FLEX/Sentinel-3 Tandem Mission Photosynthesis Study – An investigation of steady-state chlorophyll fluorescence and photosynthesis in terrestrial vegetation

The FLuorescence EXplorer (FLEX) would be the first space mission optimised for remote observation of steady-state, solar-induced chlorophyll fluorescence (SIF) in terrestrial vegetation. Within the European Space Agency’s Phase A/B1 assessment, the Photosynthesis Study considers the potential of SIF for quantifying photosynthesis, and assessing vegetation health and stress status. This report is a broad overview of the main elements and key findings of this study. The study has two components: The first developed a process-based model to quantitatively link steady-state fluorescence yield to photosynthesis; the other component evaluated the potential of steady-state fluorescence as an indicator of vegetation physiological stress, without requiring calculation of photosynthetic rates. This dual approach ensures that the full range of capabilities of the fluorescence signal might be exploited in a spaceborne mission. The modelling activity integrated state-of-the-art modules representative of physiological processes at the molecular, leaf, and canopy levels to feed the Automated SCOPE (A-SCOPE) tool. SCOPE – the Soil Canopy Observation, Photosynthesis Energy fluxes model, originally developed by C. van der Tol and colleagues – links top of canopy observations of radiance with land surface processes, and includes modules dedicated to chlorophyll fluorescence. A-SCOPE is a new Graphic User Interface software package that provides a seamless link between inputs and outputs required for running SCOPE. The SCOPE model was expanded to include novel functionalities and features, such as new leaf biochemical routines for C3 and C4 species. Outputs include fluorescence and reflectance spectra, among other products. For the other major component of the study – the use of SIF in stress detection – we focused on the stresses of plants induced by water deficit, low or high temperature extremes, and nutrient (nitrogen) insufficiency. A random-effects meta-analysis was done for studies of passively (solarinduced) and actively (laser-induced) measured chlorophyll fluorescence in detecting stress effects. Water stress tended to produce a decline in red and farred fluorescence at leaf and canopy levels. The clearest indicator of temperature stress was the ratio of red to far-red fluorescence, which declined consistently even when combining chilling and heat stress measurements. The ratio was also an effective indicator of nitrogen