To photosynthesise, plants open up their stomata to allow CO2 from the ambient air to diffuse in. The rate of photosynthesis depends on this drawdown of CO2 from the ambient air at the leaf surface, through the intercellular air spaces (Ci) and finally, to the site of carboxylation in the chloroplasts (Cc).

When state-of-the-art climate models calculate photosynthesis, they assume Ci = Cc, which means they ignore mesophyll resistance in the “middle of the leaf”. Unfortunately, this assumption is at odds with the latest research that shows notably variability in rates of mesophyll conductance, which means models may be incorrectly estimating the response of photosynthesis to atmospheric CO2.

Land surface schemes used in climate models do not simply ignore this potentially important process out of convenience. Mesophyll conductance has been shown to vary with a number of environmental stimuli and in theory, is constrained by leaf traits (e.g. cell wall thickness, chloroplast surface area). In short, we do not yet have a complete understanding of this complex physiological property but it is important that we explore how it might affect future responses of the terrestrial biosphere to increasing atmospheric CO2 concentrations.

In this paper, the researchers used an extensive literature survey to parameterise an explicit implementation of mesophyll conductance in the Germany land surface model, JSBACH. Their model results showed notable sensitivity of higher latitudes to increased atmospheric CO2 but more modest responses for temperate and tropical regions. The results suggest that accounting for mesophyll conductance may have important implications for carbon and water fluxes in boreal regions.