The fruits of Viburnum tinus, a Mediterranean flowering shrub, have a secret property that gives them a vibrant, metallic blue colour without relying on pigments. Blue fruits are uncommon in nature, due to the rarity of blue pigments, but a recent study, published in Current Biology, investigated the colour properties of the nutritionally valuable fruits of V. tinus and found it originates from unique structural features.
Viburnum tinus, a Mediterranean flowering shrub
Usually, pigmentation in fruits arises from the presence of flavonoid compounds, specifically anthocyanins. V. tinus is an important food source for birds, which are attracted to the vibrant colour. In turn for nutrition, the birds disperse the plant’s seeds.
Using microscopy and spectroscopy techniques, researchers investigating the stunning metallic properties of V. tinus fruit uncovered nanostructures of lipids in its cell walls. These structures may act as a double signal to birds, indicating these fruits are full of nutritious fats. These nanostructures differ from regular plant cell walls, which are made of cellulose, and lipids are normally only stored within the cell and used for transport. This distinctive structural property of V. tinus fruit allows it to create the blue colour without containing any pigment.
Blue fruits are uncommon in nature
“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 co-first author Miranda Sinnott-Armstrong, a researcher at the University of Colorado-Boulder. “This means that V. tinus, in addition to showing a completely novel mechanism of structural colour, is also one of the few known structurally coloured fruits.”
The researchers hope this work can help to understand how birds identify nutritious food, and that the interesting structural colour properties could be exploited to provide safe and sustainable food colourants.
“There are lots of problems connected to food coloration,” says Silvia Vignolini, senior author from the University of Cambridge. “Once this mechanism is better understood, it could potentially be used to create a healthier, more sustainable food colorant.”