Leaf anatomy does not explain the large variability of mesophyll conductance across C₃ crop species

Abstract

Increasing mesophyll conductance of CO 2 (g m ) is a strategy to improve photosynthesis in C 3 crops. However, the relative importance of different anatomical traits in determining g m in crops is unclear. Mesophyll conductance measurements were performed on 10 crops using the online carbon isotope discrimination method and the 'variable J' method in parallel. The influences of crucial leaf anatomical traits on g m were evaluated using a one-dimensional anatomical CO 2 diffusion model. The g m values measured using two independent methods were compatible, although significant differences were observed in their absolute values. Quantitative analysis showed that cell wall thickness and chloroplast stroma thickness are the most important elements along the diffusion pathway. Unexpectedly, the large variability of g m across crops was not associated with any investigated leaf anatomical traits except chloroplast thickness. The g m values estimated using the anatomical model differed remarkably from the values measured in vivo in most species. However, when the species-specific effective porosity of the cell wall and the species-specific facilitation effect of CO 2 diffusion across the membrane and chloroplast stoma were taken into account, the model could output g m values very similar to those measured in vivo. These results indicate that g m variation across crops is probably also driven by the effective porosity of the cell wall and effects of facilitation of CO 2 transport across the membrane and chloroplast stroma in addition to the thicknesses of the elements.

Greater leaf photosynthesis in the field by increasing mesophyll conductance via modified cell wall porosity and thickness in tobacco

“…g m includes CO 2 diffusion across multiple sequential barriers, each of which have an associated conductance g . The conductances across the intercellular airspace ( g ias ), cell wall ( g cw ) and membranes ( g mem ) are expected to have the largest effects on g m and can be estimated using measured values of g m , f ias , T cw , T mes , and S c 29,30 . Using the corresponding measured values presented in Fig.…”

“…The cell wall effective porosity ( p / τ ) can be determined from the cell wall conductance to CO 2 diffusion ( g cw ) provided the cell wall thickness T cw is known 73 . In turn, g cw can be estimated from g mc by accounting for the effect of other known barriers to CO 2 diffusion (specifically, the intercellular airspace, the plasma membrane, and the chloroplast envelope) 30,73 . Here we use this approach to calculate p / τ from measured values of g mc , f ias , T cw , T mes , and S c .…”

“…Here we use this approach to calculate p / τ from measured values of gmc, fias, Tcw, Tmes, and Sc. Overall, our method is similar to the one used in Ellsworth et al (2018) 73 , but differs by including the conductance across the intercellular airspace and a membrane conductance enhancement factor as in Xiong (2023) 30 . For details of the calculations, see Methods S2.…”

“…The copyright holder for this preprint this version posted February 15, 2024. ; https://doi.org/10.1101/2024.02.13.580201 doi: bioRxiv preprint conductance across the intercellular airspace and a membrane conductance enhancement factor as in Xiong (2023) 30 . For details of the calculations, see Methods S2.…”

The long and tortuous path towards improving photosynthesis by engineering elevated mesophyll conductance

“…However, these examples all involve changing the underlying biochemistry of leaves. Beyond changing biochemistry, an alternative route that has been proposed to increase photosynthetic rates is to reduce the diffusive limitations as CO2 moves from the atmosphere to the site of carboxylation (Gago et al, 2019;Salesse-Smith et al, 2023;Xiong, 2023). Of this diffusive limitation to photosynthesis, about half is the result of stomatal conductance, which confers some restriction to the rate at which CO2 can enter the leaf.…”