World’s Shiniest Living Thing Uses Structural Coloring Rather Than Pigment To Dazzle
September 11, 2012

World’s Shiniest Fruit Uses Structural Coloring Rather Than Pigment To Dazzle

Lawrence LeBlond for - Your Universe Online

A dazzling, shiny blue fruit has been described as the most colorful biological species known to science. Known as Pollia condensata, this fruit does not get its blue color from pigmentation, like most natural objects, but rather from its structural color--a method of reflecting light of particular wavelengths--research has revealed.

Researchers, describing the fruit in detail, said unlike other fruits, which have cells surrounded by cell walls containing cellulose, this particular species has cellulose that is laid down in layers, forming an asymmetrical structure that is able to interact with light and provide selective reflection of only a specific color. As a result of this process, the fruit reflects predominantly blue light.

The small, spherical, metallic blue fruit grow on the head of the 18-inch high P. condensata plant. The plant is found from Ethiopia to Angola and Mozambique, and are used widely there as decoration.

The study, published in the Proceedings of the National Academy of Sciences (PNAS),  relied on high-tech microscopic imaging to show the iridescence of the skin. The fruit itself has no blue pigment, and changes color depending on the angle from which it is observed. While blue is the predominant color, if you look closely at these small Christmas ornament-like beads, you will begin to pick up tinges of pink, red, orange, green and violet.

In the animal kingdom, such color-changing characteristics are common, as are seen in some species of butterfly, jewel beetles, some bird species, and even some dinosaurs. But in the plant kingdom, such structural colors were thought to be non-existent.

Researchers, led by Silvia Vignolini, of the University of Cambridge, discovered the fruit´s secret while working at Kew Gardens in the UK. Along with study co-leader Ulrich Steiner, Vignolini was combing the plant world looking for species that bend light in interesting ways.

Vignolini sought out the P. condensata after getting a tip from Robert Faden of the Smithsonian. Also with the aid of Kew´s Paula Rudall, Vignolini found a sample of the plant, collected from Ghana in 1974, but was still as vivid as the day it was collected (unlike pigments, structural colors do not degrade, so the fruits retain their sheen almost indefinitely). In fact, the researchers found samples of the fruit in herbarium collections dating back to the 19th century that were as colorful and vivid as ones grown today.

Vignolini said what is truly remarkable about this fruit, is that it reflects light either in a left or right direction from every cell of the fruit--a feat not seen before in single tissues. The researchers believe the fruit´s color functions are meant to mislead seed dispersers--attracting them without offering any nutritional content. Vignolini points out that P. condensata is similar to the blue berries of another plant that grows in the same region, Psychotria peduncularis, which gives a good indication of seed dispersal through trickery.

These fruits could also be attractive to birds or other animals that build unique nests to entice potential 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 study coauthor Dr Beverley Glover from the University of Cambridge´s Department of Plant Sciences.

“By taking inspiration from nature, it is possible to obtain smart multifunctional materials using sustainable routes with abundant and cheap materials like cellulose,” explained Vignolini. “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,” she added.

This research was supported by the Leverhulme Trust with some funding provided by the Engineering and Physical Sciences Research Council (EPSRC).