Structural Coloration and Epicuticular Wax Properties of the Distinctive Glaucous Leaves of Encephalartos horridus

Abstract The leaves of the cycad Encephalartos horridus exhibit a conspicuous glaucous appearance, attributed to the presence of epicuticular wax. However, the molecular and optical bases of this coloration have not been scientifically explained. In this study, we conducted a detailed analysis of the epicuticular wax composition, combined with RNA-Seq and de novo transcriptome assembly, to uncover the molecular mechanisms underlying this phenomenon. Additionally, Monte Carlo multi-layer (MCML) simulations were performed to model light interactions and explore the structural coloration generated by the epicuticular wax crystals. The wax was found to be predominantly composed of nonacosan-10-ol, forming tubular crystals that enhance reflectance in the long-wavelength UV to blue light range. However, the microstructure alone is not sufficient to produce the glaucous appearance; it arises from the interplay between the wax crystals and the underlying dark tissues rich in chlorophyll. These findings provide insights into the evolutionary adaptations of cycads to UV exposure and contribute to a broader understanding of plant surface lipid biosynthesis and structural coloration, with potential applications in biomimetic material design.