An internal circadian clock controls the distinctive concentric rings of bloom in sunflowers, maximizing visits from pollinators, according to a new study.
A sunflower head is made up of hundreds of small florets. Due to the way sunflowers grow, the youngest florets are in the center of the flower face and the most mature ones are at the edges, forming a distinctive spiral pattern from center to edge.
An individual floret blooms in a couple of days. The first day the male part of the flower opens and presents pollen; on the second day, the female stigma unfolds to receive the pollen. Somehow, the florets are coordinated so that they open in concentric rings starting from the edge and moving inward on successive days, with a ring of female flowers always outside the pollen-bearing male flowers in the earlier stage.
Pollinating bees tend to land on the ray petals around a sunflower head and walk toward the center, says Stacey Harmer, professor of plant biology in the University of California, Davis, School of Biological Sciences and lead author of the paper. in eLife.
That means they will collect pollen after they have walked on the female florets and then carry it to a different flower head.
Harmer and postdoctoral researcher Carine Marshall wanted to understand how the spiral pattern of the florets develops into concentric rings of bloom. Harmer’s lab had previously established that circadian rhythms control how growing sunflowers follow the sun during the day.
The internal circadian clock of a plant or animal works on a roughly 24-hour cycle, allowing different genes to be activated at different times of the day. Natural day/night cycles keep this internal clock in sync with the actual time of day. Changing the length of daylight or darkness can reset the clock. In sunflowers, continuous light interrupts the clock entirely.
The researchers took time-lapse videos of sunflowers grown under different light/dark or temperature conditions. They found that the plant’s circadian clock controls the opening of the florets. When the clock was disrupted by growing plants in continuous light, the florets did not open in concentric rings, but only by age, starting at the edge and moving toward the center in a continuous gradient.
When plants that had been grown on an interrupted clock were moved outdoors, they attracted fewer pollinators than normal sunflowers.
“We think being able to coordinate in this way makes them a better target for bees,” says Harmer. “It’s a strategy to attract as many insects as possible.”
As farmers adapt to a changing climate, it will become increasingly important to make pollination as efficient as possible in crops that require it, says Harmer.
Understanding how the circadian clock and the environment affect flowering will help breeders develop cultivars that flower at optimal times of day to promote pollination, despite climate change and declining insect populations, he says.
The National Science Foundation and the US Department of Agriculture-National Institute of Food and Agriculture supported the work.
Font: University of California at Davis