Fluspect-B: a model for leaf fluorescence, reflectance and transmittance spectra

We present the Fluspect-B model (generally referred to as Fluspect), which simulates leaf chlorophyll fluorescence (ChlF), reflectance and transmittance spectra. The existing PROSPECT model and its concept of a compact leaf are used as a starting point, and the differential equations for radiative transfer within the leaf are solved by an efficient doubling algorithm. Due to the simplicity of these equations, Fluspect offers a high computational speed. With incident light provided as the main input parameter, Fluspect calculates the emission of ChlF on both the illuminated and shaded side of the leaf. Other input parameters are chlorophyll and carotenoid concentrations, leaf water, dry matter and senescent material (brown pigments) content, leaf mesophyll structure parameter and ChlF quantum efficiency for the two photosystems, PS-I and PS-II. We investigated the model performance using measurements of leaf reflectance, transmittance and ChlF spectra, collected for barley and sugar beet leaves in both a laboratory and outdoors setting. The plants had been grown under various illumination conditions to increase between-leaf variability of leaf biochemical and structural properties. We retrieved the model parameters, compared them to corresponding destructive measurements and finally, used them to simulate ChlF on either side of the leaf at several light intensities. The results show that the model reproduces observed SIF accurately, especially for leaves measured under natural illumination. Most of the observed between-leaf variability of ChlF could be explained from differences in leaf biochemical and structural properties, with potential additional information held by ChlF emission efficiency parameters.