Linking canopy‐scale mesophyll conductance and phloem sugar δ¹³C using empirical and modelling approaches

Abstract

 Interpreting phloem carbohydrate or xylem tissue carbon isotopic composition as measures of water-use efficiency or past tree productivity requires in-depth knowledge of the factors altering the isotopic composition within the pathway from ambient air to phloem contents and tree-ring. One of least understood of these factors is mesophyll conductance (g m).  We formulated a dynamic model describing the leaf photosynthetic pathway including seven alternative g m descriptions and a simple transport of sugars from foliage down the trunk. We parameterized the model for a boreal Scots pine stand and compared simulated g m responses to weather variations. We further compared the simulated δ 13 C of new photosynthates among the different g m descriptions and against measured phloem sugar δ 13 C.  Simulated g m estimates of the seven descriptions varied according to weather conditions resulting in varying estimates of phloem δ 13 C during cold / moist and warm / dry periods. The model succeeded in predicting a drought response and a post-drought release in phloem sugar δ 13 C indicating suitability of the model for inverse prediction of leaf processes from phloem isotopic composition.  We suggest short-interval phloem sampling during and after extreme weather conditions to distinguish between mesophyll conductance drivers for future model development.

Limits to photosynthesis: seasonal shifts in supply and demand for CO2 in Scots pine

“…The modelling of mesophyll conductance is an important issue because g m serves as a linkage between the C isotopic composition of plant tissues and photosynthetic gas exchange, including WUE (Stangl et al, 2019;Gimeno et al, 2021;Ma et al, 2021;Schiestl-Aalto et al, 2021). The adjustment is necessary because isotopic composition is determined in the chloroplast, whereas H 2 O vapour evaporates from the intercellular space through the stomata and this path-length difference must be accounted for by incorporating the g s /g m term into the calculations.…”

“…It would be worthwhile to test these alternative models against the range of conditions from top to bottom of the canopy. We have described such a model in a recent paper, but there we assumed that g m does not change vertically within the crown (Schiestl-Aalto et al, 2021). We hope to explore the incorporation of seasonal variation and the assumption of constant mesophyll limitation in future modelling work (Schiestl-Aalto et al, 2021).…”

“…We have described such a model in a recent paper, but there we assumed that g m does not change vertically within the crown (Schiestl-Aalto et al, 2021). We hope to explore the incorporation of seasonal variation and the assumption of constant mesophyll limitation in future modelling work (Schiestl-Aalto et al, 2021). To do so here would be beyond the scope of the current study.…”

“…Consequently, g s is correlated with the rate of photosynthesis, the rate of transpiration, and the evaporative demand (vapour pressure deficit, VPD) of the ambient air (Katul et al, 2009;Medlyn et al, 2011). Mesophyll conductance and C c remain difficult to measure in the field; therefore, data on the response of g m to different environmental parameters under natural conditions is limited (Flexas, 2016;Rogers et al, 2017;Dewar et al, 2018;Stangl et al, 2019;Schiestl-Aalto et al, 2021). Nevertheless, it has received substantial interest because an increase in mesophyll conductance could enhance C uptake without increasing H 2 O loss in the process (Flexas et al, 2013;Flexas, 2016), thus having a substantial impact on water-use efficiency (WUE) (Stangl et al, 2019;Ma et al, 2021).…”

Isotopic Branchpoints: Linkages and Efficiencies in Carbon and Water Budgets

“…The isotopic parameters could thus be used either as model parameters or to provide a post hoc test of the model predictions. For example, a recently developed dynamic model of the isotopic composition of phloem contents has been tested against measurements from the European summer drought of 2018 (Schiestl‐Aalto et al., 2020). The model succeeded in predicting the pronounced changes in δ 13 C and WUE that occurred during this unusual event.…”