The internal circadian clock controls the distinctive concentric rings of inflorescences in sunflowers, increasing visits from pollinators, according to a new study from plant biologists at the University of California, Davis. The work was published on January 13 in eLife.
A sunflower head is made up of hundreds of tiny flowers. Because of the way sunflowers grow, the younger florets are in the center of the flower face and the more mature at the edges, forming a distinctive spiral pattern from center to edge.
An individual flower blooms over two days: on the first day, the male part of the flower opens and introduces pollen; On the second day, the female’s stigma unfolded to receive pollen. Somehow, the flowers are arranged so that they open in concentric rings starting at the tip and moving inward on successive days, with a ring of female flowers always outside the early-stage, pollen-bearing male flowers.
Pollinating bees tend to land on the petal rays around the head of the sunflower and walk toward the center, said senior researcher Stacy Harmer, professor of plant biology, UC Davis School of Biological Sciences. This means that they will pick up pollen after they have walked over the female florets, and then transfer it to a different flower head.
Harmer and postdoctoral researcher Karen Marshall wanted to understand how the spiral pattern of flowers transforms into concentric rings of flowers. Harmer’s lab had previously demonstrated that circadian rhythms control how growing sunflowers track sunlight during the day.
A plant or animal’s internal circadian clock runs on a cycle of about 24 hours, 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, daylight, or darkness can reset the clock. In sunflowers, constant light completely disrupts the clock.
Time-lapse video The researchers took time-lapse videos of sunflowers in different light/dark or temperature conditions. They found that the plant’s circadian clock controls the opening of flowers. When the clock was broken by the plants growing in continuous light, the florets did not open in concentric rings, but only by age, starting at the tip and moving to the center in a continuous gradient.
Two heads of yellow sunflowers on a black background. Venus on the left showing concentric rings. On the left is a sunflower head growing in natural conditions. The flowers ripen in concentric rings, day by day. The flower head on the right grew with a broken circadian clock and did not open its flowers in the correct pattern. (Harmer Lab, University of California, Davis) When plants grown with an idle clock were moved outside, they attracted fewer pollinators than did ordinary sunflowers.
“We think that being able to coordinate in this way makes them a better target for bees,” Harmer said. “It’s a strategy to attract as many insects as possible.”
Harmer said that as farmers adapt to a changing climate, it will become increasingly important to make pollination as effective as possible on crops that require it. She said understanding how the circadian clock and environment affect flowering will help breeders develop varieties that flower at optimal times of the day to enhance pollination, despite climate change and decreasing insect numbers.
Additional authors on the paper are Veronica Thompson and Nikki Crooks. Creux now works at the University of Pretoria, South Africa. The work was supported by the National Science Foundation and the US Department of Agriculture – National Institute of Food and Agriculture.