Remote Estimation of Crop Chlorophyll Content by Means of High‐Spectral‐Resolution Reflectance Techniques

Published in Agron. J. 103:1834–1842 (2011) Posted online 29 Sep 2011 doi:10.2134/agronj2011.0101 Copyright © 2011 by the American Society of Agronomy, 5585 Guilford Road, Madison, WI 53711. All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. R much attention has been paid to the development of methods to derive spatially distributed maps of Chl in vegetation using remote sensing techniques (Stagakis et al., 2010; Haboudane et al., 2008; Thoren et al., 2010). Leaf Chl provides important information about the physiological status of plants and photosynthetic activity and is related to N content, water stress, and yield forecasting (Eitel et al., 2008; Zhang et al., 2008; Schlemmer et al., 2005). Chlorophyll content in higher plants consists in different types of chlorophyll molecules (chlorophyll a and chlorophyll b, slightly different in their molecular structures) with different optical properties (Lichtenthaler, 1987). Both chlorophyll types show two large spectral absorption peaks, one in the blue and another in the red; in chlorophyll a, the two peaks are centered at 430 and 662 nm, while in chlorophyll b the two peaks are centered at 453 and 642 nm. Although light is also absorbed by many other secondary pigments that play different roles in plant physiology, photosynthesis in green plants is mostly driven by light collected primarily by chlorophylls (Heldt, 2005). While in the blue spectral region other pigments present in leaves have a considerable absorption, the spectral region between 600 and 700 nm is mostly dominated by chlorophyll a absorption only (Sims and Gamon, 2002). Therefore, this spectral region has been proposed as the most suitable spectral interval to retrieve Chl from spectral reflectance measurements. Thanks to the existence of earth observation satellites with a high temporal resolution, the physiological status of plants can be remotely estimated and monitored, for example, for yield prediction, crop classification, and land cover change detection (Weissteiner and Kuhbauch, 2005; Baez-Gonzalez et al., 2002; Chang et al., 2007; Carrao et al., 2010). The development of optical remote sensing techniques allows measurements within the spectral interval where important absorption features of pigments are present. In the last few years, several spectral indices have been proposed with the aim of quantifying different pigments from reflectance data, with the most emphasis put on Chl. These indices include the MERIS terrestrial chlorophyll index (Dash and Curran, 2004), triangular chlorophyll index (Haboudane et al., 2008), red-edge index (Gitelson et al., 2005), modified chlorophyll absorption ratio index (Daughtry et al., 2000) and the modified normalized difference vegetation index (NDVI) (Sims and Gamon, 2002). An overview and tables of Chl-sensitive indices was provided by Haboudane et al. (2008) and le Maire et al. (2008). These indices are limited to a few bands (two to five bands), however, thereby not completely detecting the subtle variation in Chl-specific spectral absorption features (Darvishzadeh et al., 2008). Moreover, most of these indices are sensitive to confounding factors such as vegetation structure or soil heterogeneity (Verrelst et al., 2010) and show saturation at high Chl values (Wu et al., 2010a). Alternatively, some studies have used the continuum shape of the reflectance profile between 600 and 700 nm to relate with ABSTRACT