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1. On the Covariation of Chlorophyll Fluorescence and Photosynthesis Across Scales

   https://doi.org/10.1029/2020GL091098  

   Magney, Troy S.; Barnes, Mallory L.; Yang, Xi (December 2020, Geophysical Research Letters)

   Abstract Recent advances in remote sensing of solar‐induced chlorophyll fluorescence (SIF) have garnered wide interest from the biogeoscience and Earth system science communities, due to the observed linearity between SIF and gross primary productivity (GPP) at increasing spatiotemporal scales. Three recent studies, Maguire et al., (2020,https://doi.org/10.1029/2020GL087858), He et al. (2020,https://doi.org/10.1029/2020GL087474), and Marrs et al. (2020,https://doi.org/10.1029/2020GL087956) highlight a nonlinear relationship between fluorescence and photochemical yields and show empirical evidence for the decoupling of SIF, stomata, and the carbon reactions of photosynthesis. Such mechanistic studies help advance our understanding of what SIF is and what it is not. We argue that these findings are not necessarily contradictory to the linear SIF‐GPP relationship observed at the satellite scale and provide context for where, when, and why fluorescence and photosynthesis diverge at smaller spatiotemporal scales. Understanding scale dependencies of remote sensing data is crucial for interpreting SIF as a proxy for GPP. 

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2. Global‐Scale Consistency of Spaceborne Vegetation Indices, Chlorophyll Fluorescence, and Photosynthesis

   https://doi.org/10.1029/2020JG006136  

   Doughty, Russell; Xiao, Xiangming; Köhler, Philipp; Frankenberg, Christian; Qin, Yuanwei; Wu, Xiaocui; Ma, Shuang; Moore, III, Berrien (June 2021, Journal of Geophysical Research: Biogeosciences)

   Abstract The new TROPOspheric Monitoring Instrument (TROPOMI) solar‐induced chlorophyll fluorescence (SIF) data provides new opportunities to corroborate and improve global photosynthesis estimates. Here we report the spatiotemporal consistency between TROPOMI SIF and vegetation indices from the bidirectional reflectance distribution function (BRDF) adjusted (MCD43) and standard MODIS (MOD09) surface reflectance products, estimates of absorbed photosynthetically active radiation by chlorophyll (APAR_chl) derived from National Centers for Environmental Prediction Reanalysis‐2 (NCEP2), MODIS MCD18, and European Reanalysis (ERA5) data, and two GPP products (GPP_VPMand GPP_MOD17). We find (a) non‐adjusted VIs were more highly correlated with SIF at mid and high latitude than BRDF‐adjusted VIs, but were less correlated in the tropics, (b) negligible differences in the correlation between SIF and non‐adjusted NIRv and EVI, but BRDF‐adjusted NIRv had higher correlations with SIF at mid to high latitude than BRDF‐adjusted EVI but lower correlations in the tropics, (c) choice of PAR data set likely to cause substantial differences in global and regional GPP estimates and the correlation between modeled GPP and SIF, (d) SIF was more highly correlated with APAR_chlat high to mid latitude than EVI but more highly correlated with EVI at lower latitudes, and (e) GPP_VPMis more highly correlated with SIF than GPP_MOD17, except in sub‐Sahara Africa. Our results highlight that spaceborne photosynthesis would likely be improved by using a non‐linear response to PAR and that the fundamental differences between the vegetation indices and PAR data sets are likely to yield important differences in global and regional estimates of photosynthesis. 

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3. Recovery of metabolites via subnivean photosynthesis in Arctic tundra plants: Implications for climate change

   https://doi.org/10.1002/ecs2.4936  

   Wright‐Osment, Nicholas; Staudhammer, Christina Lynn; Oberbauer, Steve; Mortazavi, Behzad; Starr, Gregory (July 2024, Ecosphere)

   Abstract Plants have evolved numerous strategies for surviving the harsh conditions of the Arctic. One strategy for Arctic evergreen and semi‐evergreen species is to photosynthesize beneath the snow during spring. However, the prevalence of this photosynthesis and how recent photosynthates are used is still unknown. Here we ask,how is newly acquired carbon beneath the snow allocated?To answer this question, we delivered isotopically labeled¹³CO₂to tussock tundra plants before snowmelt. Soluble sugars and starches were preferentially enriched with¹³C in all five species tested, with lipids having comparatively low¹³C enrichment. These results provide evidence of the recovery of metabolites used over the long winter. Additionally, these new soluble sugars may function in photoprotection and cold tolerance as plants release from snow cover. Climate change, by reducing the duration of subnivean photosynthesis of these species, will limit metabolite production before snowmelt, which may lead to a reduction in the ability of these species to compete effectively during the growing season, potentially leading to changes in community structure. 

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4. The physiological basis for estimating photosynthesis from Chl a fluorescence

   https://doi.org/10.1111/nph.18045  

   Han, Jimei; Chang, Christine Y‐Y.; Gu, Lianhong; Zhang, Yongjiang; Meeker, Eliot W.; Magney, Troy S.; Walker, Anthony P.; Wen, Jiaming; Kira, Oz; McNaull, Sarah; et al (May 2022, New Phytologist)
5. Quantifying high‐temperature stress on soybean canopy photosynthesis: The unique role of sun‐induced chlorophyll fluorescence

   https://doi.org/10.1111/gcb.15603  

   Kimm, Hyungsuk; Guan, Kaiyu; Burroughs, Charles H.; Peng, Bin; Ainsworth, Elizabeth A.; Bernacchi, Carl J.; Moore, Caitlin E.; Kumagai, Etsushi; Yang, Xi; Berry, Joseph A.; et al (April 2021, Global Change Biology)

   Abstract High temperature and accompanying high vapor pressure deficit often stress plants without causing distinctive changes in plant canopy structure and consequential spectral signatures. Sun‐induced chlorophyll fluorescence (SIF), because of its mechanistic link with photosynthesis, may better detect such stress than remote sensing techniques relying on spectral reflectance signatures of canopy structural changes. However, our understanding about physiological mechanisms of SIF and its unique potential for physiological stress detection remains less clear. In this study, we measured SIF at a high‐temperature experiment, Temperature Free‐Air Controlled Enhancement, to explore the potential of SIF for physiological investigations. The experiment provided a gradient of soybean canopy temperature with 1.5, 3.0, 4.5, and 6.0°C above the ambient canopy temperature in the open field environments. SIF yield, which is normalized by incident radiation and the fraction of absorbed photosynthetically active radiation, showed a high correlation with photosynthetic light use efficiency (r = 0.89) and captured dynamic plant responses to high‐temperature conditions. SIF yield was affected by canopy structural and plant physiological changes associated with high‐temperature stress (partial correlationr = 0.60 and −0.23). Near‐infrared reflectance of vegetation, only affected by canopy structural changes, was used to minimize the canopy structural impact on SIF yield and to retrieve physiological SIF yield (Φ_F) signals. Φ_Ffurther excludes the canopy structural impact than SIF yield and indicates plant physiological variability, and we found that Φ_Foutperformed SIF yield in responding to physiological stress (r = −0.37). Our findings highlight that Φ_Fsensitively responded to the physiological downregulation of soybean gross primary productivity under high temperature. Φ_F, if reliably derived from satellite SIF, can support monitoring regional crop growth and different ecosystems' vegetation productivity under environmental stress and climate change. 

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