Simple reflectance indices track heat and water stress-induced changes in steady-state chlorophyll fluorescence at the canopy scale S.Z. Dobrowski a, * , J.C. Pushnik b , P.J. Zarco-Tejada c , S.L. Ustin a a Center for Spatial Technologies and Remote Sensing, University of California, Davis, Davis, CA, USA b Department of Biological Sciences, California State University, Chico, Chico, CA, USA c Instituto de Agricultura Sostenible (IAS), Consejo Superior de Investigaciones Cientı ´ficas (CSIC), Co ´rdoba, Spain Received 2 March 2005; received in revised form 28 April 2005; accepted 22 May 2005 Abstract Non-invasive remote sensing techniques for monitoring plant stress and photosynthetic status have received much attention. The majority of published vegetation indices are not sensitive to rapid changes in plant photosynthetic status brought on by common environmental stressors such as diurnal fluxes in irradiance and heat. This is due to the fact that most vegetation indices have no direct link to photosynthetic functioning beyond their sensitivity to canopy structure and pigment concentration changes. In contrast, this study makes progress on a more direct link between passive reflectance measurements and plant physiological status through an understanding of photochemical quenching (qP) and non-photochemical quenching processes. This is accomplished through the characterization of steady-state fluorescence (Fs) and its influence on apparent reflectance in the red-edge spectral region. A series of experiments were conducted under controlled environmental conditions, linking passive reflectance measurements of a grapevine canopy (Vitis vinifera L. cv. Cabernet Sauvignon) to leaf level estimates of CO 2 assimilation (A), stomatal conductance (g), qP, and Fs. Plant stress was induced by imposing a diurnal heat stress and recovery event and by withholding water from the plant canopy over the course of the experiment. We outlined evidence for a link between Fs and photosynthetic status, identified the Fs signal in passive remote sensing reflectance data, and related reflectance-derived estimates of Fs to plant photosynthetic status. These results provide evidence that simple reflectance indices calculated in the red-edge spectral region can track temperature and water-induced changes in Fs and, consequently, provide a rapid assessment of plant stress that is directly linked to plant physiological processes. D 2005 Elsevier Inc. All rights reserved. Keywords: Plant stress monitoring; Chlorophyll fluorescence; Remote sensing; Photochemical quenching; Non-photochemical quenching; Grapevines 1. Introduction Much effort has been devoted to developing non- invasive remote sensing techniques for monitoring plant stress and photosynthetic status. Most remotely sensed vegetation indices are used for characterizing the amount and spatial distribution of vegetation (Baret & Guyot, 1991; Price, 1992). Vegetation indices have also been used to estimate potential levels of canopy photosynthesis and net primary productivity with mixed success (Choudhury, 2001; Gamon et al., 1995; Verma et al., 1993). Nevertheless, the majority of published vegetation indices are not sensitive to rapid changes in plant photosynthetic status brought on by common environmental stressors such as diurnal fluxes in irradiance and heat. This is due to the fact that most vegetation indices have no direct link to photosynthetic functioning beyond their sensitivity to canopy structure (e.g., leaf angle) and pigment concentrations. Consequently, measurements of canopy reflectance have proven less useful for real-time monitoring of plant photosynthesis and/or water status at the whole plant level (Gamon et al., 1990; Pen ˜uelas et al., 1995). One exception to this involves remote estimates of the xanthophyll cycle captured by the photochemical reflec- 0034-4257/$ - see front matter D 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.rse.2005.05.006 * Corresponding author. Tel.: +1 530 752 5092. E-mail address: szdobrowski@ucdavis.edu (S.Z. Dobrowski). Remote Sensing of Environment 97 (2005) 403 – 414 www.elsevier.com/locate/rse