Biologists have discovered that the Pollia condensata fruit does not get its blue color from pigment but instead uses structural color – a method of reflecting light of particular wavelengths.
Pollia condensata (© Mark Hyde, Bart Wursten and Petra Ballings / David Gwynne-Evans / Flora of Zimbabwe, via EOL: Encyclopedia of Life / CC BY-NC 3.0)
Most colors around us are the result of pigments. However, a few examples in nature – including the peacock, the scarab beetle and now the Pollia condensata fruit – use structural color as well.
Fruits are made of cells, each of which is surrounded by a cell wall containing cellulose. However, the researchers found that in the Pollia condensata fruit the cellulose is laid down in layers, forming a chiral structure that is able to interact with light and provide selective reflection of only a specific color. As a result of this unique structure, it reflects predominately blue light.
The scientists also discovered that each individual cell generates color independently, producing a pixilated or pointillist effect. This color is produced by the reflection of light of particular wavelengths from layers of cellulose in the cell wall. The thickness of the layers determines which wavelength of light is reflected. As a result, some cells have thinner layers and reflect blue; others have thicker layers and reflect green or red.
The biologists believe that the plants invest in the complicated coloring structure as a mechanism for seed dispersal. Although the Pollia fruit does not provide any nutritional value, birds are attracted to its bright coloring – possibly as a means of decorating their nests or impressing their mates.
“This obscure little plant has hit on a fantastic way of making an irresistible shiny, sparkly, multi-colored, iridescent signal to every bird in the vicinity, without wasting any of its precious photosynthetic reserves on bird food. Evolution is very smart!” said Dr Beverley Glover from the University of Cambridge’s Department of Plant Sciences, a co-author of the study published in the Proceedings of the National Academy of Sciences.
Because of how it is created, the color of the Pollia condensata fruit does not fade. The researchers found that samples of the fruit in herbarium collections dating back to the 19th century were as colorful and shiny as ones grown today.
“By taking inspiration from nature, it is possible to obtain smart multifunctional materials using sustainable routes with abundant and cheap materials like cellulose,” explained lead author Dr Silvia Vignolini of the University of Cambridge’s Department of Physics.
“We believe that using cellulose to create colored materials can lead to many industrial applications. As an example, edible cellulose-based nanostructures with structural color can be used as substitutes for toxic dyes and colorants in food. Moreover, the fact that the processes involved in cellulose extraction and manipulation are already used in the paper industry facilitates the use of such materials for industrial applications such as security labeling or cosmetics.”
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Bibliographic information: Silvia Vignolini et al. Pointillist structural color in Pollia fruit. PNAS, published online before print September 10, 2012; doi: 10.1073/pnas.1210105109
