Nonphotochemical quenching does not alter the relationship between sun-induced fluorescence and gross primary production under heatwave

Sun-induced fluorescence (SIF) is a remote sensing signal that has recently received substantial attention due to its origin from plants' photosynthetic apparatus, which makes it more related to photosynthesis than reflectance-based vegetation indices (Mohammed et al., 2019). Upon absorption by the light-harvesting complex of photosystems, the energy can be used by four major paths: (1) photochemistry (p), which mainly, but not exclusively, results in gross primary production (GPP; or simply photosynthetic assimilation of carbon); (2) light-intensity-dependent heat dissipation, commonly referred to as nonphotochemical quenching (npq); (3) light-intensity-independent (or basal) heat dissipation (d); and (4) fluorescence (f), which is, in natural conditions, termed SIF. These four processes compete for the absorbed energy, and p and npq together normally constitute c. 80% of the energy use (Lazár, 2015). Only 1–2% of absorbed energy is normally emitted as f. The widespread notion of using this small proportion of emitted energy for the estimation of photosynthesis originates in the covariance of f and p that originate from reaction centres (RCs) due to npq that decreases the amount of energy reaching RCs, therefore available for further partitioning into p, f and d (Van Der Tol et al., 2014; Magney et al., 2020). However, the assumption of the proportional impact of npq on f and p and the stable share of p being used for GPP is close to the truth only in optimal conditions. The range of environmental stresses affecting plants decreases the SIF vs GPP correlation, and during severe stress, this correlation may cease to exist completely (Wieneke et al., 2018). One of the most impactful studies in regard to not only reporting the broken SIF vs GPP correlation during heat stress but also interpreting the plant physiology behind the broken correlation by leaf-level active Chl f measurements was published by Martini et al. (2022). However, it could not escape our attention, that this study somewhat misinterpreted the results and not very correctly assigned the broken correlation to nonphotochemical quenching (NPQ) of maximal fluorescence saturation. Therefore, we wrote this short commentary to point out overlooked factors from the article of Martini et al. (2022), its supplementary materials and raw data (10.5281/zenodo.5773208), and bring an impulse for a different interpretation of this interesting and important dataset.

Despite the title of Martini et al. (2022) suggests that the heatwave (HW) which occurred at the beginning of August 2018 caused the broken correlation of SIF and GPP, a look at the time series of the used data suggests that the correlation was also broken in the ‘normal’, pre-HW, conditions (Fig. 1). Before noon (9:00–12:00 h), SIF and GPP do not correlate as GPP is steadily decreasing from morning till afternoon, but SIF is increasing with increasing photosynthetically active radiation (PAR) and decreasing solar zenith angle (SZA) and peaking at noon. In the afternoon hours (14:00–16:00 h), when SIF decreases following PAR decrease and SZA increase, SIF and GPP may exhibit a positive correlation. During the HW and particularly on 3 and 6 August, SIF did not decrease in the afternoon but kept rising, which drives the negative correlation of SIF and GPP in HW. The positive correlation of pre-HW SIF and GPP is driven solely by differences among days. The within-day correlations are not significant for any of the 8 pre-HW days. The last 3 d in the pre-HW period even show a negative SIF vs GPP trend (Fig. 2). That, and decreasing SIF and GPP (Fig. 1), suggest that at least 31 July and 1 August were not pre-HW, but rather the beginning of HW days.

Modifications of xanthophylls, known as the xanthophyll cycle, are one of the major components of npq. As the xanthophyll pool of plants is not infinite and the other npq-related changes in photosynthetic apparatus (for a detailed description, see Ruban, 2016) cannot continue forever, the safe energy dissipation by npq may saturate under severe stress and high light intensity (Lazár, 2015; Ruban, 2016). However, despite the claims of Martini et al. (2022) about the role of NPQ in energy partitioning and SIF vs GPP correlation modulation making theoretical sense, the presented data do not support them. One of the main points of the publication is the NPQ saturation in the HW. Nevertheless, fig. 4 of Martini et al. (2022) does not support this idea when NPQ clearly does not saturate with decreasing GPP and also does not reach the saturation point with increasing VPD. The apparent saturation of SIF and NPQ is, paradoxically, driven by low values of NPQ. We would like to draw attention to the fact that the parameter NPQ is a ratio of the quantum yield of light-dependent heat dissipation (φNPQ) and the sum of the quantum yield of basal heat dissipation and quantum yield of fluorescence (Van Der Tol et al., 2014; Lazár, 2015; Kalaji et al., 2017). That means that NPQ also contains information about the fluorescence emission; therefore, it is not correct to use NPQ as an energy partitioning parameter explaining changes in SIF and GPP. Instead, φNPQ or, as the authors called it, NPQ yield, should be used to explain the use of absorbed energy for different pathways (Lazár, 2015; Kalaji et al., 2017). Correlating φNPQ with a quantum yield of photochemistry further supports no saturation of φNPQ in measured data and suggests that the excess energy was dissipated by npq also during HW (data not shown, but available at 10.5281/zenodo.5773208).

Martini et al. (2022) further argued that during the HW, there was a change in the energy allocation from NPQ towards SIF. This seems to be very unlikely not only for the reasons stated above but also because of the disproportionally high decrease in SIF described below.

If NPQ was not the cause of broken linearity between SIF and GPP, then the logical question is: what was causing the decoupling of SIF's and GPP's daily courses?

As mentioned earlier, in normal conditions, SIF and GPP correlate because the SIF intensity and amount of fixed carbon are both regulated by the amount of energy reaching RCs. Hence, they are both dependent on the absorbed PAR, which in turn largely depends on PAR reaching the top of the canopy (Van Der Tol et al., 2014; Magney et al., 2020). However, during heat and drought stress, photosynthesis is not limited by PAR but by water availability, which regulates the stomatas’ opening and closure (Farquhar et al., 1980). The limitation of gas exchange between the atmosphere and leaf interior induces a cascade of changes in photosynthetic apparatus activity, including upregulation of alternative electron sinks that can use the energy passed to p, causing a situation whereby SIF increases due to energy reaching RCs, but GPP does not increase because of electrons being used for alternative processes (Alric & Johnson, 2017).

The data from Martini et al. (2022) and the associated dataset (10.5281/zenodo.5773208) help us to understand the broken linearity of SIF and GPP. While SIF was strongly correlated with PAR, especially in the pre-HW period (Supporting Information Fig. S1B), GPP was not significantly correlated with PAR for any of the analysed days (Fig. S1A). However, GPP was negatively dependent on vapour pressure deficit (VPD, which is largely temperature-dependent) every day in pre-HW and HW periods (Fig. S1C). By contrast, SIF did not correlate significantly with VPD in the pre-HW period but correlated strongly in 3 of the 5 HW days (Fig. S1D). As high VPD is a main driver of stomata closure, these results indicate that stomatal limitation of photosynthesis was the main cause of the broken correlation between SIF and GPP during pre-HW, but stomatal conductance alone cannot explain the reverse relationship during HW (Grossiord et al., 2020). The below-discussed changes in SIF must be taken into account in severe heat conditions.

One of the strongest effects of HW reported by Martini et al. (2022) was a significant decrease in SIF. This decrease is undoubtedly caused by plant physiology; however, as seen in Fig. 1, the SIF retrieval stopped being reliable, as some of the reported values were negative. Negative SIF is a theoretical nonsense as negative emission of photons is impossible and practically had to result from lowering the signal-to-noise ratio to such an extent that even as simple a method as improved Fraunhofer line depth (iFLD) stopped yielding trustworthy results (Alonso et al., 2008). Correcting SIF by the near-infrared reflectance of vegetation (NIRv)-derived fluorescence escape ratio and absorbed PAR yielded even more negative values than top-of-canopy SIF itself. Such results point to our general lack of understanding of top-of-canopy SIF and top-of-canopy SIF-derived parameters during severe stress, such as temperatures above 40°C. Therefore, more attention should be paid to the impact of environmental and plant stress conditions on retrieved SIF reliability, which is an important outcome of the Martini et al. (2022) study that did not get enough attention in the publication.

The decrease in SIF in HW was disproportionally larger than the decrease in GPP or increase in NPQ. This would cause a large error in linear regression-based GPP estimation using either instantaneous or mid-day SIF. Moreover, such a large decrease in SIF due to severe stress is not well understood and should be further examined. One possible, although rather speculative, explanation for the observed large decrease in SIF is the temperature dependency of fluorescence emission after crossing the physiological threshold of plants (Kouřil et al., 2004). Kouřil et al. (2004) also reported an increase in fluorescence after leaves were heated to 40–45°C, similar to data presented in Fig. 1 and Fig. S1D during HW, especially on 3 and 6 August. Therefore, more consideration should be given to temperature as a factor influencing SIF in future studies performed in extreme heat conditions.

We believe that this Correspondence will induce a new way of thinking about the SIF : GPP : NPQ relationship, the absorbed energy partitioning between photochemical and nonphotochemical processes, the role of stomata in the SIF : GPP relationship and the changes in SIF emission under severe heat or other abiotic and biotic stress conditions.

None declared.

MA, RJ and AR developed the concept. MA prepared the figures and wrote the first draft. RJ and AR reviewed and edited the text.

These data were derived from the following resources available in the public domain: 10.5281/zenodo.5773208.

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