Continuous Monitoring of Canopy Level Sun-Induced Chlorophyll Fluorescence During the Growth of a Sorghum Field

A field platform dedicated to fluorescence measurements (INRA, Avignon, France) was used to monitor the fluorescence emission of a sorghum field during its growing period. The measurements were performed continuously at the canopy level, from seeding to maturity. A passive instrument based on three spectrometers was used to monitor the evolution of fluorescence fluxes and vegetation indexes such as Photochemical Reflectance Index (PRI) and Normalized Difference Vegetation Index (NDVI). Fluorescence fluxes were retrieved from radiances, using the filling-in of the atmospheric oxygen absorption bands, at 687 and 760 nm. In parallel, leaf fluorescence spectra, canopy height, and leaf chlorophyll contents were acquired during the growth. Both PRI and NDVI indexes varied with the development of the sorghum field: we observed that NDVI was more sensitive during the early stage of the growth. However, NDVI saturates before the PRI index. Fluorescence fluxes at 687 nm (Fs687) and 760 nm (Fs760) showed an overall increase: Fs687 increased more rapidly at the beginning of growth but trends to saturate while Fs760 still increase. During the growth, the Fs687/Fs760 ratio at the canopy level is found lower than at leaf level. At canopy level, the ratio decreased when the leaf chlorophyll content increases. A decrease was also observed at leaf level with a lower extend. This more important decrease of the fluorescence ratio at canopy level is attributed to a reabsorption of red fluorescence (Fs687) during its transfer through the canopy layers. In the context of forthcoming large-scale remote sensing application, the modification of the leaf level fluorescence emission by the structure of the canopy observed in this article is one of the major issues that must be addressed to interpret the fluorescence signal.