Chlorophyll Fluorescence and Dynamic Xanthophyll Reflectance as Methods to Estimate Photosynthesis with Remote Sensors - A Modelling Approach

Chlorophyll fluorescence (ChlF) has been proven valuable in studying the dynamics of photosynthesis: together with the photochemical reflectance index (PRI), which reflects the changes in reflectance around 535 nm due to the xanthophyll cycle effect, it offers a powerful way to non-destructively quantify plant photosynthetic and dissipation activity from leaf to potentially global scale. We present an extended simulation model, Fluspect, for the transfer of light within the leaf as a function of its pigment contents and structure. Output of the model are reflectance and transmittance spectra of the leaf as well as ChlF spectra. From ChlF spectra, fluorescence quantum efficiency parameters (η) of photosystems I and II can be retrieved. Moreover, we extended and parameterized Fluspect by including a physically based description of the dynamic xanthophyll reflectance (XR). We introduced a new, retrievable parameter, V2Z, for the status of the XR. Finally, we combined the model Fluspect with a simple enzyme kinetics model of photosynthesis. In this study, we explore the combined model by using relations among photochemical quenching (PQ), non-photochemical quenching (NPQ), V2Z and η. We explore these relationships by using multiple available datasets, where measurements were by different combinations of the following methods: (1) hyperspectral leaf reflectance in visible to NIR region together with ChlF spectra, (2) gas exchange, and (3) Pulse Amplitude Modulated (PAM) fluorescence. The aim of the study is to explore the relationships between the two fundamentally different components, namely the transfer of light within the leaf and the enzyme kinetics of photosynthesis. Moreover, it is now possible to simulate dynamic xanthophyll reflectance and chlorophyll fluorescence quenching simultaneously, which is relevant for the remote sensing of photosynthesis.