Sensitivity of Sentinel-2 Red-Edge Bands to Leaf Chlorophyll Concentration in Winter Wheat.

The present work addresses the comparison between the sensitivity of several narrow-band and broad-band vegetation indices (VI) to leaf chlorophyll concentration at the canopy scale in winter wheat field trials with different nitrogen fertilization levels. Spectral signatures were collected on three dates (Feekes growth stages 5-6-9) over experimental plots with a portable spectroradiometer (ASD FieldSpec HH), along with Minolta SPAD measures of leaf optical thickness as a proxy of leaf chlorophyll concentration. Considered broad-band indices included classical slope-based VI (i.e.: NDVI, SR and OSAVI) and some indices incorporating green reflectance (i.e. Green NDVI, Green SR, CVI and MTVI), whereas narrow-band indices included VI specifically proposed as leaf chlorophyll estimators at the canopy scale (i.e. MCARI/OSAVI, TCARI/OSAVI, TCI/OSAVI, NOAC, REIP and MTCI). Broad-band VI were obtained from average reflectance in spectral ranges corresponding to SPOT HRG bands and from average reflectance in Sentinel-2 visible and NIR bands whereas narrow-band VI were obtained from reflectance at the original spectral resolution of the spectroradiometer and from average reflectance in Sentinel-2 Red-Edge bands. Among VI obtained from Sentinel-2 bands REIP and MTCI indices were the best narrow-band estimators of winter wheat leaf chlorophyll concentration, whereas CVI was the best estimator among broad-band VI.