Signature analysis of leaf reflectance spectra: Algorithm development for remote sensing of chlorophyll

The goal of the study is to investigate the basic spectral properties of plant leaves to develop spectral indices more sensitive to chlorophyll concentration than the presently widely used Normalized Difference Vegetation Index. These indices can serve as indicators of stress, senescence, and disease in higher plants.The spectral reflectance of senescing leaves of two deciduous species (maple and chestnut) as well as their pigment content were measured. Spectral indices were developed using reflectances corresponding to wavelengths with maximum and minimum sensitivity to variation in pigment concentration. The signature analysis of reflectance spectra indicated that, for a wide range of leaf greenness (completely yellow to dark green leaves), the maximum sensitivity of reflectance coincides with the maximum absorption of chlorophyll a at 670 nm. However, for yellow-green to green leaves (minimum chlorophyll a as low as 3-5 nmol/cm²), the reflectance near 670 nm is not sensitive to chlorophyll concentration due to saturation effects. Therefore, it seems inappropriate to use this spectral band for pigment estimation in yellow-green to green vegetation. The spectral bands ranging from 400 to 480 nm and above 730 nm are not sensitive to chlorophyll concentration as found for 670 nm. The reflectances at these wavelengths could be used as references in the vegetation indices. Maximum sensitivity to chlorophyll a concentration was found at 550-560 nm and 700-710 nm. Reflectances at 700 nm correlated very well with that at 550 nm for a wide range of chlorophyll concentrations for both plant species studied. The inverse reflectance, (R₅₅₀)¹ and (R₇₀₀)¹ are proportional to chlorophyll a concentration; therefore indices R₇₅₀/R₅₅₀ are directly proportional (correlation r²>0.95) to chlorophyll concentration. These indices were tested for a wide range of chlorophyll a concentration, using several independent data sets. The estimation error in the derivation of chlorophyll concentrations form the indices is assessed to be less than 1.2 nmol/cm².