Deriving diurnal variations in sun-induced chlorophyll-a fluorescence in winter wheat canopies and maize leaves from ground-based hyperspectral measurements

Remote sensing of solar-induced chlorophyll-a fluorescence (SIF) is a promising method for quantifying photosynthetic activity at the Earth's surface. Some recent studies therefore aim to clarify to what extend passive SIF measurements can be used to track photosynthetic processes. The present study contributes to that research by investigating the diurnal relationship between short-term variability of SIF and gas exchange rates in terms of stomatal conductance (g(s)). Quantification of SIF was performed using the Fraunhofer line discriminator (FLD) principle in the O-2-A absorption band and by applying a novel approach using the second derivative of the reflectance. Experimental measurements were conducted for two crops, namely winter wheat and maize under stressed and unstressed conditions. Overall, 47 high-spectral-resolution signatures and measurements of g(s) were collected on three different days during the 2014 growing season. During two days of unstressed conditions, diurnal variability of g(s) could - to a large extent (R-2 = 0.62 and 0.78) - be explained by the fraction of SIF in the target radiance flux between 655 and 665 nm (F-fraction). In an experiment with heat and water stress no diurnal relationship appeared between g(s) and F-fraction, which can be attributed to the occurrence of non-photochemical quenching under stress conditions.