Blue fruits are rare because the required pigment compounds are relatively uncommon in nature. Viburnum tinus does it differently, however.
Researchers have found that the evergreen shrub, which is popular for landscaping throughout Europe, owes the dazzling blue colour of its fruit to fat in its cellular structure: the first time this type of colour production has been observed.
While most plants have cell walls made of cellulose, V. tinus fruit cells have much thicker walls with thousands of globular lipids arranged in layers that reflect blue light.
“Structural colour is very common in animals, especially birds, beetles, and butterflies, but only a handful of plant species have ever been found to have structural colour in their fruits,” says Miranda Sinnott-Armstrong, from Yale University, US, co-first author of a paper in the journal Current Biology.
Senior author Silvia Vignolini, a physical chemist at the University of Cambridge, UK, thought something was different when she first saw the plants in an Italian garden 10 years ago, but initial analysis revealed nothing of note.
The chance to study them using electron microscopy changed the game. “Before we got the images, we were just seeing all these blobs,” Vignolini says. “When we found out that those blobs were lipids, we got very excited.”
Analysis showed that the lipid structures are incorporated into the cell wall of the outer skin, or epicarp, of the fruits.
In addition, a layer of dark red anthocyanin pigments lies underneath the complex structure, and any light that is not reflected by the lipid structure is absorbed by the dark red pigment beneath. This prevents any backscattering of light, making the fruits appear even more blue.
While most fruits have low fat content, some – such as avocadoes, coconuts and olives – do contain lipids, providing an energy-dense food source for animals. This is not a direct benefit to the plant, the researchers say, but it can increase seed dispersal by attracting birds.
They thus suspect that the colour of the V. tinus fruits may serve as a signal of its nutritional content. A bird could look at a fruit and know whether it is rich in fat or in carbohydrates based on whether or not it is blue.
“Honest signals are rare in fruits as far as we know,” says Sinnott-Armstrong. “If the structural colour of Viburnum tinus fruits are in fact honest signals, it would be a really neat example where colour and nutrition come at least in part from the same source: lipids embedded in the cell wall.
“We’ve never seen anything like that before, and it will be interesting to see whether other structurally coloured fruits have similar nanostructures and similar nutritional content.”
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