FLEX / SENTINEL-3 TANDEM MISSION PHOTOSYNTHESIS STUDY – AN INVESTIGATION OF STEADY-STATE CHLOROPHYLL FLUORESCENCE AND PHOTOSYNTHESIS IN TERRESTRIAL VEGETATION Gina Mohammed (1) , Alexander Ač (2) , Fabrice Daumard (3 ), Matthias Drusch (4) , Alexander Gallé (5,6) , Yves Goulas (3) , Federico Magnani (7) , Zbyněk Malenovský (2,8) , Jose Moreno (9) , Julie Olejníčková (2) , Dan Pernokis (1) , Uwe Rascher (6) , Juan Pablo Rivera (9) , Christiaan van der Tol (10) , Wouter Verhoef (10) , Jochem Verrelst (9) , Antonio Volta (7) (1) P & M Technologies, 66 Millwood St., Sault Ste. Marie, Ontario, Canada P6A 6S7, Email:gina.mohammed@pmtech.ca; Email: dan.pernokis@pmtech.ca (2) Global Change Research Centre AS CR, Belidla 986/4a, 603 00 Brno, Czech Republic, Email: ac.a@czechglobe.cz; Email: zbynek.malenovsky@gmail.com; Email: olejnickova.j@czechglobe.cz (3) Laboratoire de Météorologie Dynamique, Ecole Polytechnique, 91128 Palaiseau cedex, France, Email: fabrice.daumard@lmd.polytechnique.fr; Email: yves.goulas@lmd.polytechnique.fr (4) Mission Science Division (EOP-SME), European Space Agency, ESTEC, Postbus 299, 2200 AG Noordwijk, The Netherlands, Email: Matthias.Drusch@esa.int (5) Bayer CropScience NV, Innovation Center, Technologiepark 38, 9052 Zwijnaarde, Belgium, Email: alexander.galle@gmail.com (6) Institute of Bio- and Geosciences, IBG-2: Plant Sciences, Forschungszentrum Jülich GmbH, Leo-Brandt-Straße, 52428 Jülich, Germany, Email: u.rascher@fz-juelich.de; Email: alexander.galle@gmail.com (7) Forest Ecology Lab, Department of Agricultural Sciences, University of Bologna, via Fanin 46, I-40127 Bologna, Italy, Email: federico.magnani@unibo.it; Email: antonio.volta@unibo.it (8) School of Biological Sciences, University of Wollongong, Northfields Ave., 2522 Wollongong, NSW, Australia, Email: zbynek.malenovsky@gmail.com (9) Laboratory for Earth Observation, Department of Earth Physics and Thermodynamics, Faculty of Physics, University of Valencia, C/Dr. Moliner, 50, 46100 Burjassot, Valencia, Spain, Email: jose.moreno@uv.es, Email: juanri2@alumni.uv.es; Email: jochem.verrelst@uv.es (10) Department of Water Resources, Faculty of Geo-Information Science and Earth Observation (ITC), University of Twente, Hengelosestraat 99, P.O. Box 217, 7500 AE, Enschede, Netherlands, Email: c.vandertol@utwente.nl; Email: w.verhoef@utwente.nl ABSTRACT 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 C 3 and C 4 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 (solar- induced) and actively (laser-induced) measured chlorophyll fluorescence in detecting stress effects. Water stress tended to produce a decline in red and far- red 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