A model for chlorophyll fluorescence and photosynthesis at leaf scale C. van der Tol a, *, W. Verhoef a , A. Rosema b a International Institute for Geo-Information Science and Earth Observation, P.O. Box 6, 7500 AA Enschede, The Netherlands b EARS Remote Sensing Consultants, Kanaalweg 1, P.O. Box 449, 2600 AK Delft, The Netherlands 1. Introduction Passive measurements of chlorophyll fluorescence from satellite platforms could theoretically provide information about growth, transpiration and the energy balance of terrestrial vegetation. Chlorophyll in green vegetation and algae absorbs energy in the visible part of the spectrum of radiation, providing energy for photosynthesis. Part of the excess absorbed energy is re-emitted as chlorophyll fluores- cence, in a spectral range between 650 and 800 nm. Together with reflected radiation, chlorophyll fluorescence forms the spectral signature of optical radiation of vegetation. Although chlorophyll fluorescence is small compared to the spectrum of reflected radiation, it is now possible to detect it using accurate equipment with a high spectral resolution (Zarco-Tejada et al., 2000; Moya et al., 2004; Meroni and Colombo, 2006). Improvements in measurements techniques during the last decade have led to the development of the fluorescence explorer (FLEX) mission, which aims to launch a satellite to monitor steady-state fluorescence of terrestrial vegetation. Recent research focused on the development of sensors (Moya et al., 2004; Meroni and Colombo, 2006), the relationship between chlorophyll fluorescence and photosynthesis (He and Edwards, 1996), the scaling from organelle to canopy level (Zarco-Tejada et al., 2000), and atmospheric correction and retrieval (Verhoef and Bach, 2003; Schaepman et al., 2005). The aim is to eventually monitor the status of vegetation and photosynthesis over large areas with passive measurements of chlorophyll fluorescence from satellites. This paper focuses on the relationship between fluores- cence and photosynthesis at leaf level. A photon absorbed by a leaf can be dissipated as heat, or used for photo- chemistry, or emitted as fluorescence. The probability of each depends on biochemical and environmental condi- tions. For the interpretation of the chlorophyll fluorescence signal, it is necessary to describe the relationship between agricultural and forest meteorology 149 (2009) 96–105 article info Article history: Received 2 November 2007 Received in revised form 22 July 2008 Accepted 23 July 2008 Keywords: Photosynthesis Photosynthetically active radiation Carbon assimilation Remote sensing abstract This paper presents a leaf biochemical model for steady-state chlorophyll fluorescence and photosynthesis of C3 and C4 vegetation. The model is a tool to study the relationship between passively measured steady-state chlorophyll fluorescence and actual photosynth- esis, and its evolution during the day. Existing models for chlorophyll fluorescence and photosynthesis are integrated into a relatively simple deterministic model to quantify chlorophyll fluorescence, electron transport, carboxylation and deactivation of antennae in case of light and moisture stress. The model explains the behaviour of the relationship between fluorescence and photosynthesis that has been reported in the literature. Simula- tions, a sensitivity analysis, and measurements show that variations in total chlorophyll fluorescence correlate well with variations in actual photosynthesis in the late morning and afternoon. Then, photosynthesis is light saturated and limited by stomatal regulation. To calculate the actual photosynthesis rate, an estimate of the maximum carboxylation capacity is needed beside chlorophyll fluorescence. # 2008 Elsevier B.V. All rights reserved. * Corresponding author. E-mail address: tol@itc.nl (C. van der Tol). available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/agrformet 0168-1923/$ – see front matter # 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.agrformet.2008.07.007