Why flowers near the equator ‘get a tan’

Many flowers that appear uniform in color to humans have dark patterns only visible in ultraviolet—and to pollinators, such as bees. (Credit: Joshua Mayer/Flickr)

The discovery by German zoologist Constantin Wilhelm Lambert Gloger in 1833 that warm-blooded animals closer to the equator tend to be darker applies to plants, too, new research confirms.

One of the reasons investigators have not previously pursued proof of Gloger’s rule in flowers is that pollinators, such as bees, don’t see what we see when they look at a flower.

They see in the ultraviolet as well as visible ranges. What appears bright yellow to a person can appear dark or patterned to a bee.

flower in visible light and UV light
Many flowers that appear uniform in color to humans (left) have patterns in the ultraviolet spectrum (right) that are used by pollinators. Interestingly, these patterns can also protect pollen from damage caused by solar UV radiation. (Credit: U. Pittsburgh)

Sweet centers

For the study, published in Nature Plants, researchers studied the flowers of Argentina anserina, a plant in the rose family, across four lines of latitude—three in the Northern and one in the Southern Hemisphere.

Using ultraviolet imaging, they examined the “bull’s-eye” centers of the flowers (that look dark to pollinators) and discovered that the dark areas were larger the closer to the equator the flowers grew.


Larger “bull’s-eyes” are associated with higher levels of ultraviolet light, which is more intense near the equator, says Matthew Koski, a PhD student in the lab of Tia-Lynn Ashman, a professor and associate chair of biological sciences at the University of Pittsburgh.

The researchers then hypothesized that bigger bull’s-eyes absorb more ultraviolet light, functioning as a protective trait, because high ultraviolet light levels are known to damage DNA. In the study, they confirmed that extreme ultraviolet light reduces the viability of pollen (gametes) in Argentina anserina.

One might predict then that as the Earth receives more ultraviolet light at extreme northern and southern climes due to depletion of the ozone layer, flowers farther from the equator are likely to begin to evolve traits, such as larger ultraviolet light-absorbing bull’s-eyes, that are beneficial to their survival.

However, this may come at a cost: bigger bull’s-eyes obscure the “sweet center” of the flower where pollen and nectar rewards are found, thus making poorer targets for pollinators.

“Spring is coming earlier, and plants and pollinators are no longer in sync,” Ashman says. “Increased ultraviolet radiation is causing the same sort of disruption.”

Source: University of Pittsburgh