Disentangling changes in the spectral shape of chlorophyll fluorescence: Implications for remote sensing of photosynthesis

Authors: MAGNEY, TROY - Jet Propulsion Laboratory; FRANKENBERG, CHRISTIAN - Jet Propulsion Laboratory; KOHLER, PHILIPP - Occidental College; NORTH, GRETCHEN - Occidental College; DAVIS, THOMAS - Colorad0 State University; DOLD, CHRISTIAN - Orise Fellow; DUTTA, DEBSUNDER - Occidental College; FISHER, JOSHUA - Jet Propulsion Laboratory; GROSSMANN, KATJA - University Of California; HARRINGTON, ALEXIS - Occidental College; Hatfield, Jerry; STUTZ, JOCHEN - University Of California; SUN, YING - Cornell University; PORCAR-CASTELL, ALBERT - University Of Helsinki

Submitted to: Journal of Geophysical Research-Biogeosciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/26/2019
Publication Date: 5/7/2019
Citation: Magney, T.S., Frankenberg, C., Kohler, P., North, G., Davis, T.S., Dold, C., Dutta, D., Fisher, J.B., Grossmann, K., Harrington, A., Hatfield, J.L., Stutz, J., Sun, Y., Porcar-Castell, A. 2019. Disentangling changes in the spectral shape of chlorophyll fluorescence: Implications for remote sensing of photosynthesis. Journal of Geophysical Research-Biogeosciences. 124(6):1491-1507. https://doi.org/10.1029/2019JG005029.
DOI: https://doi.org/10.1029/2019JG005029

Interpretive Summary: Satellite remote sensing provides a global picture of photosynthetic activity, allowing for an assessment of when, where, and how much carbon dioxide plants are extracting from the atmosphere. This is done by using a technique where satellites measure the small emission of energy from plants called chlorophyll fluorescence. This measurement is possible with only a few wavebands but reveals how efficient plant photosynthesis can be and when they become stressed for water or nutrients, this efficiency decreases. We evaluated the dynamics of this process to show how these measurements could be reliably used to infer photosynthesis for a number of crops. Fluorescence information from multiple wavelengths could provide insight into short-term responses of plants to stress, and long-term changes in canopy biochemistry and structure. These insights will help us improve satellite estimates of global photosynthesis and benefit scientists and policy makers in understanding how and why changes are occurring in global productivity of food.

Technical Abstract: Novel satellite measurements of solar-induced chlorophyll fluorescence (SIF) can improve our understanding of global photosynthesis; however, little is known about how to interpret the controls on its spectral variability. To address this, we disentangle simultaneous drivers of fluorescence spectra by coupling active and passive fluorescence measurements with photosynthesis. We show empirical and mechanistic evidence for where, why, and to what extent leaf fluorescence spectra change. Three distinct components explain more than 95% of the variance in leaf fluorescence spectra under both steady-state and changing illumination conditions. A single shape of fluorescence explains approximately 85% of the variance across a wide range of species. The magnitude of this shape responds to absorbed light and photosynthetic up/down regulation; meanwhile, chlorophyll concentration and non-photochemical quenching (NPQ) control approximately 7% and 2% of the remaining spectra variance, respectively. The spectral shape of fluorescence is remarkably stable where most current satellite retrievals occur (‘far-red’, >740nm), and dynamic downregulation of photosynthesis reduces fluorescence magnitude similarly across the 670-850nm range. When placed in context of future satellite missions, a small decrease in the red (approximately 685nm) to far-red (approximately 750nm) ratio of fluorescence – where remote sensing retrievals typically occur - tracks stress-induced photosynthetic downregulation at both leaf and canopy scales. However, at both of these scales, the subtle changes in red:far-red fluorescence is overshadowed in the longer term changes in canopy chlorophyll and structure. Our exploratory canopy-level SIF analysis informs how the fluorescence spectral shape can be used to disentangle simultaneous contributions from physiology, biochemistry, and canopy structure.