in the morning when bees visit.
The researchers wanted to see if these effects were due to temperature, and exposed the plants to portable heaters to warm the heads. They found that when west-facing heads were warmed with the heater, they began to behave more like east-facing heads.
Sunflowers have hundreds or even thousands of individual flowers, which start to develop at the edge of the head as florets, and form a spiral pattern. The large head of sunflowers are a composite flower, called an inflorescence, which is made up of two kinds of tiny florets. There are about 30 types of sunflowers.