Diurnal Variation in Mesophyll Conductance and Its Influence on Modelled Water-use Efficiency in a Mature Boreal Pinus Sylvestris Stand

Mesophyll conductance (g) is a critical variable for the use of stable carbon isotopes to infer photosynthetic water-use efficiency (WUE). Although g is similar in magnitude to stomatal conductance (g), it has been measured less often, especially under field conditions and at high temporal resolution. We mounted an isotopic CO analyser on a field photosynthetic gas exchange system to make continuous online measurements of gas exchange and photosynthetic C discrimination (ΔC) on mature Pinus sylvestris trees. This allowed the calculation of g, g, net photosynthesis (A), and WUE. These measurements highlighted the asynchronous diurnal behaviour of g and g. While g declined from around 10:00, A declined first after 12:00, and g remained near its maximum until 16:00. We suggest that high g played a role in supporting an extended A peak despite stomatal closure. Comparing three models to estimate WUE from ∆C, we found that a simple model, assuming constant net fractionation during carboxylation (27‰), predicted WUE well, but only for about 75% of the day. A more comprehensive model, accounting explicitly for g and the effects of daytime respiration and photorespiration, gave reliable estimates of WUE, even in the early morning hours when WUE was more variable. Considering constant, finite g or g/g yielded similar WUE estimates on the diurnal scale, while assuming infinite g led to overestimation of WUE. These results highlight the potential of high-resolution g measurements to improve modelling of A and WUE and demonstrate that such g data can be acquired, even under field conditions.