A three‐dimensional model of CO₂transport in airspaces and mesophyll cells of a silver birch leaf

Abstract

A realistic numerical three-dimensional (3D) model was constructed to study CO 2 transport inside a birch leaf. The model included chloroplasts, palisade and spongy mesophyll cells, airspaces, stomatal opening and the leaf boundary layer. Diffusion equations for CO 2 were solved for liquid(mesophyll) and gaseous(air) phases. Simulations were made in typical ambient field conditions varying stomatal opening, photosynthetic capacity and temperature. Doubled ambient CO 2 concentration was also considered. Changes in variables caused non-linear effects in the total flux, especially when compared with the results of double CO 2 concentration. The reduction in stomatal opening size had a smaller effect on the total flux in doubled concentrations than ambient CO 2 . The reduced photosynthetic capacity had a similar effect on the flux in both cases. The palisade and spongy mesophyll cells had unequal roles mainly due to the light absorption profile. Results from the 3D simulation were also compared to the classical onedimensional resistance approach. Liquid and gas phase resistances were estimated and found strongly variable according to changes in temperature and degree of stomatal opening. For the birch leaves modelled, intercellular airspace resistance was small (2% of the total resistance in saturating irradiance conditions at 25 ∞ ∞ ∞ ∞ C at stomatal opening diameter of 4 m m m m m) whereas the liquid phase resistance was significant (23% for mesophyll and chloroplasts in the same 'base case'). The absorption of CO 2 into water at cell surfaces caused additional (strongly temperature dependent) resistance which accounted for 36% of the total resistance in the base case.

Excess Diffuse Light Absorption in Upper Mesophyll Limits CO2 Drawdown and Depresses Photosynthesis

“…Compared with models based on the circuitresistance analog, FEM can more accurately describe the interactive processes of CO 2 diffusion and photosynthetic reaction within a spatially explicit leaf geometry (Parkhurst, 1994;Aalto and Juurola, 2002;Tholen and Zhu, 2011;Ho et al, 2016). We assume that since sunflower leaves have a relatively high stomatal density and a relatively porous, isotropic mesophyll airspace network, a 1-D model accurately represents the interaction between CO 2 diffusional limitation and reactive demand.…”

“…Compared with models based on the circuit-resistance analog, a finite element model (FEM) can more accurately describe the interactive processes of CO 2 diffusion and photosynthetic reaction within a spatially explicit leaf geometry (Parkhurst, 1994;Aalto and Juurola, 2002;Tholen and Zhu, 2011;Ho et al, 2016). Using this model, we investigated if the spatial differences in light absorption profiles observed in sun-and shade-grown leaves could explain their photosynthetic responses to diffuse versus direct light.…”

The Mechanistic Basis of Internal Conductance: A Theoretical Analysis of Mesophyll Cell Photosynthesis and CO2 Diffusion

“…Traditionally, the r i has been divided into two major components : the intercellular air space resistance (from substomatal cavities to the cell surface) and the liquid-phase resistance (from the cell walls to the site of carboxylation). Theoretical and Atkins et al (1972aAtkins et al ( , 1972b; Price et al (1994); Peltier et al (1995); Gillon and Yakir (2000) experimental results suggested that the resistance of the intercellular air spaces is negligible in many cases (Genty et al, 1998;Aalto and Juurola, 2002;Piel et al, 2002), although it accounts for a significant portion of r i in thick, hypostomatous leaves (Parkhurst and Mott, 1990;Parkhurst, 1994). The resistances of the cell wall (25%-50%) and chloroplast envelopes (24%-76%) are considered to make major contributions to the total r i Gillon and Yakir, 2000).…”

“…Paradoxically, this could mean that a decrease in the rate of photosynthesis results in a lower g i , instead of the other way around. In addition, the conductance approach usually considers only one dimension and therefore cannot incorporate the effects of the full 3D leaf structure on diffusion (Parkhurst, 1994;Aalto and Juurola, 2002). It remains challenging to modify biochemical models of photosynthesis with an adequate 3D description of diffusion (Farquhar et al, 2001).…”

“…One possible approach is to describe diffusion processes inside a structure using partial differential equations. Several such reaction-diffusion models have been presented, mainly for analyzing air space conductance (Gross, 1981;Parkhurst, 1994;Vesala et al, 1996;Aalto and Juurola, 2002;Galloët and Herbin, 2005).…”