Figure 3. Relationship between carotenoid and chlorophyll a concentration ratio ( C c /C a ) and SI PI for the best
set of wavebands ([Xj, A 9 ]=[445 nm, 680 nm]) and the exponential model (7). R 2 = 0.987, RMSE=0 .112;
«=35. The axes are in logarithmic scales.
By construction, SIPI removes any additive and multiplicative factors. Therefore, no absolute calibration of the
sensors is needed. Only relative calibration is required.
Results show' that chlorophyll a and carotenoids are strongly correlated for medium to high
concentrations. The maximum scatter around the main relation is observed for low? concentration levels. In these
conditions, the ratio CjC a is the most sensitive to errors in the estimate of the individual pigment
concentrations, and that can induce important errors when adjusting or retrieving CJC a from reflectance
measurements.
The question to be addressed in further studies should be the development of a more robust
theoretical background to retrieve directly individual pigment concentrations such as C a and C c from reflectance
spectra observed at leaf level. This modeling effort should also concurrently force finding a more meaningful
physiological indicator as compared to the simple CjC a ratio.
6. ACKNOWLEDGEMENTS
This research was supported by a Grant from CICYT AGR90-458 (Spain) to J. Penuelas. We gratefully
acknowledge a FPI (Spain) fellowship to I. Filella. Many thanks also to people that completed the data set, and
particularly to S. Jacquemoud, B. Andrieu, M. Steven, T. Mai thus, M. Danson, K. Jaggard, J. Schellberg and U.
Rinderle.
7. REFERENCES
Baret, F., B. Andrieu, and G. Guyot. 1988. A simple model for leaf optical properties in visible and
near infrared : application to the analysis of spectral shifts determinism. In H. K. Lichtenthaler (Eds.),
Applications of chlorophyll fluorescence. Kluwer Academic publishers ,pp. 345-351.
Baret F., S. Jacquemoud , G. Guyot , and C. Leprieur. 1992. Modeled analysis of the biophysical
nature of spectral shifts and comparison with information content of broad bands. Remote Sensing of
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Benedict, H.M. and R. Swidler. 1961. Nondestructive method for estimating chlorophyll content of
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Chappelle, E.W., S.K. Moon, and J.E. McMurtrey III. 1992. Ratio analysis of reflectance spectra
(RARS). An algorithm for the remote estimation of the concentrations of chlorophyll a, Chlorophyll
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