Evaluating the Strengths and Limitations of Fluorescence to Constrain Photosynthetic Processes: A New Statistical Inference Framework

Abstract:

Solar-induced chlorophyll fluorescence (SIF) measurements are increasingly being used as a proxy for photosynthesis and to constrain carbon cycle models. While many studies have utilized SIF to estimate key biophysical parameters and gross photosynthesis^1,2,3, few have done so at the leaf level in a robust statistical inference framework⁴. The relationship of SIF to photosynthesis is mediated by a cascade of processes including some highly dynamic and adaptive processes such as non-photochemical quenching (NPQ), hence one could easily attribute changes in SIF to incorrect processes and/or parameters (e.g. V_cmax).

Here, we present a novel data assimilation system at the leaf level that can utilize any combination of measurements of active and passive fluorescence (e.g. spectral SIF), reflectance, transmittance, and gas exchange (e.g. net CO₂ assimilation rate). This uses the radiative transfer model Fluspect-B⁵ and a model for photosynthesis, abstracted from the multi-layer canopy model SCOPE. We apply this framework to fit leaf level measurements of SIF spectra and gas exchange, independently and simultaneously, to assess the ability of the model to capture variability in SIF across multiple species, temperature, CO₂ concentration and light conditions. With this we can evaluate the consistency in parameter estimates when using SIF measurements or gas exchange measurements, providing insight into the model assumptions. We further evaluate the influence of NPQ dynamics on the fluorescence-photosynthesis link using synthetic experiments, showing that small errors in NPQ formulation can result in inaccurate estimates of parameters and photosynthetic rate, even with a good fit to SIF. This study provides a new framework for the inference of photosynthetic processes from a range of measurements at the leaf level. Overall this provides a guide for future research on the most important and uncertain processes the govern the link between fluorescence and photosynthesis.

¹ MacBean et al., doi: 10.1038/s41598-018-20024-w

² Norton et al., doi: 10.5194/bg-16-3069-2019

³ Zhang et al., doi: 10.1016/j.rse.2018.03.031

⁴ Vilfan et al., doi: 10.1111/nph.15782

⁵ Vilfan et al., doi: 10.1016/j.rse.2016.09.017