Improved method for measuring the apparent CO₂ photocompensation point resolves the impact of multiple internal conductances to CO₂ to net gas exchange

Abstract

There is a growing interest in accurate and comparable measurements of the CO2 photocompensation point (Γ*), a vital parameter to model leaf photosynthesis. The Γ* is measured as the common intersection of several CO2 response curves, but this method may incorrectly estimate Γ* by using linear fits to extrapolate curvilinear responses and single conductances to convert intercellular photocompensation points (Ci *) to chloroplastic Γ*. To determine the magnitude and minimize the impact of these artefacts on Γ* determinations, we used a combination of meta-analysis, modelling and original measurements to develop a framework to accurately determine Ci *. Our modelling indicated that the impact of using linear fits could be minimized based on the measurement CO2 range. We also propose a novel method of analysing common intersection measurements using slope-intercept regression. Our modelling indicated that slope-intercept regression is a robust analytical tool that can help determine if a measurement is biased because of multiple internal conductances to CO2 . Application of slope-intercept regression to Nicotiana tabacum and Glycine max revealed that multiple conductances likely have little impact to Ci * measurements in these species. These findings present a robust and easy to apply protocol to help resolve key questions concerning CO2 conductance through leaves.

Variable Mesophyll Conductance among Soybean Cultivars Sets a Tradeoff between Photosynthesis and Water-Use-Efficiency

“…S2). The mean C i * here falls between the value of G* from tobacco (Nicotiana tabacum; Bernacchi et al, 2002) commonly used for soybean (Bernacchi et al, 2005;Rosenthal et al, 2014;Köhler et al, 2017) and a value calculated from soybean Rubisco kinetic properties (Gallé et al, 2013), and it further matches exactly the C i * estimated for soybean with similar methodology (Walker and Ort, 2015). Thus, the mean C i * value was used as a proxy for G* in all calculations.…”

“…C i * was taken as the absolute value of the slope from the second regression, while R d was taken as the y intercept. We found no differences between C i * and R d calculated by the slope-intercept method of Walker and Ort (2015) and the traditional common-slope method (paired Student's t tests, P . 0.1 for both) or the SD values of these estimates.…”

“…C i * and R d were calculated by the modified slope-intercept method of Walker and Ort (2015). For each cultivar, a linear regression was performed on the aggregated CO 2 -response curves for a given PPFD.…”

The Costs of Photorespiration to Food Production Now and in the Future

“…This extended model partitions photorespired carbon loss between reentry into the chloroplast and release into the intercellular air space. It is still not certain whether such an extended model is necessary to interpret net carbon dioxide exchange around ambient carbon dioxide levels, and a recent advance in interpreting measurements of * indicates that the simple model presented in Equation 4 may explain most of the carbon dioxide transfer to the chloroplast, at least in soybean and wheat (81). Because the present model of mesophyll conductance appears to adequately describe leaf-level gas exchange, it seems reasonable to continue using it for higher-scale modeling until additional evidence is presented to the contrary.…”

What gas exchange data can tell us about photosynthesis

“…In this issue of PC&E, Walker and Ort (2015) provide a method for analysing Γ* data so that the effects of small errors and uncertainties are minimized, giving a more robust estimate of Γ *.…”