Stacked scattering: The key to bright flowers lies in the mesophyll

The coloration of flowers is caused by wavelength-selective absorption by pigments and scattering of light by floral structures. Although the molecular, physiological, and chemical properties of floral pigments have been studied in considerable detail, how floral structures contribute to the visual signal remains largely unknown. A flower can be considered as a stack of layers, where each layer is characterized by specific pigmentation and scattering properties. Quantifying the contribution of different floral layers to visual signalling aids our understanding of the origin and maintenance of Earth's resplendent flora.

We quantified the contribution of the cuticle, epidermal cell layer, and the mesophyll to the reflection of light by flowers for nasturtium (Tropaeolum majus), a St. John's wort hybrid (Hypericum 'Hidcote'), and the large-flowered evening primrose (Oenothera glazioviana). The obtained experimental and modelling data allowed the quantification of the absorption and scattering of light by different floral layers.

The mesophyll was by far the strongest reflecting layer. The reflectance from the epidermal layer was minor, and the cuticle reflected a very small percentage of the total. The strong scattering by the mesophyll is caused by its inhomogeneity and thickness.

The strong light-scattering by the mesophyll crucially determines a flower's visual signal, whereas the cuticle and epidermal cells contribute less than generally assumed. Mesophyll thickness, cell properties and inhomogeneity, including porosity, are essential components of the flower's optical toolkit.