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

ABSTRACT: 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 CO₂ analyser on a field photosynthetic gas exchange system to make continuous online measurements of gas exchange and photosynthetic ¹³C discrimination (?¹³C) 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 ?¹³C, 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.

SUBMITTER: Stangl ZR 

PROVIDER: S-EPMC6612512 | biostudies-literature | 2019 Jul

REPOSITORIES: biostudies-literature