EGU General Assembly 2022
© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.

Photosynthetic acclimation under CO2 fertilization: new perspectives from current experiments

Yunke Peng1,2, Iain Colin Prentice3,4,5, Kevin Van Sundert6,7,8, Sara Vicca6, and Benjamin Stocker1,2
Yunke Peng et al.
  • 1Department of Environmental Systems Science, ETH, Universitätsstrasse 2, 8092 Zurich, Switzerland (
  • 2Swiss Federal Institute for Forest, Snow and Landscape Research WSL, Zürcherstrasse 111, 8903 Birmensdorf, Switzerland
  • 3Georgina Mace Centre for the Living Planet, Department of Life Sciences, Imperial College London, Silwood Park Campus, Buckhurst Road, Ascot SL5 7PY, UK
  • 4Department of Biological Sciences, Macquarie University, North Ryde, NSW 2109, Australia
  • 5Department of Earth System Science, Tsinghua University, Beijing 100084, China
  • 6Department of Biology, Plants and Ecosystems, University of Antwerp, Wilrijk, Belgium
  • 7Department of Earth System Science, Stanford University, USA
  • 8Ecological Synthesis Lab, Northern Arizona University, USA

Photosynthetic acclimation under CO2 fertilization is still incompletely understood. Observed reductions in the maximum rate of carboxylation (Vcmax) and electron transport (Jmax) under elevated CO2 (often called ‘down-regulation’) have been explained in various ways, including limited soil nitrogen (N) and phosphorus (P) availability, or a reduced demand for N. However, there remains large variation of the Vcmax decline and some non-sensitive or even positive responses are documented. The least cost hypothesis (Prentice et al., 2014) states that optimal photosynthesis is achieved at balanced unit costs of capacities of carboxylation and transpiration and predicts the acclimation of Vcmax and Jmax in response to the environment. Under elevated CO2, Vcmax is predicted to decline, while the ratio Jmax/Vcmax is predicted to increase - independent of N supply from the soil. In contrast, common model parametrisations conceive Vcmax to be controlled by soil N supply.

Here, we analyse a compilation of experimental results in an attempt to better understand photosynthetic acclimation to elevated CO2 and balance the evidence for contrasting model formulations. Within 38 CO2 fertilization plots investigated at forest, grassland and cropland,  Vcmax and Jmax are shown to decrease in concert, while the ratio Jmax/Vcmax increases with higher CO2 concentration, consistent with predictions from the least cost hypothesis. However, the predicted increase in the Jmax/Vcmax ratio is too large and the observed change in Vcmax is correlated with the change in soil inorganic N. Observed leaf N responses are broadly consistent with changes in Vcmax and Jmax. These findings support the idea acclimation of photosynthetic traits under enhanced CO2 is modulated by soil N supply. This can be explained by the direct decline of soil N availability at higher CO2 concentrations. However, it may also be caused by increase rates of net primary production (NPP) and N uptake that increase N sequestered in biomass under elevated CO2, in such a way to constrain labile soil N available for leaf-level photosynthesis.

Vcmax and Jmax responses to CO2 were also found to be negatively related to increases of above- and below-ground net primary production (ANPP, BNPP). This pattern might be explained by a ‘dilution effect’, due to a CO2-induced increase of leaf area index (LAI). However, it might also be due to plants having different stomatal responses to CO2. According to this hypothesis, at one end of the spectrum, the ratio of leaf intercellular CO2 (Ci) relative to ambient CO2 (Ca) remains constant; optimal photosynthesis increases, while optimal Vcmax declines. At the other end of the spectrum, Ci/Ca  decreases enough that Ci remains constant; then there is no increase in optimal photosynthesis, and no change in optimal Vcmax. Testing this hypothesis would require concomitant measurements of all of the relevant quantities (LAI, NPP, Ci/Ca) in multiple experiments.

How to cite: Peng, Y., Prentice, I. C., Sundert, K. V., Vicca, S., and Stocker, B.: Photosynthetic acclimation under CO2 fertilization: new perspectives from current experiments, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1858,, 2022.

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