Importance of leaf anatomy in determining mesophyll diffusion conductance to CO2 across species: quantitative limitations and scaling up by models

TLDR: The results demonstrate the major role of anatomy in constraining mesophyll diffusion conductance and, consequently, in determining the variability in photosynthetic capacity among species.

Abstract: Abbreviations: α, leaf absorptance; β, fraction of absorbed light that reaches photosystem II; Γ*, CO2 compensation point in the absence of mitochondrial respiration; ФPSII, effective quantum efficiency of the PSII photochemistry; Δ Lias, effective diffusion path length in the gas phase; ϵPSII, fraction of electrons absorbed by PSII; ς, diffusion path tortuosity; Amass, photosynthetic capacity per dry mass; AN, net CO2 assimilation rate; Ca, atmospheric CO2 concentration; Cc, chloroplastic CO2 concentration; Ci, substomatal CO2 concentration; Ci-Cc, CO2 drawdown from intercellular airspace to chloroplasts; Da, diffusion coefficient for CO 2 in the gas phase; DL, leaf density; Dw, aqueous phase diffusion coefficient for CO 2; fias, volume fraction of intercellular air spaces; Fm’, maximum fluorescence in lightadapted state; Fs, steady-state fluorescence emission; g cel, partial liquid phase conductance for different portions along cell walls; gcyt, cytosol conductance; genv, chloroplast envelope conductance; gias, intercellular air space conductance to CO2 (gas phase conductance); gliq, sum of liquid and lipid phase conductances; gm, mesophyll diffusion conductance; gpl, plasma membrane conductance; gs, stomatal conductance to CO2; gtot, total conductance to CO2 from ambient air to chloroplasts; H/(RTk), dimensionless form of Henry’s law constant; JF, linear electron transport rate from chlorophyll fluorescence; J max, maximum photosynthetic electron transport rate; Kc, Michaelis–Menten constant for the carboxylation activity of Rubisco; Ko, Michaelis–Menten constant for the oxygenation activity of Rubisco; lb, biochemical limitation; Lchl, length of chloroplasts exposed to intercellular air spaces; Lcyt, diffusion pathway length in the cytoplasm; lias, gas-phase limitation; lm, mesophyll limitation; ls, stomatal limitation; MA, leaf mass per area; O, leaf internal oxygen concentration; pi, effective porosity in the given part of the diffusion pathway; Q, incident quantum flux density; R, gas constant; R d, leaf respiration in the dark; rf,i, proportional reduction of Dw in the cytosol and in the stroma compared with free diffusion in water; RL, leaf respiration in the light; SC/O, Rubisco specificity factor; S c/S, chloroplast surface area exposed to intercellular air spaces per unit of leaf area; Sc/Sm, ratio of exposed chloroplasts to mesophyll surface areas; Sm/S, mesophyll surface area exposed to intercellular air spaces per unit of leaf area; Ss, cross-sectional area of mesophyll cells in micrograph; SE, standard error; Tchl, chloroplast thickness; Tcw, cell wall thickness; Tcyt, cytoplasm thickness; Tk, absolute temperature; TL, leaf thickness; tmes, mesophyll thickness; Vcmax, maximum rates for the carboxylation activity of Rubisco; W, width of the leaf anatomical section.