Variations in photosynthesis still cause substantial uncertainties in predicting photosynthetic CO2 uptake rates and monitoring plant stress. Changes in actual photosynthesis that are not related to greenness of vegetation are difficult to measure by reflectance based optical remote sensing techniques. Several activities are underway to evaluate the sun-induced fluorescence signal on the ground and on a coarse spatial scale using space-borne imaging spectrometers. Intermediate-scale observations using airborne-based imaging spectroscopy, which are critical to bridge the existing gap between small-scale field studies and global observations, are still insufficient. Here we present the first validated maps of sun-induced fluorescence in that critical, intermediate spatial resolution, employing the novel airborne imaging spectrometer HyPlant. HyPlant has an unprecedented spectral resolution, which allows for the first time quantifying sun-induced fluorescence fluxes in physical units according to the Fraunhofer Line Depth Principle that exploits solar and atmospheric absorption bands. Maps of sun-induced fluorescence show a large spatial variability between different vegetation types, which complement classical remote sensing approaches. Different crop types largely differ in emitting fluorescence that additionally changes within the seasonal cycle and thus may be related to the seasonal activation and deactivation of the photosynthetic machinery. We argue that sun-induced fluorescence emission is related to two processes: (i) the total absorbed radiation by photosynthetically active chlorophyll; and (ii) the functional status of actual photosynthesis and vegetation stress.

Department of Earth Physics and Thermodynamics University of Valencia Dr Moliner 50 46100 Burjassot Valencia Spain

European Space Agency ESTEC Keplerlaan 1 2200 AG Noordwijk the Netherlands

Field Lab Campus Klein Altendorf Agricultural Faculty University of Bonn Klein Altendorf 3 53359 Rheinbach Germany

Global Change Research Centre AS CR Bělidla 986 4a 603 00 Brno Czech Republic

Institute for Astronomy Astrophysics Space Applications and Remote Sensing National Observatory of Athens 15236 Athens Greece

Institute for Space Sciences Free University of Berlin Carl Heinrich Becker Weg 6 10 12165 Berlin Germany

Institute of Bio and Geosciences IBG 2 Plant Sciences Forschungszentrum Jülich GmbH Leo Brandt Str 52425 Jülich Germany

Remote Sensing Laboratories University of Zurich Winterthurerstrasse 190 8057 Zurich Switzerland

Remote Sensing of Environmental Dynamics Lab DISAT Università degli Studi Milano Bicocca Milano Italy

Specim Spectral Imaging Ltd Teknologiantie 18A 90590 Oulu Finland

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