Remote sensing of chlorophyll fluorescence for estimation of stress in vegetation. Recommendations for future missions Julia Amorós-López, Joan Vila-Francés, Luis Gómez-Chova, Luis Alonso, Luis Guanter, Secundino del Valle- Tascón, Javier Calpe, and José Moreno University of Valencia Dr. Moliner 50, 46100, Burjasot (Valencia), Spain. E-mail: julia.amoros@uv.es Abstract— Vegetation monitoring is a key issue in Earth Observation due to its relation with the global CO 2 cycle. Chlorophyll fluorescence (ChF) emitted by the vegetation is an accurate indicator of the plant status and their photosynthetic activity. This work analyses the diurnal evolution of the ChF emission spectrum and the fluorescence yield in order to determine the best conditions for remote sensing of ChF from a satellite platform. The ChF evolution is studied at leaf level during several diurnal cycles, in simulated conditions, for two species under different stress conditions. The analysis of the signal levels gives an estimation of the values of ChF emission which could be observed from a remote sensing platform, and determines the best overpass time for this observation. Chlorophyll fluorescence, emission spectrum, field spectroradiometer, remote sensing, vegetation stress, diurnal cycle. I. INTRODUCTION Global vegetation monitoring is one of the key objectives of remote sensing due to its strong link with land-atmosphere carbon dioxide (CO2) exchange. Several studies present the chlorophyll fluorescence (ChF) as an accurate physiological indicator of the plant state that is, moreover, directly related to plant photosynthesis [1-3]. ChF consists on the emission of red and far-red light from photosynthetic green plant tissues in response to the absorption of photosynthetically active radiation (PAR) from 400 to 700 nm [4]. This ChF emission is characterized by two broad peaks that span from 650 to 800 nm and present their maxima at 690 nm and 740 nm, respectively. Traditionally, ChF has been analyzed actively through the fluorescence yield parameter, which is measured using instruments based on the Pulse Amplitude Modulation technique (PAM-instruments) [5]. These instruments can measure ChF in presence of bright background illumination, but they consider the whole ChF emission spectrum as a single broad band from 710 nm to 850 nm (i.e., taking into account only the second peak of ChF), and give a relative value of the fluorescence yield that is not related to absolute radiometric values [6]. Passive measuring of the ChF is difficult to carry out due to the low intensity of the ChF signal (only 1% to 2% of absorbed light) with respect to the reflected incoming radiation, which makes the decoupling of both signals a challenging problem. However, present remote sensing systems make possible the passive measurement of the fluorescence signal under solar illumination in the solar Fraunhofer lines or narrow atmospheric absorption bands using the Fraunhofer Line Discrimination (FLD) principle [7]. The quantification of the ChF signal levels while identifying vegetation stress state is a key observational requirement to monitor vegetation photosynthesis. This work analyses the diurnal evolution of the ChF in absolute radiometric values and the fluorescence yield in order to determine the best conditions for the remote sensing of ChF from a satellite platform. The ChF evolution is studied at leaf level during several diurnal cycles at the laboratory in simulated conditions, for two species under various stress conditions. The paper is outlined as follows. Section II describes the employed methodology for the ChF measurement and the experimental setup. The results are presented in Section III, and the conclusions are given in Section IV. II. METHODOLOGY The diurnal evolution of the chlorophyll fluorescence was measured in the laboratory simulating the photosynthetically active radiation (PAR) that a plant would receive at two different moments of the year: winter and summer. We used two different plants that were not watered during the experiment while several diurnal cycles were applied to each plant to produce different water stress conditions. The ChF was measured at leaf level by means of two different methodologies. First, an active Pulse Amplitude Modulation (PAM) fluorometer was used to measure the relative fluorescence yield (FY), the quantum yield (QY), and the non- photochemical quenching (NPQ). At the same time, passive measurement of the ChF emission spectrum was performed using a spectroradiometer. For the passive measurements, the plant was illuminated in the PAR region with a filtered light that does not present any contribution at the ChF emission range (Fig. 1). This technique allows the independent analysis of the two peaks of the ChF emission spectrum, the radiometric quantification of the ChF emission, and the absolute measurement of the fluorescence yield [8]. In addition to these measurements, we calculate the photochemical reflectance This work has been sponsored by the Spanish Ministry of Science and Education’s under the CICYT programme (projects DATASAT and BIO- 2005-09252-002-2) and the ESA SEN2FLEX project (ESA ESRIN/Contract N o 19187/05/I-EC).