Large, deciduous shrubs have lined construction sites in the United States and Europe for decades – their flowering domes have an underrated attractiveness. But after the flowers of the Viburnum tinus plant fade, the shrub does something unusual: shiny fruit and blue glow.
Scientists had noticed that the pigments associated with those in blueberries exist in the fruits of viburnum, and assumed that this must be the source of their strange coloration. Blue trees, after all, are rare. But researchers reported last week in Current Biology that viburnum blue is actually created from layers of molecules arranged beneath the surface of the skin, a form of what scientists call structural dyes. In unfamiliar words, plant cells create thin plates of fat arranged in a stack, like flakes of puffy paste, and their distinctive luster is the result.
Rox Middleton, a researcher at the University of Bristol in England and author of the new paper, had studied the African plant polia, which produces its own exotic blue fruits. But the fruits of viburnum were everywhere, and she realized that their blueness had not been well studied. Together with Miranda Sinnott-Armstrong, a researcher at the University of Colorado, Boulder and other colleagues, she tried to take a closer look at the skin of the fruit.
Polis fruit blue is a form of structural color in which light emits a regular arrangement of small structures, such that certain wavelengths, usually those that look blue or green, are reflected back to the viewer. In polia fruits, the color comes from the interaction of light with thin cellulose sheets packed together. At first the team thought there would be something similar in viburnum. But they saw no cellulose stacks.
Experimenting with different ways to see the fruit, they used a transmission electron microscope to get a side view of the plant cells. The cell nucleus was actually wrapped in pigment blocks. But between it and the surface of the skin there was a large object, as thick as the nucleus itself. A closer look revealed that it was made of regular layers in space.
This was a strong sign that structural color, which requires microscopic materials, regularly placed so that light bounces off, was included. “Once you see a repetitive structure, that exact size – yes, it should be,” said Dr. Middleton.
The layers seemed to be made of a globular, uneven substance. Mathematical patterns of the layers showed that this lump helped provide the special cloudy blue of the viburnum fruit. If the layers were softer, the blue would have been purer, more like that of a beetle’s wings, said Dr. Middleton. The pigments in the nucleus, while binding to those in blueberries, are actually a very deep, dark red, allowing structural blue to predominate.
The layers, to the surprise of the researchers, were made of fat molecules.
“We have never seen anything like this before,” said Dr. Sinnott-Armstrong.
Waxes and proteins are most often found in plant cell walls. But fats are rare in cell walls, and it is not clear how layers are formed.
The seeds of viburnum fruit are distributed by the birds that eat them and digest them. The brilliant color created by the fats could provide a signal to the birds that the fruit is nutritious and worth eating, researchers suggest.
“We’ve found a couple of other species that look like they have structures similar to this,” said Dr. Sinnott-Armstrong. She is planning to investigate whether, too, they are fat.
As for blue fruits with unusual coloring strategies, viburnum has certainly been easier to care for than polis fruits, Dr. thought. Middleton.
“It was nice,” she said, “to have one I could choose in my backyard.”