The chloroplast as a photonic structure

In 2017 the Whitney and Oulton groups collaborated to show that modified chloroplasts (known as iridoplasts) in the leaves of Begonia species have lamellar structures that arrange into a periodic photonic structure capable of reflecting blue light. It was shown that these iridoplasts were more photosynthetically efficient at low light levels, with the hypothesis that the photonic structure modified the absorption to scavenge light at the right wavelength at low light levels. This work was the first of its kind to show that plants have evolved photonic structures to aid photosynthesis.

These structures were discovered because of the blue appearance of the leaves. However, in other light environments there would be good reason to suggest that plants would evolve chloroplasts that reflect in the green area of the spectrum. This effect may be very widespread, and therefore an important factor in the evolution of photosynthesising plants. However, accidental discovery via leaf inspection would be almost impossible. Nevertheless, some tantalising evidence of highly reflective green chloroplasts (alongside the blue ones) have also been discovered by the candidate during his final year project.

The candidate's PhD project will involve systematically searching for and testing different species of plants and algae to discover whether photonic structures are widespread in chloroplasts. He will begin by searching known databases of electron microscopy images of plants and algae to look for periodic structures, indicating that the structure may be photonic. Specimens will then be sought out and reflectivity measured as a function of angle to determine whether the chloroplasts have a structural colour as well as that due to pigment. We anticipate that structural colour may have evolved separately several times. If this were the case, it would provide another avenue to genetically engineering more efficient crops, as well as changing our fundamental understanding of one of the most important energy processes on the planet.