Diurnal variation in mesophyll conductance and its influence on modelled water-use efficiency in a mature boreal Pinus sylvestris stand

Mesophyll conductance (g(m)) is a critical variable for the use of stable carbon isotopes to infer photosynthetic water-use efficiency (WUE). Although g(m) is similar in magnitude to stomatal conductance (g(s)), it has been measured less often, especially under field conditions and at high temporal resolution. We mounted an isotopic CO(2) analyser on a field photosynthetic gas exchange system to make continuous online measurements of gas exchange and photosynthetic (13)C discrimination (Δ(13)C) on mature Pinus sylvestris trees. This allowed the calculation of g(m), g(s), net photosynthesis (A(net)), and WUE. These measurements highlighted the asynchronous diurnal behaviour of g(m) and g(s). While g(s) declined from around 10:00, A(net) declined first after 12:00, and g(m) remained near its maximum until 16:00. We suggest that high g(m) played a role in supporting an extended A(net) peak despite stomatal closure. Comparing three models to estimate WUE from ∆(13)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(m) 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(m) or g(m)/g(s) yielded similar WUE estimates on the diurnal scale, while assuming infinite g(m) led to overestimation of WUE. These results highlight the potential of high-resolution g(m) measurements to improve modelling of A(net) and WUE and demonstrate that such g(m) data can be acquired, even under field conditions. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (10.1007/s11120-019-00645-6) contains supplementary material, which is available to authorized users.