Iridoplasts as a photonic structure

Photonic structure is used extensively in nature to achieve colourful displays for visual signalling in diverse inter- and intra-specific relationships. This has been well studied for many animal and plant species with the use of structural colouration ranging from camouflage, signalling for warning, mating, seed dispersal, pollination and even social status. This research sets out to investigate the modified chloroplast found commonly in Begonia species which, using ordered multilayer structures acts as a photonic structure to manipulate its interaction with light. Crucially, whilst the photonic structure here does cause colouration which can be visible under specific conditions, it is not likely to be the primary purpose. Previous research has shown that the photonic structure in iridoplasts may contribute towards an increased photosynthetic yield in low light conditions. Here iridoplasts in Begonia species have been investigated using Fourier-plane microspectrometry, autofluorescence confocal microscopy and transmission electron microscopy to measure structural properties and compare optical modelling to measured spectra. Novel green-reflecting iridoplasts have been characterised and their optical properties compared to more common blue-reflecting iridoplasts showing that these structures may be far more common in nature as they are not visible to the naked eye. The response of iridoplast ultrastructure was tested in acclimation to different wavelengths of light showing that the photonic structure is conserved under greatly varying spectral quality, unlike mesophyll chloroplasts which show drastic ultrastructural changes. In one species iridoplasts grown under blue light showed a consistent structural response indicating some wavelength tuning is possible. The effects of natural disorder found in real iridoplast images was analysed and discussed in comparison to recent theoretical studies. The findings show that the optical characteristics are robust against even the highest levels of disorder and, in contrast to a recent theoretical study, this work finds no strong evidence for the presence of correlated disorder which may have implications for the way iridoplasts form