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 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.

Estimating canopy gross primary production by combining phloem stable isotopes with canopy and mesophyll conductances

“…Under these conditions, the respiration effect is small (Barbour, Ryazanova, & Tcherkez, 2017; Tcherkez et al, 2017). This agrees with observed Δ e , the respiratory discrimination effect, by Stangl et al (2019), which averaged only 0.13‰ at our site. Including this value in the WUE i calculation would increase WUE i by 1.5 ± 0.2% and 1.6 ± 0.1% in the F and the R plots respectively.…”

“…Simplest would be to assume a constant mean value (Keenan, Sabate, & Gracia, 2010). For example, we estimated GPP with a constant g m = 0.31 mol CO 2 m −2 s −1 measured at the site (Stangl et al, 2019).…”

Revisiting the carbon isotope discrimination and water use efficiency relation: the influence of mesophyll conductance

“…We nevertheless recognize that variations in gsc/gm amongst functionally similar species or between studies is the main challenge in the use of iWUEmes. Current studies show that responses of gm to environmental factors are highly complex and such factors include CO2 (Flexas et al, 2007), temperature (von Caemmerer & Evans, 2013;Shrestha et al, 2019) and VPD (Loucos et al, 2017, Stangl et al, 2019. Up to now, variations in gm cannot be easily related to anatomical features and it is believed that biochemical processes (e.g.…”

“…In fact, the main limitation of the use of iWUEsim is the simplifying assumption that mesophyll conductance (gm, the diffusive conductance of CO2 from intercellular space to the site of carboxylation) is infinite (Seibt et al, 2008;Franks et al, 2013;Stangl et al, 2019). This assumption, theoretically, leads to an overestimation of iWUE (see Theory below) because gm is not conservatively high (for reviews see Flexas et al, 2008Flexas et al, , 2012 and thus this causes an increased CO2 drawdown from the atmosphere to chloroplasts and thereby decreases the actual isotope fractionation Δ.…”

“…In addition, while stomatal limitation of photosynthetic gas exchange has been found to be the dominant factor in the early stage of drought, the decrease in gm further restricts photosynthesis later on (Perez-Martin et al, 2014). Vapor pressure deficit (VPD), a key factor that influences both plant water potential and stomatal conductance, has been found to decrease (Loucos et al, 2017) or have no influence on gm (Warren, 2008;Stangl et al, 2019). In principle, a low sensitivity of gm to water deficit is beneficial for maintaining a high CO2 assimilation rate (Cano et al, 2014).…”