The flowers of the Silverweed Cinquefoil, Argentina anserina, are darker in plants growing nearer to the equator reports a study published online in the first issue of Nature Plants. This is the first time that Gloger’s rule, where organisms are darker coloured when living closer to the equator, has been demonstrated in plants.
In 1833 the German ornithologist Constantin Lambert Gloger observed that birds in warmer climates tended to have darker plumage. Gloger’s rule was subsequently seen in many animals including people, where it is thought to arise as a protection against damage by ultraviolet (UV) light, balanced by the need for UV in the synthesis of vitamin D.
Matthew Koski and Tia-Lynn Ashman study the Silverweed Cinquefoil, Argentina anserina, which has small, dish-shaped yellow flowers. When viewed under ultraviolet light these have a dark centre called a bullseye, as is common with many flowers. The authors measured the size of these bullseyes in four regions in the Northern and Southern hemispheres. In every region they measured, the bullseyes were larger in populations nearer the equator.
When grown under laboratory conditions in the presence of UV light, plants with smaller bullseyes produced less viable pollen. When UV light was filtered out there was no such reduction in viability. This finding suggests that the larger bullseyes of plants closer to the equator serve to absorb excess UV light and helps protect the plants’ pollen from UV damage.
However, Innes Cuthill points out in his accompanying News & Views “…protecting pollen from UV damage is a cause of floral colour variation is an important conclusion, although it does not rule out other factors from being as (or more) significant, including the effects of UV on other plant functions.”