Downscaling of far-red solar-induced chlorophyll fluorescence of different crops from canopy to leaf level using a diurnal data set acquired by the airborne imaging spectrometer HyPlant

Abstract

Remote sensing-based measurements of solar-induced chlorophyll fluorescence (SIF) are useful for assessing plant functioning at different spatial and temporal scales. SIF is the most direct measure of photosynthesis and is therefore considered important to advance capacity for the monitoring of gross primary production (GPP) while it has also been suggested that its yield facilitates the early detection of vegetation stress. However, due to the influence of different confounding effects, the apparent SIF signal measured at canopy level differs from the fluorescence emitted at leaf level, which makes its physiological interpretation challenging. One of these effects is the scattering of SIF emitted from leaves on its way through the canopy. The escape fraction ( $f_{\mathrm{esc}}$ ) describes the scattering of SIF within the canopy and corresponds to the ratio of apparent SIF at canopy level to SIF at leaf level. In the present study, the fluorescence correction vegetation index (FCVI) was used to determine $f_{\mathrm{esc}}$ of far-red SIF for three structurally different crops (sugar beet, winter wheat, and fruit trees) from a diurnal data set recorded by the airborne imaging spectrometer HyPlant. This unique data set, for the first time, allowed a joint analysis of spatial and temporal dynamics of structural effects and thus the downscaling of far-red SIF from canopy ( ${\mathrm{SIF}}_{760}^{\mathrm{canopy}}$ ) to leaf level ( ${\mathrm{SIF}}_{760}^{\mathrm{leaf}}$ ). For a homogeneous crop such as winter wheat, it seems to be sufficient to determine $f_{\mathrm{esc}}$ once a day to reliably scale SIF₇₆₀ from canopy to leaf level. In contrast, for more complex canopies such as fruit trees, calculating $f_{\mathrm{esc}}$ for each observation time throughout the day is strongly recommended. The compensation for structural effects, in combination with normalizing SIF₇₆₀ to remove the effect of incoming radiation, further allowed the estimation of SIF emission efficiency ( ${\epsilon }_{\mathrm{SIF}}$ ) at leaf level, a parameter directly related to the diurnal variations of plant photosynthetic efficiency.

Keywords: Diurnal course; FCVI; Fluorescence correction vegetation index; Fluorescence escape fraction; HyPlant; Photosynthetically active radiation; SIF; Solar-induced chlorophyll fluorescence.