Circadian Clock Controls Sunflower Blooms, Optimizing for Pollinators

By Andy Fell

An internal circadian clock controls the distinctive concentric rings of flowering in sunflowers, maximizing visits from pollinators, shows a new study from plant biologists at the University of California, Davis. The work is published Jan. 13 in eLife.

A sunflower head is made up of hundreds of tiny florets. Because of the way sunflowers grow, the youngest florets are in the center of the flower face and the most mature at the edges, forming a distinctive spiral pattern from the center to the edge.

An individual floret blooms over a couple of days: On the first day, it opens the male part of the flower and presents pollen; on the second day, the female stigma unfold to receive pollen. Somehow, florets coordinate 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 earlier-stage, pollen-bearing male flowers.

Pollinating bees tend to land on the ray petals around a sunflower head and walk toward the center, said senior author Stacey Harmer, professor of plant biology, UC Davis College of Biological Sciences. That means that they will pick up pollen after they have walked over the female florets, then carry it to a different flower head.