Estimating corn leaf chlorophyll concentration from leaf and canopy reflectance

Farmers must balance the competing goals of supplying adequate N for their crops while minimizing N losses to the environment. To characterize the spa tial variability of N over large fields, traditional methods (soil testing, plant tissue analysis, and chlorophyll meters) require many point samples. Because of the close link between leaf chlorophyll and leaf N concentratio n, remote sensing techniques have the potential to evaluate the N variabili ty over large fields quickly. Our objectives were to (1) select wavelengths sensitive to leaf chlorophyll concentration, (2) stimulate canopy reflecta nce using a radiative transfer model, and (3) propose a strategy for detect ing leaf chlorophyll status of plants using remotely sensed data. A wide ra nge of leaf chlorophyll levels was established in field-grown corn (Zea may s L.) with the application of 8 N levels: 0%, 12.5%, 25%, 50%, 75%, 100%, 1 25%, and 150% of the recommended rate. Reflectance and transmittance spectr a of fully expanded upper leaves were acquired over the 400-nm to 1,000-nm wavelength range shortly after anthesis with a spectroradiometer and integr ating sphere. Broad-band differences in leaf spectra were observed near 550 nm, 715 nm, and >750 nm. Crop canopy reflectance was simulated using the S AIL (Scattering by Arbitrarily Inclined Leaves) canopy reflectance model fo r a wide range of background reflectances, leaf area indices (LAI), and lea f chlorophyll concentrations. Variations in background reflectance and LAI confounded the detection of the relatively subtle differences in canopy ref lectance due to changes in leaf chlorophyll concentration. Spectral vegetat ion indices that combined near-infrared reflectance and red reflectance (e. g. OSAVI and NIR/Red) minimized contributions of background reflectance, wh ile spectral vegetation indices that combined reflectances of near-infrared and other visible bands (MCARI and NIR/Green) were responsive to both leaf chlorophyll concentrations and background reflectance. Pairs of these spec tral vegetation indices plotted together produced isolines of leaf chloroph yll concentrations. The slopes of these isolines were linearly related to l eaf chlorophyll concentration. A limited test with measured canopy reflecta nce and leaf chlorophyll data confirmed these results. The characterization of leaf chlorophyll concentrations at the field scale without the confound ing problem of background reflectance and LAI variability holds promise as a valuable aid for decision making in managing N applications. Published by Elsevier Science Inc.