Gross Primary Production of a Wheat Canopy Relates Stronger to Far Red Than to Red Solar-Induced Chlorophyll Fluorescence

Abstract

Sun-induced chlorophyll fluorescence (SIF) is a radiation flux emitted by chlorophyll molecules in the red (RSIF) and far red region (FRSIF), and is considered as a potential indicator of the functional state of photosynthesis in remote sensing applications. Recently, ground studies and space observations have demonstrated a strong empirical linear relationship between FRSIF and carbon uptake through photosynthesis (GPP, gross primary production). In this study, we investigated the potential of RSIF and FRSIF to represent the functional status of photosynthesis at canopy level on a wheat crop. RSIF and FRSIF were continuously measured in the O 2 -B (SIF687) and O 2 -A bands (SIF760) at a high frequency rate from a nadir view at a height of 21 m, simultaneously with carbon uptake using eddy covariance (EC) techniques. The relative fluorescence yield (Fyield) and the photochemical yield were acquired at leaf level using active fluorescence measurements. SIF was normalized with photosynthetically active radiation (PAR) to derive apparent spectral fluorescence yields (ASFY687, ASFY760). At the diurnal scale, we found limited variations of ASFY687 and ASFY760 during sunny days. We also did not find any link between Fyield and light use efficiency (LUE) derived from EC, which would prevent SIF from indicating LUE changes. The coefficient of determination (r 2 ) of the linear regression between SIF and GPP is found to be highly variable, depending on the emission wavelength, the time scale of observation, sky conditions, and the phenological stage. Despite its photosystem II (PSII) origin, SIF687 correlates less than SIF760 with GPP in any cases. The strongest SIF-GPP relationship was found for SIF760 during canopy growth. When canopy is in a steady state, SIF687 and SIF760 are almost as effective as PAR in predicting GPP. Our results imply some constraints in the use of simple linear relationships to infer GPP from SIF, as they are expected to be better predictive with far red SIF for canopies with a high dynamic range of green biomass and a low LUE variation range.

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

“…At the leaf scale, we show that the spectrally integrated spectrum is only modulated by 8% through the reabsorption of emitted ChlF, but by up to about 20% in the red region (Figure c); notably, the spectral and sample weights would be significantly amplified for highly chlorotic leaves, which are not included in this study. However, at the canopy scale, we do observe an amplification of the red:far‐red SIF ratio during senescence (Figure d) because more red photons are prone to escape the canopy when leaves become highly chlorotic (Figures S9 and S10; Fournier et al, ; Goulas et al, ; Yang & van der Tol, ). Further, while we attribute PC2 at steady‐state changes primarily to chlorophyll concentration, confounding effects of leaf‐level radiative transfer should not be negated.…”

“…The seasonal trends in red:far‐red SIF are highest early and late in the season, and consistently low during peak greenness—negatively related to NDVI for most of the season, and positive during rapid vegetative growth (first 10 days; Figures d and e). This suggests that high red:far‐red SIF ratios early in the season are likely the result of low green leaf area; meanwhile, Chl concentration (spectroscopically retrieved using the equation described in Datt, ) is high at this time—which we would expect to enhance the reabsorption of red photons at the leaf scale (Goulas et al, ). As the crop begins to senesce later in the season, the red:far‐red SIF ratio also increases due to a depletion of Chl pigments (Figures e and f).…”

Variability of sun‐induced chlorophyll fluorescence according to stand age‐related processes in a managed loblolly pine forest

“…(), which found the red fluorescence as the most sensitive to the canopy net photosynthesis. Unfortunately, only a few recent studies (Cheng et al., ; Goulas et al., ; Joiner, Yoshida, Guanter, & Middleton, ; Louis et al., ; Middleton et al., ; Rossini et al., ; Wieneke et al., ) exploiting both red SIF and far‐red SIF have been conducted, and future investigations are necessary to consolidate the results found in this study.…”

“…The fact that red fluorescence, rather than far-red, seems more sensitive to describe these physiological processes can be considered in line with the recent study of Verrelst et al (2016), which found the red fluorescence as the most sensitive to the canopy net photosynthesis. Unfortunately, only a few recent studies (Cheng et al, 2013;Goulas et al, 2017;Joiner, Yoshida, Guanter, & Middleton, 2016;Louis et al, 2005;Middleton et al, 2015;Rossini et al, 2015;Wieneke et al, 2016) Verhoef, Timmermans, Verhoef, & Su, 2009) coupled with ecosystem process model for estimating storage and flux of carbon, nitrogen and water (e.g., BIOME-BGC, Running & Gower, 1991) in future research may help in better describing and understanding the role of fluorescence in age-related processes.…”

“…which will be used to derive the photosynthetic activity of natural and managed ecosystems. Fluorescence is considered the most direct proxy of actual photosynthetic activity available from remote sensing techniques and as such it has been used extensively to track plant status at leaf and canopy level (Ac et al, 2015;Cheng et al, 2013;Damm et al, 2010;Daumard et al, 2010;Goulas et al, 2017;Joiner et al, 2014;Koffi, Rayner, Norton, Frankenberg, & Scholze, 2015;Meroni & Colombo, 2006;Meroni et al, 2008;Middleton, Huemmrich, Cheng, & Margolis, 2012;Moya et al, 2004;Rascher et al, 2009;Rossini et al, 2010Rossini et al, , 2015Zarco-Tejada, Gonz alez-Dugo, & Berni, 2012;Zarco-Tejada, Gonz alez-Dugo, & Fereres, 2016;Zhang et al, 2014).…”