title: Photonic multilayer structure of Begonia chloroplasts enhances photosynthetic efficiency.
abstract: Enhanced light harvesting is an area of interest for optimizing both natural photosynthesis and artificial solar energy capture1,2. Iridescence has been shown to exist widely and in diverse forms in plants and other photosynthetic organisms and symbioses3,4, but there has yet to be any direct link demonstrated between iridescence and photosynthesis. Here we show that epidermal chloroplasts, also known as iridoplasts, in shade-dwelling species of Begonia5, notable for their brilliant blue iridescence, have a photonic crystal structure formed from a periodic arrangement of the light-absorbing thylakoid tissue itself. This structure enhances photosynthesis in two ways: by increasing light capture at the predominantly green wavelengths available in shade conditions, and by directly enhancing quantum yield by 5-10% under low-light conditions. These findings together imply that the iridoplast is a highly modified chloroplast structure adapted to make best use of the extremely low-light conditions in the tropical forest understorey in which it is found5,6. A phylogenetically diverse range of shade-dwelling plant species has been found to produce similarly structured chloroplasts7-9, suggesting that the ability to produce chloroplasts whose membranes are organized as a multilayer with photonic properties may be widespread. In fact, given the well-established diversity and plasticity of chloroplasts10,11, our results imply that photonic effects may be important even in plants that do not show any obvious signs of iridescence to the naked eye but where a highly ordered chloroplast structure may present a clear blue reflectance at the microscale. Chloroplasts are generally thought of as purely photochemical; we suggest that one should also think of them as a photonic structure with a complex interplay between control of light propagation, light capture and photochemistry.
authors: School of Biological Sciences, University of Bristol, Life Sciences Building, Tyndall Avenue, Bristol BS8 1TQ, UK.
authors: Matthew
authors: M
authors: Jacobs
authors: Department of Electrical and Electronic Engineering, University of Bristol, Bristol BS8 1TH, UK.
authors: Martin
authors: M
authors: Lopez-Garcia
authors: School of Biological Sciences, University of Bristol, Life Sciences Building, Tyndall Avenue, Bristol BS8 1TQ, UK.
authors: O-Phart
authors: OP
authors: Phrathep
authors: School of Biological Sciences, University of Essex, Wivenhoe Park, Colchester, CO4 3SQ, UK.
authors: Tracy
authors: T
authors: Lawson
authors: Department of Electrical and Electronic Engineering, University of Bristol, Bristol BS8 1TH, UK.|HH Wills Physics Laboratory, University of Bristol, BS8 1TL, UK.
authors: Ruth
authors: R
authors: Oulton
authors: School of Biological Sciences, University of Bristol, Life Sciences Building, Tyndall Avenue, Bristol BS8 1TQ, UK.
authors: Heather M
authors: HM
authors: Whitney
mesh_terms: D031112:Begoniaceae / Q000502:physiology*; D002736:Chloroplasts / Q000378:metabolism* / Q000648:ultrastructure; D020285:Cryoelectron Microscopy; D008855:Microscopy, Electron, Scanning; D046529:Microscopy, Electron, Transmission; D010788:Photosynthesis; D019441:Plant Epidermis / Q000378:metabolism / Q000648:ultrastructure; D020524:Thylakoids / Q000378:metabolism / Q000648:ultrastructure