Large, leafy viburnum bushes have lain thousands upon thousands in the United States and Europe – their domes of flowering have an understated appeal. But once the flowers of the Viburnum tinus plant disappear, the shrub makes something unusual: shiny, brilliant blue fruit.
Scientists had noticed that pigments related to those in blue animals exist in viburnum fruit, and assumed that this must be the source of their odd hue. Blue fruit, by all means, is rare. But researchers reported last week in Current Biology that the blue of viburnum is actually made by layers of molecules arranged below the surface of the skin, a form of what scientists call structural color. With the means still unknown, the cells of the plant make thin slices of fat arranged in a stack, like the flakes of puff pastry, and their own luster is the result.
Rox Middleton, a researcher at the University of Bristol in England and an author of the new paper, had studied the African pollia plant, which produces its own exotic blue fruit. But viburnum fruit was everywhere, and she realized that her blue was not well studied. Together with Miranda Sinnott-Armstrong, a researcher at the University of Colorado, Boulder, and other colleagues, they set out to take a closer look at the skin of the fruit.
The blue of the pollia fruit is a form of structure color, in which light bounces off a regularly aligned arrangement of small structures so that certain wavelengths, usually those looking at us blue or green, return to the viewer. In pollia fruit, the color of light interacts with thin sheets of cellulose intertwined. At first the team thought that there would be something similar in Viburnum. But they saw no cellulose stacks.
Experimenting with different ways of looking at the fruit, they used a transmission electron microscope to get a side view of the plant’s cells. The nucleus of the cell was in fact lard with leaves of pigment. But between it and the surface of the skin there was an enormous object, as thick as the core itself. On closer inspection it turned out that it was made of neatly spaced layers.
This was a strong sign that structural color, which requires microscopic, regularly spaced materials for light to emerge, was involved. “Once you see a repetitive structure, that’s exactly the right size – yes, it should be,” said Drs. Middleton.
The layers appeared to be made of a spherical, uneven fabric. Mathematical models of the layers showed that these bumps helped to produce the particular cloudy blue of the viburnum fruit. If the layers were smoother, the blue would have been purer, more so than that of the wings of a cage, Drs. Middleton. The pigments in the nucleus, although similar to those in blue bushes, are in fact a very deep, dark red, which can dominate the structural blue.
The layers, to the researchers’ surprise, were made of fat molecules.
“We have never seen such a thing before,” said Drs. Sinnott-Armstrong.
Waxes and proteins are more commonly found in the cell walls of plants. But fats are rare in cell walls, and it is not clear how the layers form.
The seeds of viburnum fruits are spread by birds that eat and deepen them. The brilliant color created by the fats can provide a signal to birds that the fruit is useful and worth eating, the researchers suggest.
“We have found a few other species that appear to have similar structures to this one,” said Drs. Sinnott-Armstrong. She plans to investigate if they are also made of fat.
When it comes to blue fruit with unusual color strategies, viburnum has certainly been easier to get than the pollen fruit, thought Dr. Middleton.
‘It was nice,’ she said, ‘to have someone I could pick in my backyard.’
