Solar‐Induced Fluorescence Does Not Track Photosynthetic Carbon Assimilation Following Induced Stomatal Closure

Abstract

Since 2006, six satellites measuring solar‐induced chlorophyll fluorescence (SIF) have been launched to better constrain terrestrial gross primary productivity (GPP). The promise of the SIF signal as a proxy for photosynthesis with a strong relationship to GPP has been widely cited in carbon cycling studies. However, chlorophyll fluorescence originates from dynamic energy partitioning at the leaf level and does not exhibit a uniformly linear relationship with photosynthesis at finer scales. We induced stomatal closure in deciduous woody tree branches and measured SIF at a proximal scale, alongside leaf‐level gas exchange, pulse amplitude modulated (PAM) fluorescence, and leaf pigment content. We found no change in SIF or steady‐state PAM fluorescence, despite clear reductions in stomatal conductance, carbon assimilation, and light‐use efficiency in treated leaves. These findings suggest that equating SIF and photosynthesis is an oversimplification that may undermine the utility of SIF as a biophysical parameter in GPP models.

On the Covariation of Chlorophyll Fluorescence and Photosynthesis Across Scales

“…The third recent study (Marrs et al, 2020) used a controlled experiment to test the relationship between fluorescence (PAM and SIF), stomatal conductance, and net photosynthesis. Marrs and colleagues induced stomatal closure (through pressure cuffs and abscisic acid) over a short time period (1 day) to show that no changes in steady‐state chlorophyll fluorescence or SIF were observed while clear reductions in stomatal conductance and carbon assimilation occurred.…”

“…Over longer time scales, and especially those observed from space (temporal frequency >1 week), it is likely that the relationship between electron transport rate (Step 1) and carbon assimilation (Step 2) will be similar (Genty et al, 1989); however, more research along these lines is warranted, particularly under severe water and temperature stress. Notably, the Marrs et al (2020) paper suggests that the observed linear relationship between SIF and GPP at increasing scales is likely due to their common drivers: light absorbed by chlorophyll molecules and photochemical regulation. Understanding these scaling nuances are thus crucial for interpreting satellite measurements of SIF and its relationship to GPP.…”

“…More recently, three studies at the leaf and canopy scales in this journal have sought to clarify the link between chlorophyll fluorescence and photosynthesis (He et al, 2020; Maguire et al, 2020; Marrs et al, 2020). To summarize these findings, we outline three steps necessary to translate a fluoresced photon at the photosystem scale to an estimate of GPP at the ecosystem scale: Step (1) understanding the fate of absorbed photons; Step (2) translating fluoresced photons to carbon assimilation; and Step (3) the scale dependence of the SIF‐GPP relationship (Figure 1).…”

“…(c, f ) Step 3: Scaling the SIF:Photosynthesis relationship across time and space. (c) The time plot is hypothetical and represents findings from Marrs et al (2020) and suggests that following a natural or imposed stress to plant hydraulics, stomata are likely to respond prior to a change in pigments, canopy structure or photosystems. Over longer time periods the relative change in SIF will be much greater than that in ΦF due to changes in canopy structure and absorbed photosynthetically active radiation by chlorophyll (APAR chl ).…”

Partitioning Net Ecosystem Exchange (NEE) of CO2 Using Solar‐Induced Chlorophyll Fluorescence (SIF)

“…This equation adopts the hyperbolic light‐response‐curve used in DT but substitutes the global radiation ( R g ) with SIF, where a and b are empirical parameters to be fitted. This formulation has the flexibility to characterize both the linear and nonlinear SIF‐GPP patterns reported in literature (Li et al., 2018; Marrs et al., 2020; Y.Zhang et al., 2016; Sun et al., 2017). The nonlinearity usually occurs at short time scales (e.g., subdaily, when PAR dominates the co‐variations of SIF and GPP) and/or under stress, modulated by physiological factors, such as q L , NPQ, and/or CO 2 availability in the vicinity of Rubisco (Gu et al., 2019b).…”