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. 2012 Sep 25;109(39):15712-5.
doi: 10.1073/pnas.1210105109. Epub 2012 Sep 10.

Pointillist structural color in Pollia fruit

Silvia Vignolini  1 , Paula J Rudall, Alice V Rowland, Alison Reed, Edwige Moyroud, Robert B Faden, Jeremy J Baumberg, Beverley J Glover, Ullrich Steiner
Affiliations

Affiliation

  • 1 Cavendish Laboratory, Department of Physics, University of Cambridge, Cambridge, United Kingdom.

Pointillist structural color in Pollia fruit

Silvia Vignolini et al. Proc Natl Acad Sci U S A. .
. 2012 Sep 25;109(39):15712-5.
doi: 10.1073/pnas.1210105109. Epub 2012 Sep 10.

Authors

Silvia Vignolini  1 , Paula J Rudall, Alice V Rowland, Alison Reed, Edwige Moyroud, Robert B Faden, Jeremy J Baumberg, Beverley J Glover, Ullrich Steiner

Affiliation

  • 1 Cavendish Laboratory, Department of Physics, University of Cambridge, Cambridge, United Kingdom.

Abstract

Biological communication by means of structural color has existed for at least 500 million years. Structural color is commonly observed in the animal kingdom, but has been little studied in plants. We present a striking example of multilayer-based strong iridescent coloration in plants, in the fruit of Pollia condensata. The color is caused by Bragg reflection of helicoidally stacked cellulose microfibrils that form multilayers in the cell walls of the epicarp. We demonstrate that animals and plants have convergently evolved multilayer-based photonic structures to generate colors using entirely distinct materials. The bright blue coloration of this fruit is more intense than that of any previously described biological material. Uniquely in nature, the reflected color differs from cell to cell, as the layer thicknesses in the multilayer stack vary, giving the fruit a striking pixelated or pointillist appearance. Because the multilayers form with both helicoidicities, optical characterization reveals that the reflected light from every epidermal cell is polarized circularly either to the left or to the right, a feature that has never previously been observed in a single tissue.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Fig. 1.

Fig. 1.

Photographs of Pollia condensata fruits.…

Fig. 1.

Photographs of Pollia condensata fruits. ( A ) Single fruit from dried herbarium…

Fig. 1.
Photographs of Pollia condensata fruits. (A) Single fruit from dried herbarium specimen collected in Ghana (Kew Herbarium: Faden and Lock 74/37, 1974). The blue color of the fruit is not uniform but has a brilliant pixelated iridescent appearance with green and purple/red speckles. (B) Infructescence (cluster of fruits) from alcohol-preserved specimen collected in Ethiopia (Kew Herbarium: Moult 24, 1974). The diameter of each fruit is about 5 mm.
Fig. 2.

Fig. 2.

Anatomy of Pollia condensata fruit.…

Fig. 2.

Anatomy of Pollia condensata fruit. ( A ) SEM image of the fruit…

Fig. 2.
Anatomy of Pollia condensata fruit. (A) SEM image of the fruit surface showing smooth cuticular layer. (B) TEM cross-section showing three distinct tissue zones: (1) an outer epicarp of 3–4 layers of thick-walled cells, (2) an intermediate region of 2–3 layers of tanniniferous cells, and (3) a zone of thin-walled cells. (C) TEM of a single thick-walled cell from layer 1. (D) TEM of the cellulose microfibrils that constitute the thick cell wall in layer 1. The red lines highlight the twisting direction of the microfibrils. (E) (Left) Scheme showing a wedge of an LH helicoid with the arched pattern exposed on the oblique face (adapted from ref. (24)). (Right) Circularly polarized beams of light, with [Image: see text] the wave vector of the light. The handedness of the transmitted and reflected light depends on the handedness of the helicoid. Here, light transmitted through the structure ([Image: see text] pointing down) is LH circularly polarized, while the reflected light ([Image: see text] pointing up) is RH. (F) 3D representation of the orientation of cellulose microfibril assembly and a transmitted circularly polarized beam.
Fig. 3.

Fig. 3.

Polarized reflection of Pollia condensata …

Fig. 3.

Polarized reflection of Pollia condensata fruit. ( A ) LH and ( B …

Fig. 3.
Polarized reflection of Pollia condensata fruit. (A) LH and (B) RH optical micrographs of the same area of the fruit under epi-illumination. The insets show a zoom of the central areas, with white lines delimiting the cells. (C) The same area of the fruit surface was also imaged between crossed polarizers. All three images were obtained using a × 10 objective. (D) Schematic representation of light reflection from a curved multilayer, representing the ellipsoidal shape of the epicarp cells. Only light from the central part of the cell is reflected into the numerical aperture of the objective (NA = 0.3), resulting in a color stripe in the center of each cell, seen in A and B. (E) Spectra from two different cells (continuous and dotted lines, respectively) for the two polarization channels (red and blue color, respectively). Auxiliary minor spectral features leading to the double-peak structure arise from the stacked nature of the cells in the epicarp (Fig. 2B). This leads to spectral contributions from underlying cells with different p values.
Fig. 4.

Fig. 4.

Color-filtered images corresponding with Fig.…

Fig. 4.

Color-filtered images corresponding with Fig. 3 A and B . The color dispersion…

Fig. 4.
Color-filtered images corresponding with Fig. 3A and B. The color dispersion of LH and RH reflected light was imaged by inserting a tunable liquid crystal filter in front of the CCD camera. Ten-nanometer-wide reflection bands were imaged at (A and B) 430 nm, (C and D) 530 nm, and (E and F) 630 nm. Images with different colors were individually normalized (images with the same color have the same normalization).
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Comment in

References

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