Sun‐Induced Chlorophyll Fluorescence, Photosynthesis, and Light Use Efficiency of a Soybean Field from Seasonally Continuous Measurements

Abstract

Recent development of sun‐induced chlorophyll fluorescence (SIF) technology is stimulating studies to remotely approximate canopy photosynthesis (measured as gross primary production, GPP). While multiple applications have advanced the empirical relationship between GPP and SIF, mechanistic understanding of this relationship is still limited. GPP:SIF relationship, using the standard light use efficiency framework, is determined by absorbed photosynthetically active radiation (APAR) and the relationship between photosynthetic light use efficiency (LUE) and fluorescence yield (SIFy). While previous studies have found that APAR is the dominant factor of the GPP:SIF relationship, the LUE:SIFy relationship remains unclear. For a better understanding of the LUE:SIFy relationship, we deployed a ground‐based system (FluoSpec2), with an eddy‐covariance flux tower at a soybean field in the Midwestern U.S. during the 2016 growing season to collect SIF and GPP data simultaneously. With the measurements categorized by plant growth stages, light conditions, and time scales, we confirmed that a strong positive GPP:SIF relationship was dominated by an even stronger linear SIF:APAR relationship. By normalizing both GPP and SIF by APAR, we found that under sunny conditions our soybean field exhibited a clear positive SIFy:APAR relationship and a weak negative LUE:SIFy relationship, opposite to the positive LUE:SIFy relationship reported previously in other ecosystems. Our study provides a first continuous SIF record over multiple growth stages for agricultural systems and reveals a distinctive pattern related to the LUE:SIFy relationship compared with previous work. The observed positive relationship of SIFy:APAR at the soybean site provides new insights of the previous understanding on the SIF's physiological implications.

Disentangling Changes in the Spectral Shape of Chlorophyll Fluorescence: Implications for Remote Sensing of Photosynthesis

“…This implies that the spectral shape is quite stable and that all wavelength ranges respond to changes in variable PSII fluorescence yield driven by PQ and NPQ; (2) far‐red SIF is more closely correlated to photosynthesis and APAR than red SIF (Figures ), because it is less prone to re‐absorption by Chl at both leaf and canopy scales (Fournier et al, ; Yang & van der Tol, ). At the canopy scale, the strong correlation between SIF and APAR (Figures and ) confirms previous results in crop canopies (Miao et al, ; Yang et al, ), which makes intuitive sense since APAR covaries with seasonal changes in leaf area in these systems, and is prone to less error than calculated GPP fluxes; and (3) Since wavelengths in the far‐red spectrum are less impacted by confounding factors such as chlorophyll or canopy structure (Figure S12), current satellite retrievals in this region may be better suited to assess stress induced downregulation of photosynthesis than changes in the ChlF spectral shape alone.…”

Sun-induced fluorescence and gross primary productivity during a heat wave

“…Radiative energy absorbed by chlorophyll molecules in excess of what can be used to carboxylate CO 2 , is either dissipated as heat or re-emitted as fluorescence 13 , which underlies the idea of inferring photosynthesis on the basis of measurements of chlorophyll fluorescence 28 . Even though the relationship between chlorophyll fluorescence and leaf photosynthesis is not unique 14 , tower- and satellite-based SIF measurements have been shown to scale with GPP as it changes during the season and/or with eco-climatological factors that govern the distribution of global biomes 15,17,18 . To a large degree, the explanatory power of these SIF-GPP relationships derives from the underlying significant changes in APAR across season and latitude 19 , which together with the light-use efficiency (LUE), determines GPP 29 .…”

“…Thus, even though fluorescence competes for the same excitation energy as photosynthesis, their relationship is neither unique nor simple 14 . Nevertheless, SIF has been repeatedly demonstrated to scale with GPP across broad gradients in productivity 15,16 and/or along the seasonal cycle 17,18 , driven mainly by underlying large changes in the magnitude of absorbed photosynthetically active radiation (APAR) 19 . It remains to be demonstrated, however, how well ecosystem-scale SIF is able to track changes in GPP in situations when APAR is constant, but GPP declines in response to stress.…”

Varying Contributions of Drivers to the Relationship Between Canopy Photosynthesis and Far‐Red Sun‐Induced Fluorescence for Two Maize Sites at Different Temporal Scales

“…SIF has been found strongly correlated with the gross primary production (GPP) in a quasi‐linear pattern at large spatial and temporal scales (Frankenberg et al, ; Guanter et al, ; Li et al, ; Miao et al, ; Wood et al, ; Yang et al, ; K. Yang, Ryu, et al, ; Yang et al, ). Some studies suggest the potential of a universal GPP:SIF relationship across various biomes (Frankenberg et al, ; Li et al, ; Sun et al, ; but see Zhang et al, ), and others used SIF to constrain photosynthetic parameters (Guan et al, ; Perez‐Priego et al, ; Zhang et al, ; Zhang et al, ).…”

“…Previous studies found that APAR is the primary driver that leads to the linear GPP:SIF relationship (Rossini et al, ; K. Yang, Ryu, et al, ; X. Yang, Shi, et al, ; Yang et al, ; Zhang et al, ). Some studies also suggested that LUE and SIF y contribute to the relationship and a diverse LUE:SIF y relationship implies a potentially diverse GPP:SIF relationship (Badgley et al, ; Miao et al, ; Wieneke et al, ). However, the contributions from PAR in , fPAR, LUE, and SIF y to GPP and SIF variations as well as the GPP:SIF relationship have not been specifically quantified and compared across a sufficiently wide range of conditions to provide a clear conclusion, and more studies of this issue are warranted.…”