As the world’s climate changes, plants and animals have adapted by expanding into new territory and even shifting their breeding seasons. Now, research suggests that over the past 75 years, flowers have also adapted to rising temperatures and declining ozone by altering ultraviolet (UV) pigments in their petals.
Flowers’ UV pigments are invisible to the human eye, but they attract pollinators and serve as a kind of sunscreen for plants, says Matthew Koski, a plant ecologist at Clemson University. Just as UV radiation can be harmful to humans, it can also damage a flower’s pollen. The more UV-absorbing pigment the petals contain, the less harmful radiation reaches sensitive cells.
Previously, Koski and colleagues found that flowers exposed to more UV radiation—usually those growing at higher elevations or closer to the equator—had more UV pigment in their petals. He then wondered whether two factors affected by human activity, damage to the ozone layer and temperature changes, also influenced the UV pigments.
To find out, Koski and colleagues examined plant collections from North America, Europe, and Australia dating back to 1941. In all, they examined 1238 flowers from 42 different species. They photographed flower petals from the same species collected at different times throughout their natural range using a UV-sensitive camera, which captured changes in UV pigment. They then matched these changes to data on the local ozone level and temperature.
On average, pigment in flowers at all locations increased over time—an average of 2% per year from 1941 to 2017, they reported this month in Current Biology. But changes varied depending on flower structure. In saucer-shaped flowers with exposed pollen, like buttercups, UV-absorbing pigment increased when ozone levels went down and decreased in locations where ozone went up. But flowers with pollen concealed within their petals, such as the common bladderwort, decreased their UV pigment as temperatures went up—regardless of whether ozone levels changed.
Though surprising, the finding “makes total sense,” says Charles Davis, a plant biologist at Harvard University who was not involved with the work. Pollen hidden within petals is naturally shielded from UV exposure, but this extra shielding can also act like a greenhouse, trapping heat. When these flowers are exposed to higher temperatures, their pollen is in danger of being cooked, he says. Reducing UV pigments in the petals causes them to absorb less solar radiation, bringing down temperatures.
Although such pigment changes may be indistinguishable to the human eye, they stand out like a beacon to pollinators like hummingbirds and bees. Koski says most pollinators prefer flowers with a “bull’s-eye” pattern: UV-reflecting petal tips and UV-absorbing pigments near the center of the flower. Though scientists don’t fully understand the appeal of this pattern, they think it could help distinguish flowers from the UV-absorbing background of other plants.
As a result, flowers with less pigment may pop even more to pollinators, Koski says. But flowers that dial up their pigment could lose that contrast, ultimately making them less attractive to passing flyers. These pigment changes may help protect pollen, Davis says, but “pollinators might miss the flowers entirely.”