Re-absorption and scattering of chlorophyll fluorescence in canopies: A revised approach

The measurement of chlorophyll fluorescence in remote way represents a tool that is becoming increasingly important in relation to the diagnosis of plant health and carbon budget on the planet. However, the detection of this emission is severely affected by distortions, due to processes of light re-absorption both in the leaf and in the canopy. Even though some advances have been made to correct the signal in the far-red, the whole spectral range needs to be addressed, in order to accurately assess plant physiological state. In 2018, we introduced a model to obtain fluorescence spectra at leaf level, from what was observed at canopy level. In this present work, we publish a revision of that physical model, with a more rigorous and exact mathematical treatment. In addition, multiple scattering between the soil and the canopy, and the fraction of land covered by vegetation have also been taken into consideration. We validate this model upon experimental measures, in three types of crops of agronomic interest (Pea, Rye grass and Maize) with different architecture. Our model accurately predicts both the shape of fluorescence spectra at leaf level from that measured at canopy level and the fluorescence ratio. Furthermore, not only do we eliminate artifacts affecting the spectral shape, but we are also able to calculate the quantum yield of fluorescence corrected for re-absorption, from the experimental quantum yield at canopy level. This represents an advance in the study of these systems because it offers the opportunity to make corrections for both the fluorescence ratio and the intensity of the observed fluorescence.