Iridescence is displayed by many plants and other photosynthetic organisms, but the biological function of this eye-catching feature is not yet fully understood. A study by Matthew Jacobs and collaborators from the University of Bristol and the University of Essex in the UK now suggests a direct link between iridescence and photosynthesis. The team reported that the iridoplasts (a type of epidermal chloroplast; top right image) found in the blue iridescent leaves of some species of Begonia are characterized by photonic crystal structures responsible for selective light absorption and increased quantum yield in low-light conditions (Nat. Plants 2, 16162; 2016).

Credit: NATURE PUBLISHING GROUP

The iridescent Begonia species grows in the tropical forest understorey, where light conditions for photosynthesis are rather extreme: indeed, overhead foliage and branches are responsible for light attenuation up to 60–70 dB and for significant light absorption around 460 nm and 680 nm. “From a plant science perspective, the interaction of photonics and photosynthesis is virtually unexplored, so there is lots of potential to understand and perhaps improve how plants deal with light,” Jacobs told Nature Photonics.

The authors used transmission and cryogenic scanning electron microscopy to observe the ultrastructure of individual iridoplasts (bottom right image), which consist of regularly spaced stacks, or grana, of smaller light-absorbing compartments known as thylakoids. To investigate the potential photosynthetic function of these highly ordered iridoplasts, Jacobs and co-workers developed an optical transfer matrix method model that indicated that the reflectance peak wavelength of a single iridoplast — typically around 470 nm — depends on the spacing between adjacent grana. Further data analysis showed that iridoplasts exhibit enhanced light absorption between 500 and 700 nm and a reduced absorbance below 500 nm, the cut-off wavelength again being determined by the spacing between grana. This response mirrors the light conditions of the forest understorey in that the observed enhancement in light absorption occurs for those wavelengths that are not filtered by higher foliage. As for the effect of this light management on photosynthetic processes, Jacobs and collaborators found that the quantum yield — the efficiency with which absorbed light can be used for electron transport and photosynthesis — in shade conditions is higher by 5 to 10% for iridoplasts than it is for other types of chloroplast.

When asked about potential applications beyond a plant science viewpoint, Jacobs said that “photonic structures similar to those found in iridoplasts are being investigated for use in solar energy devices. The iridoplast structure is interesting in this respect since the photonic structure and the light-harvesting structure are the same, and this idea could then inspire new light-harvesting applications.” Future work will also look into photonic chloroplasts that are not iridescent.