Coloration of the Chilean Bellflower, Nolana paradoxa, interpreted with a scattering and absorbing layer stack model

Abstract

Main conclusionAn absorbing-layer-stack model allows quantitative analysis of the light flux in flowers and the resulting reflectance spectra. It provides insight in how plants can optimize their flower coloration for attracting pollinators.The coloration of flowers is due to the combined effect of pigments and light-scattering structures. To interpret flower coloration, we applied an optical model that considers a flower as a stack of layers, where each layer can be treated with the Kubelka–Munk theory for diffusely scattering and absorbing media. We applied our model to the flowers of the Chilean Bellflower, Nolana paradoxa, which have distinctly different-colored adaxial and abaxial sides. We found that the flowers have a pigmented, strongly scattering upper layer, in combination with an unpigmented, moderately reflecting lower layer. The model allowed quantitative interpretation of the reflectance and transmittance spectra measured with an integrating sphere. The absorbance spectrum of the pigment measured with a microspectrophotometer confirmed the spectrum derived by modeling. We discuss how different pigment localizations yield different reflectance spectra. The absorbing layer stack model aids in understanding the various constraints and options for plants to tune their coloration.

How to colour a flower: on the optical principles of flower coloration

“…We measured the reflectance and transmittance of the floral elements with an integrating sphere (for equipment details, see [14]). The dominant coloured flower areas were mounted with the display side (generally the adaxial surface) facing the detector.…”

“…The combined averaged reflectance (R) and transmittance (T ) of the flowers yielded the absorptance A ¼ 1 2 (R þ T ). Absorbance spectra of floral elements immersed in water were measured with a microspectrophotometer (MSP), as in [14].…”

“…We modelled the reflectance and transmittance of the petals by considering a petal as a stack of absorbing and scattering layers [14]. In this optical model, we combined the KubelkaMunk theory [15] with a calculation procedure for a stack of absorbing and reflecting layers [16 -18].…”

“…the fractions of reflected, transmitted, and absorbed light, flower absorbance spectra and types of pigment localization. To understand the functional consequences of different pigment localizations and amounts of scattering, we applied our recently developed optical model that treats a petal as a stack of layers, with each layer having specific absorption and scattering characteristics [14]. The optical properties of each individual layer (e.g.…”

“…Flower coloration is due to the combined effect of wavelength-selective absorption by pigments and light scattering by the petal interior [11][12][13][14]. The reflected light in the wavelength ranges complementary to the pigment's absorption band determines the flower's hue and saturation.…”