Multi-angle hyperspectral observations using fluorescence and PRI to detect plant stress and productivity in a cornfield

Abstract

The effects of seasonal water stress and nitrogen availability, which significantly impact plant productivity, can be monitored remotely using reflectance and fluorescence measurements which are commonly acquired under varying viewing and illumination geometries. During the growing seasons of 2012 and 2014, we collected canopy fluorescence, hyperspectral reflectance spectra, and biophysical measurements in corn plots established for nitrogen (N) and water augmentation within a USDA/Beltsville experimental cornfield also hosting an eddy covariance flux tower. There were four N application levels (0% N, 50% N, 100% N, 200% N) of the optimal level of 140 kg/ha, and two water availabilities (watered, not-watered) to produce eight treatment plots. We examined the behaviour of the Solar Induced Fluorescence (SIF) and the Photochemical Reflectance Index (PRI) during morning and afternoon periods for nine days throughout the 2012 growing season, including early, vegetative, reproductive, and senescent phenological growth stages. The SIF was retrieved from high resolution spectra using the 3-wavelength Fraunhofer Line Depth method in both oxygen absorption bands associated with the two chlorophyll fluorescence peaks in the red at 687 nm and the far-red at 760 nm. PRI was calculated as a Normalized Difference Reflectance Index using two wavelengths at 531 and 570 nm. At each collection, directional measurements were obtained for the SIFs and PRI at 15° view-zenith angle increments along the solar principal plane, in order to quantify the maximum anisotropy associated with the cornfield's bidirectional reflectance and fluorescence distribution functions. Along the solar principal plane, the maximum and minimum values at the hotspot and the coldspot, respectively, were determined and used to calculate an Anisotropy Difference Index (ANDI) as coldspot minus hotspot. We found that the various SIF values and PRI all exhibited significant anisotropy across the growing season, and these seasonal ANDI profiles differed significantly among these four observation types. Anisotropy for far-red SIF₇₆₀ always exceeded that for red SIF₆₈₇ (<0.001 W/(m²sr)), at each phenologic stage, and was greatest in mid-season during early reproductive phases. Also at mid-season, ANDI was highest for the SIF Ratio (SIF₆₈₇/SIF₇₆₀) but lowest for the PRI. ANDI for SIFs was most variable for watered plots over the season. Nitrogen application rates did not significantly affect the observed anisotropy. We include examples of data acquired in 2014 for cornfield anisotropy obtained with our custom tower-mounted FUSION system, and the new FluoWat leaf clip.