Abstract

Abstract The point of maximum slope in a reflectance spectrum of vegetation occurs at the boundary between red and near infrared wavelengths and is known as the “red edge”. There is a strong relationship between the red edge and the chlorophyll concentration of leaves and canopies. The aim of this research was to determine the effect of a second leaf pigment, red amaranthin, on the relationship between red edge and chlorophyll concentration. The red edge, chlorophyll concentration, and amaranthin concentration were recorded for 163 amaranth leaves in the laboratory. Experimental treatments with nitrate and salts caused a very large range in red edge (686–724 nm), chlorophyll concentration (0–20 mg/g), and amaranthin concentration (0–0.47 mg/g). There was a near-linear relationship between red edge and chlorophyll concentration for leaves with low amaranthin concentration (< 0.075 mg/g). This relationship was strongest for leaves from vegetative plants and was similar in form to that observed for a canopy. By contrast the red edge was at longer wavelengths and independent of chlorophyll concentration for leaves with high amaranthin concentration (0.075 mg/g), as a result of the strong absorption of visible light by amaranthin and the resultant movement of the red/near infrared boundary to longer wavelengths at higher amaranthin concentrations. This is the first reported study on the effect of a second leaf pigment on the relationship between red edge and chlorophyll concentration. The results suggest that the presence of a second leaf pigment would limit the use of a remotely sensed red edge for the estimation of chlorophyll concentration.