Metallic Blue

Viburnum tinus metallic blue fruit

Viburnum hedges have lined yards in the United States and Europe for decades — their domes of blossoms have an understated attractiveness. But once the flowers of the Viburnum tinus plant fade, the shrub makes something unusual: shiny, brilliantly blue fruit.

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Scientists had noticed that pigments related to those in blueberries exist in viburnum fruit, and assumed that this must be the source of their odd hue. Blue fruit, after all, is rare. But researchers reported recently in Current Biology that viburnum’s blue is actually created by layers of molecules arranged under the surface of the skin, a form of what scientists call structural color. By means still unknown, the plant’s cells create thin slabs of fat arranged in a stack, like the flakes of puff pastry, and their peculiar gleam is the result.

Rox Middleton, a researcher at University of Bristol in England and an author of the new paper, had been studying the African Pollia condensata plant, which produces its own exotic blue fruit. But viburnum fruit were everywhere, and she realized that their blue had not been well-studied. Along with Miranda Sinnott-Armstrong, a researcher at the University of Colorado, Boulder, and other colleagues, she set out to take a closer look at the fruit’s skin.

Pollia condensata metallic blue fruit

Pollia condensata

The tiny, rock-hard fruits of Pollia condensata, a wild plant that grows in the forests of Ethiopia, Mozambique, Tanzania and other African countries, can’t be eaten raw, cooked or turned into a beverage. In Western Uganda and elsewhere, though, the plant’s small metallic fruits have long been used for decorative purposes because of an unusual property: They stay a vibrant blue color for years or even decades after they’ve been picked. A specimen at the Kew Botanical Gardens in London that was gathered in Ghana in 1974 still retains its iridescent hue.

Intrigued, a team of researchers from Kew, the University of Cambridge and the Smithsonian Natural History Museum decided to look into how this plant produces such a dazzling and persistent color. When they attempted to extract a pigment to study, though, they were surprised to discover the fruit had none.

Pollia condensata

When they examined P. condensata on a cellular level (above), they realized that the fruit produces its characteristic color through structural coloration, a radically different phenomenon that is well-documented in the animal kingdom but virtually unknown in plants. They determined that the fruit’s tissue is more intensely colored than any previously studied biological tissue—reflecting 30 percent of light, as compared to a silver mirror, making it more intense than even the renowned color of a Morpho butterfly’s wings. Their findings were revealed in a 2012 study published in the Proceedings of the National Academy of Sciences.

The vast majority of colors in the biological world are produced by pigments—compounds produced by a living organism that selectively absorb certain wavelengths of light, so that they appear to be the color of whichever wavelengths they reflect. For example, most plants are green because of the pigment chlorophyll, used in photosynthesis, which absorbs most wavelengths of visible light except green, reflecting that color into our eyes. As a consequence, plant colors created by pigmentation appear to be the exact same hue no matter which angle we view them from, and the color degrades when the plant dies.

P. condensata, however, produces its vibrant blue via tiny, nanoscale-size cellulose strands that are stacked inside its skin. These strands are arranged in layers of twisting, arced helix shapes, which interact with each other to scatter light and produce the fruit’s deep blue coloration. Here’s a view of the fruit through an electron microscope, revealing the presence of the color on a cellular level.

Viburnum tinus

Experimenting with different ways to look at the fruit, the researchers studying Viburnum tinus used a transmission electron microscope to get a side view of the plant’s cells. The cell’s nucleus was indeed larded with blobs of pigment. But between it and the surface of the skin there was an enormous object, as thick as the nucleus itself. A closer look revealed that it was made of neatly spaced layers.

Viburnum tinus fruit clusters

That was a strong sign that structural color, which requires microscopic, regularly spaced materials for light to bounce off, was involved. “Once you see a repetitive structure, that exact size — yeah, it’s got to be,” Dr. Middleton said.

The layers seemed to be made of a globular, uneven substance. Mathematical models of the layers showed that this bumpiness helped provide the particular cloudy blue of the viburnum fruit. If the layers had been smoother, the blue would have been purer, more like that of a beetle’s wings, Dr. Middleton said. The pigments in the nucleus, while related to those in blueberries, are in fact a very deep, dark red, allowing the structural blue to dominate.

The layers, to the researchers’ surprise, were made of fat molecules.

Viburnum tinus

“We’ve never seen anything like that before at all,” Dr. Sinnott-Armstrong said.

Waxes and proteins are more commonly found in plants’ cell walls. But fats are rare in cell walls, and it is not clear how the layers form.

The seeds of viburnum fruits are scattered by birds that eat and digest them. The brilliant color created by the fats may provide a signal to birds that the fruit is nutritious and worth eating, the researchers suggest.

“We’ve found a couple of other species that look like they have similar structures to this,” Dr. Sinnott-Armstrong said. She is planning to investigate if they, too, are made of fat.

In terms of blue fruits with unusual coloration strategies, viburnum has certainly been easier to get ahold of than the pollia fruit, Dr. Middleton reflected.

“It was nice,” she said, “to have one that I could pick in my backyard.”

Pokeweed (left, Phytolacca americana) and Elderberry (right Sambucus nigra subsp. canadensis) are two blue Florida natives that one might encounter in late summer to fall

Sambucus nigra subsp. cerulea

Sambucus nigra, commonly called European elder, is a deciduous, somewhat sprawling, multi-stemmed shrub (occasionally a small tree) that is native to Europe, southwestern Asia and northern Africa. It typically grows to 8-20’ (less frequently to 30') tall. It is particularly noted for its aromatic late spring flowers and its edible blue fruits (elderberries).

In Florida Sambucus nigra subspecies canadensis is common. It is a fast-growing shrub. In early Central Florida Elderberry is smothered with 6-10 inch wide clusters of yellowish-white blooms, spring to fall. These blooms are followed by a multitude of small, dark blue-purple berries which are quite popular with birds.

Florida Elderberry bloom (Sambucus nigra canadensis)

Subsp. cerulea, often commonly called blueberry elder or blue elder, is native to mountain areas in western North America. It is a deciduous tree or large multi-stemmed shrub which typically grows 15-30' (less frequently to 50') tall with a rounded crown.

Genus name comes from the Latin name, perhaps connected with sambuca a kind of harp.

Specific epithet comes from the Latin word for black in obvious reference to berry color. Subspecies name means blue for fruit color.