Plants possess a safety valve called Non-Photochemical Quenching (NPQ) to actively dissipate excess light energy as heat during the energy conversion process of photosynthesis. In recent years, there has been progress in understanding the molecular mechanism of NPQ, highlighting its important role in environmental adaptation. However, the impact of heat dissipation through NPQ has yet to be well-studied or quantified.
A research team led by Associate Professor Kenji Takizawa from the Astrobiology Center of the National Institutes of Natural Sciences, Assistant Professor Eunchul Kim from National Institute for Basic Biology, Professor Jun Kikawada, and Aoi Murakami from the Graduate University for Advanced Studies calculated the amount of heat emitted by NPQ in standard plants under sunlight. They estimated the effect of the resulting temperature increase inside leaves due to NPQ and the effect of the ground temperature increase when averaged on a global scale.
While the heat generated by NPQ is relatively small compared to the overall energy balance at both the cellular and global levels, the study demonstrated that it is non-negligible and could contribute to temperature increases under conditions where heat transfer is restricted.
This result was published in Frontiers in Plant Science (March 20, 2024).
Research Findings:
- Impact of NPQ on leaf internal temperature:
Using a combination of the amount of solar irradiance at noon in middle-latitude areas with absorption rates by light-harvesting complex and energy distribution rate at the photosynthetic reaction centers , they estimated that the thermal energy released as NPQ is approximately 64 Wm-2. By modeling the leaf structure consisting of epidermal tissue, palisade tissue, and spongy tissue, and calculating the internal temperature gradient when this heat is emitted from the central palisade tissue, they found that the normal temperature increase is slight (less than 0.1 degrees). However, under special conditions where heat conduction is limited to the air layer within the spongy tissue, the internal leaf temperature could potentially rise by up to about 1 degree Celsius. - Impact of NPQ on Earth’s Surface Temperature:
Considering variations in solar irradiance due to latitude, season, and time of day, as well as vegetation coverage, they estimated the overall average calorific value due to NPQ to be 2.2 Wm-2. This corresponds to approximately 0.55% of the total infrared radiation emitted from the Earth’s surface. While this percentage is small, it suggests that NPQ could have a comparable impact on Earth’s environment to recent greenhouse gas effects.
Prospects for the future
Perspectives in Plant Physiology :
Although the heat generated by NPQ is not effective in warming the entire leaf, it may locally and temporarily increase the temperature within the cell and chloroplast. The development of tiny temperature sensors such as metal nanoparticles is expected to clarify the temperature gradient around the chloroplast thylakoid membrane, thereby elucidating the energy distribution in the photosynthetic reaction process and its control by temperature.
Perspectives in Astrobiology :
Most plants on earth have evolved to maximize light collection and release excess energy as heat. Vegetation cover of the earth’s surface reduces reflectance and thus offsets the cold temperatures caused by the absorption of carbon dioxide. If plants that reflect most of the solar radiation and emit less heat evolve on an exoplanet different from Earth, the expansion of terrestrial vegetation could lead to more rapid cooling than on Earth.
Publication Journals
Magazine name：Frontiers in Plant Science
Publication date：March 20, 2024
Paper Title： How much heat does non-photochemical quenching produce?
Authors：Murakami A, Kim E, Minagawa J, and Takizawa K
