Extending Fluspect to simulate xanthophyll driven leaf reflectance dynamics

The xanthophyll cycle regulates the energy flow to photosynthetic reaction centres of plant leaves. Changes in the de-epoxidation state (<i>DEPS</i>) of xanthophyll cycle pigments can be observed as changes in the leaf absorption of light with wavelengths between 500 to 570 nm. These spectral changes can be a good remote sensing indicator of the photosynthetic efficiency, and are traditionally quantified with a two-band physiologically based optical index, the Photochemical Reflectance Index (PRI). In this paper, we present an extension of the plant leaf radiative transfer model Fluspect (Fluspect-CX) that reproduces the spectral changes in a wide band of green reflectance: a radiative transfer analogy to the PRI. The idea of Fluspect-CX is to use <i>in vivo</i> specific absorption coefficients for two extreme states of carotenoids, representing the two extremes of the xanthophyll de-epoxidation, and to describe the intermediate states as a linear mixture of these two states. The ‘photochemical reflectance parameter’ (C_x) quantifies the relative proportion of the two states. Fluspect-CX simulates leaf chlorophyll fluorescence (<i>ChlF</i>) excitation-emission matrices, as well as reflectance (R) and transmittance (T) spectra as a function of leaf structure, pigment contents and C_x. We describe the calibration of the model and test its performance using various experimental datasets. Furthermore, we retrieved C_x from optical measurements of various datasets. The retrieved C_x correlates well with xanthophyll DEPS (R² = 0.57), as well with non-photochemical quenching (NPQ) of fluorescence (R² = 0.78). The correlation with NPQ enabled us to incorporate Fluspect-CX in the model SCOPE to scale the processes to the canopy level. Introducing the dynamic green reflectance into a radiative transfer model provides new means to study chlorophyll fluorescence and PRI dynamics on leaf and canopy scales, which is crucial for the remote sensing.