Abstract
Non-photochemical quenching (NPQ) is a protective mechanism used by plants to safely dissipate excess absorbed light energy as heat, minimizing photo-oxidative damage. Although the importance of NPQ as a safety valve for photosynthesis is well-known, the physiological and environmental effects of the heat produced remain unclear because the amount of heat produced by NPQ is considered negligible, and its physiological effects have not been directly observed. Here, we calculated the heat produced by NPQ and evaluated its impact on the leaf and global warming based on simplified models. Our evaluation showed that the heat produced by NPQ in a given leaf area is 63.9 W m−2 under direct sunlight. Under the standard condition, NPQ warms up the leaf at less than 0.1°C, but it could be 1°C under particular conditions with low thermal conductance. We also estimated the thermal radiation of vegetation’s NPQ to be 2.2 W m−2 par global averaged surface area. It is only 0.55% of the thermal radiation by the Earth’s surface, but still significant in the current climate change response. We further discuss the possible function of NPQ to plant physiology besides the safety valve and provide strategies with artificial modification of the NPQ mechanism to increase food production and mitigate global warming.
Introduction
Green plants operate photosynthesis to convert light energy into chemical energy to produce organic compounds that support life on Earth. However, excessive light energy can lead to the production of reactive oxygen species and other harmful byproducts, which can damage photosynthetic machinery and impair photosynthetic efficiency (). Non-photochemical quenching (NPQ) is a protective process through which plants safely dissipate excess absorbed light energy as heat, thereby minimizing photo-oxidative damage (). However, despite its importance as a safety valve for photosynthesis, the effects of the heat produced by NPQ on photosynthetic reactions and surrounding environments remain unclear. Although NPQ is a complex phenomenon involving a variety of processes and is defined differently depending on the measurement method and object of observation (), it is used here as a term for regulative heat dissipation mechanisms induced under light irradiation, including the quickly reversible component and long-sustained component ().
Green plants utilize atmospheric carbon dioxide to produce organic carbon. Evidently, carbon dioxide has a greenhouse effect (), so planting trees in the desert is expected to be a mitigation strategy to reduce global warming. However, afforestation has multiple biophysical effects, such as cooling by evaporation and a warming effect due to lower albedo, therefore, it can no longer be said to mitigate global warming in general (). The effect of these plants on cooling or warming against the climate varies depending on the climate, latitude, and many unknown aspects in microhabitat and physiological response.
In this article, we estimated heat produced by NPQ and explored its significance in terms of physiological and environmental impacts. We used a simplified model to calculate the heat produced by NPQ because of its complexity, which is influenced by various environmental and biological factors, including variations in solar radiation and physiological responses. Solar radiation reaching the Earth’s surface is affected by the altitude angle and weather, and the amount of light reaching the leaf varies in complex ways depending on the canopy structure and the leaf angle. Even under the same light intensity, NPQ varies depending on the other environmental conditions, plant type, growth stage, and physiological conditions. Here, we present the simple and reliable estimation of heat produced by NPQ and its possible and maximum effects on plant physiology and the environment.
By advancing our understanding of the role of heat by NPQ, we can contribute to the development of more efficient and sustainable agricultural practices and a broader understanding of the role of photosynthesis in the Earth’s ecosystems and environment.
Energy conversion in photosynthesis and heat production by NPQ
Solar radiation reaching the Earth’s land surface, in terms of direct and circumsolar radiation, has a standard flux (ASTM G173-03) of 888 W m−2 (). Visible light (400–700 nm) constitutes approximately 42.3% of this radiation and is the portion that can be absorbed by leaves (Figure 1A). In general, leaves can only capture approximately 84% of the available visible light (). The absorbed energy is divided almost equally between the reaction centers in photosystem I (PSI) and photosystem II (PSII) () (Figure 1B). On the PSII side, the absorbed energy goes through three distinct pathways. First, charge separation occurs in the reaction center, facilitating the conversion of light energy into chemical energy. Second, intrinsic dissipation takes place within the chlorophyll. Finally, in the third pathway, excess energy is dissipated as heat through processes collectively referred to as NPQ, which regulates and protects the plant from potential damage (; ).
Figure 1
Most of the absorbed light energy (317 Wm-2 in Figure 1C) besides NPQ is converted to heat. A small fraction of energy (≈35% at the maximum) of the charge separation in PSI and PSII is ultimately fixed as sugar; the rest is released as heat during the intermediate process. The intrinsic energy dissipation is also mostly thermal except for a few percent of fluorescence emission (). The amount of heat released by NPQ is less than that of the other processes combined, so it has not received much attention. We dare to focus on NPQ and examine in the subsequent sections whether its heat generation effect is negligible.
Under moderate direct sunlight, photosynthetic activity is saturated, and more than half of the excitation energy around PSII flows into the NPQ pathway (Supplementary Figure S1). In addition to light intensity, NPQ varies with factors such as temperature, CO2 concentration, and canopy structure. However, the general trend of previous experiments is for NPQ energy partitioning to be 40-60% when PSII quantum yield is 20-30% (; ; ; ). In this calculation, we assumed the yield of NPQ at PSII as 50% and the solar radiation corresponds to the midday in the average latitude for the contiguous USA. According to the simplified energy conversion model shown in Figure 1C, the amount of heat produced by NPQ is 63.9 W m−2, equivalent to 20.3% of absorbed light energy and 7.2% of solar radiation on the land surface.
Increase in leaf temperature by NPQ
Plant leaves are known to keep their temperature appropriate for photosynthesis even in direct sunlight () through transpiration, convection, and radiation. The heating effect of NPQ has been considered negligible compared to the cooling effects. When the stomata is closed and there is no transpiration, leaf surface temperature significantly increase under illumination. The previous thermoimaging analysis of tobacco leaves showed the leaf temperature increase by 1-5°C, and the enhancing NPQ increased leaf temperature increase by 10% (). Under these conditions, the inside leaf temperature could be higher than the surface temperature. However, the amount of heat accumulated in leaves has not been fully evaluated because of the lack of effective probes for sensing the temperature inside the leaf tissues and in a single cell.
We assumed leaf condition to form a maximum temperature difference between the outside, closed stomata, no transpiration, and no convection in the intercellular spaces. In this case, the heat transfer is evaluated on the 1D-geometry model, as shown in Figure 2A, using the simplified Fourier’s heat transfer equation:
Figure 2
Where ΔT, ϕq, k, and dx are the temperature difference (K), heat flux (W m−2), thermal conductivity (W m−1 K–1), and length of heat transfer (m) respectively.
A typical leaf tissue structure consists of a tightly packed palisade mesophyll layer on the upper (adaxial) side and a spongy mesophyll layer on the lower (abaxial) side (Figure 2A). When we assume that the heat produced by NPQ is mainly generated in the palisade layer, the value of ΔT formed by abaxial side heat transfer via cells and adaxial side heat transfer via air (ΔTabaxial+ΔTadaxial=ΔTleaf) can be driven from the following equation:
Assuming the relative thickness of the upper epidermis, palisade, and spongy mesophyll to be 12%, 36%, and 52%, respectively, as in the case of Arabidopsis (
Although the above calculations show that NPQ could increase leaf tissue temperature by 1°C at the maximum under the extreme conditions, the temperature inside the chloroplast could be much higher. The temperature dynamics and subcellular variation inside living cells have been obscure for a long time. In mammalian cells, recently developed nanothermometry probes enable the measurement of intracellular temperature. It is revealed that the mitochondrial temperature could increase by 10°C by stimulating respiration, which is 105 times higher than the theoretical estimation (
Increase in global temperature by NPQ
The Earth’s thermal equilibrium is determined by the balance between the incoming shortwave (300-3,000 nm) radiation (SWR) from the sun and the outgoing longwave (5-25 μm) radiations (LWR) from the land surface and the atmosphere. The development of land vegetation not only has a cooling effect as it absorbs atmospheric CO2 and latent heat by transpiration but also has a warming impact due to the absorption of light energy and its conversion to heat. In this section, we calculated the share of NPQ in the global heat budget and estimated the effect on global temperature.
In previous sections, we calculated that the heat produced by NPQ is 63.9 W m–2 at noon in the mid-latitude, where the solar zenith angle is 48.19 degrees. This section evaluates thermal radiation by NPQ globally by considering changing solar zenith angle by diurnal cycle, annual cycle, and latitude. Time-averaged SWR at each latitude of the Earth (Q(ϕ)) is calculated using Python’s climlab package (
Where ta, rland, rveg, rNPQ, and εveg are atmospheric transmittance, the ratio of land area to global area, the ratio of vegetation area to the land area, the ratio of NPQ thermal energy to solar radiation on the land surface, and emissivity of vegetation respectively. ta is estimated to be 0.47 from the ratio of the surface irradiance at 160 W m-2 to the incoming solar irradiance at 340 W m-2 (
A weighted average of INPQ (ϕ) for each latitude (ϕ i) from 90 degrees north to 90 degrees south gives a global average value (INPQglobal).
The calculation results in 2.20 W m–2 for INPQglobal. The global average of Q (ϕ) calculated from the weighted average is at 341 W m-2, consistent with the latest estimation by IPCC at 340 W m-2 (
Finally, the relationship between the thermal radiation by NPQ and the Earth’s surface temperature is calculated from Stefan-Boltzmann’s law in the following equation:
Where I, ε, σ, and T are radiant emittance (W m–2), emissivity (0.98), Stefan–Boltzmann constant (5.67×10–8 W m–2 K–4), and temperature (K), respectively. Considering that LWR from the land surface at 398 W m–2, the surface temperature is 290.9 K (17.8°C). If the radiant emittance corresponding to NPQ (2.2 W m-2) is removed, the surface temperature decreases by 0.4 K to 290.5 K. The result indicates that NPQ contributes to an increase in the Earth’s surface temperature of 0.4°C.
Discussion
Heat dissipation by NPQ is a primary feedback regulation mechanism in photosynthesis under excessive light, and its mechanism has been intensely studied for decades (
Conversely, we cannot ignore the possibility that thermal energy, including NPQ, is involved in the mechanisms to optimize photosynthetic functions. Illumination of dark-adapted plants induces a pronounced thermal energy dissipation and activates the light-harvesting antenna complex, transferring excitations toward the reaction centers (
If NPQ significantly raises the intracellular temperature, it could affect biochemical and physiological reactions in photosynthesis. Heat production should be beneficial for plants to protect them from chilling stress. Snow algae and some cold-tolerant plants can perform photosynthesis under sub-freezing temperatures. It could be possible that these plants maintain local temperatures around photosynthetic reaction centers. Evergreen plants from high latitudes can sustain high rates of NPQ during winter (
Thermal production at moderate and high temperatures does not accelerate photosynthesis but might downregulate it safely. High temperature and high light stress often co-occur, and they can induce similar photoprotective mechanisms, such as state transitions (
The artificial control of NPQ can also be used as a strategy to modulate global temperature. The radiation from plant NPQ is about 2.2 W m–2 par surface area of the earth. It is less than 1% of the thermal radiated from the Earth’s surface, but it cannot be ignored. The climate forcing caused by the doubling of atmospheric CO2 by human activities has perturbed Earth’s energy balance about 4 W m–2 (
The environment and living organisms have coevolved in the Earth’s long history. The evolution and expansion of terrestrial plants could have made the environment cooler and caused the Ice Age (
Statements
Data availability statement
The original contributions presented in the study are included in the article/Supplementary Material. Further inquiries can be directed to the corresponding author/s.
Author contributions
AM: Investigation, Writing – original draft, Writing – review & editing. EK: Investigation, Writing – original draft, Writing – review & editing. JM: Conceptualization, Writing – review & editing. KT: Investigation, Project administration, Writing – original draft, Writing – review & editing.
Funding
The author(s) declare financial support was received for the research, authorship, and/or publication of this article. This work was supported by JSPS KAKENHI Grant Number JP21H05040 to JM, JP23H04960 to EK and JM, and JP23K14216 to EK; Takeda Science Foundation to EK.
Conflict of interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Publisher’s note
All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.
Supplementary material
The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fpls.2024.1367795/full#supplementary-material
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Summary
Keywords
photosynthesis, non-photochemical quenching (NPQ), heat transfer, heat budget, global warming
Citation
Murakami A, Kim E, Minagawa J and Takizawa K (2024) How much heat does non-photochemical quenching produce?. Front. Plant Sci. 15:1367795. doi: 10.3389/fpls.2024.1367795
Received
09 January 2024
Accepted
08 March 2024
Published
20 March 2024
Volume
15 - 2024
Edited by
Artur Nosalewicz, Polish Academy of Sciences, Poland
Reviewed by
Dusan Lazar, Palacký University, Czechia
Conrad Mullineaux, Queen Mary University of London, United Kingdom
Wieslaw Gruszecki, Maria Curie-Sklodowska University, Poland
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© 2024 Murakami, Kim, Minagawa and Takizawa.
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*Correspondence: Kenji Takizawa, kenji-t@nibb.ac.jp
†These authors have equally contributed to this work and share first authorship
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All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article or claim that may be made by its manufacturer is not guaranteed or endorsed by the publisher.