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Remote Sens. 2017, 9(5), 415; doi:10.3390/rs9050415

Multiangular Observation of Canopy Sun-Induced Chlorophyll Fluorescence by Combining Imaging Spectroscopy and Stereoscopy

1
Institute of Bio- and Geosciences, IBG-2: Plant Sciences, Forschungszentrum Jülich GmbH, 52428 Jülich, Germany
2
Global Wheat Program, International Maize and Wheat Improvement Center (CIMMYT), 56237 Texcoco, Mexico
3
INRES-Plant Breeding, University of Bonn, 53115 Bonn, Germany
4
Experimental Station of University of Bonn in Klein-Altendorf, 53359 Rheinbach, Germany
*
Author to whom correspondence should be addressed.
Academic Editors: Jose Moreno, Clement Atzberger and Prasad S. Thenkabail
Received: 16 February 2017 / Revised: 19 April 2017 / Accepted: 23 April 2017 / Published: 28 April 2017
View Full-Text   |   Download PDF [28105 KB, uploaded 28 April 2017]   |  

Abstract

The effect that the canopy structure and the viewing geometry have on the intensity and the spatial distribution of passively measured sun-induced chlorophyll fluorescence at canopy scale is still not well understood. These uncertainties constrain the potential use of fluorescence to quantify photosynthesis at this level. Using a novel technique, we evaluated the diurnal changes in the spatial distribution of sun-induced fluorescence at 760 nm (F760) within the canopy as a consequence of the spatial disposition of the leaves and the viewing angle of the sensor. High resolution spectral and stereo images of a full sugar beet canopy were recorded simultaneously in the field to estimate maps of F760 and the surface angle distribution, respectively. A dedicated algorithm was used to align both maps in the post-processing and its accuracy was evaluated using a sensitivity test. The relative angle between sun and the leaf surfaces primarily determined the amount of incident Photosynthetic Active Radiation (PAR), which in turn was reflected in different values of F760, with the highest values occurring in leaf surfaces that are perpendicularly oriented to the sun. The viewing angle of the sensor also had an impact in the intensity of the recorded F760. Higher viewing angles generally resulted in higher values of F760. We attribute these changes to a direct effect of the vegetation directional reflectance response on fluorescence retrieval. Consequently, at leaf surface level, the spatio-temporal variations of F760 were mainly explained by the sun–leaf–sensor geometry rather than directionality of the fluorescence emission. At canopy scale, the diurnal patterns of F760 observed on the top-of-canopy were attributed to the complex interplay between the light penetration into the canopy as a function of the display of the various leaves and the fluorescence emission of each leaf which is modulated by the exposure of the individual leaf patch to the incoming light and the functional status of photosynthesis. We expect that forward modeling can help derive analytical simplified skeleton assumptions to scale canopy measurements to the leaf functional properties. View Full-Text
Keywords: sun-induced chlorophyll fluorescence; imaging spectroscopy; stereo imaging; 3D reconstruction; leaf angle distribution; top-of-canopy irradiance dynamics; remote sensing of vegetation; PAR (photosynthetic active radiation); Fraunhofer Line Depth (FLD); directionality effects; solar-induced fluorescence sun-induced chlorophyll fluorescence; imaging spectroscopy; stereo imaging; 3D reconstruction; leaf angle distribution; top-of-canopy irradiance dynamics; remote sensing of vegetation; PAR (photosynthetic active radiation); Fraunhofer Line Depth (FLD); directionality effects; solar-induced fluorescence
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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. (CC BY 4.0).

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MDPI and ACS Style

Pinto, F.; Müller-Linow, M.; Schickling, A.; Cendrero-Mateo, M.P.; Ballvora, A.; Rascher, U. Multiangular Observation of Canopy Sun-Induced Chlorophyll Fluorescence by Combining Imaging Spectroscopy and Stereoscopy. Remote Sens. 2017, 9, 415.

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