Resistances along the CO2 diffusion pathway inside leaves

Abstract

CO(2) faces a series of resistances while diffusing between the substomatal cavities and the sites of carboxylation within chloroplasts. The absence of techniques to measure the resistance of individual steps makes it difficult to define their relative importance. Resistance to diffusion through intercellular airspace differs between leaves, but is usually of minor importance. Leaves with high photosynthetic capacity per unit leaf area reduce mesophyll resistance by increasing the surface area of chloroplasts exposed to intercellular airspace per unit leaf area, S(c). Cell walls impose a significant resistance. Assuming an effective porosity of the cell wall of 0.1 or 0.05, then cell walls could account for 25% or 50% of the total mesophyll resistance, respectively. Since the fraction of apoplastic water that is unbound and available for unhindered CO(2) diffusion is unknown, it is possible that the effective porosity is <0.05. Effective porosity could also vary in response to changes in pH or cation concentration. Consequently, cell walls could account for >50% of the total resistance and a variable proportion. Most of the remaining resistance is imposed by one or more of the three membranes as mesophyll resistance can be altered by varying the expression of cooporins. The CO(2) permeability of vesicles prepared from chloroplast envelopes has been reduced by RNA interference (RNAi) expression of NtAQP1, but not those prepared from the plasma membrane. Carbonic anhydrase activity also influences mesophyll resistance. Mesophyll resistance is relatively insensitive to the manipulation of any step in the pathway because it represents only part of the total and may also be countered by pleiotropic compensatory changes. The parameters in greatest need of additional measurements are S(c), mesophyll cell wall thickness, and the permeabilities of the plasma membrane and chloroplast envelope.

Enhanced Stomatal Conductance by a Spontaneous Arabidopsis Tetraploid, Me-0, Results from Increased Stomatal Size and Greater Stomatal Aperture

“…Gas exchange affects CO 2 uptake, photosynthesis, and biomass production (Horie et al, 2006;Evans et al, 2009;Tanaka et al, 2014). Stomatal conductance (gs) is used as an indicator of gas-exchange capacity (Franks and Farquhar, 2007).…”

Elements Required for an Efficient NADP-Malic Enzyme Type C4 Photosynthesis

“…In this model, the D cyt of C3P is assumed to be 5.25 3 10 210 m 2 s 21 (Sowinski et al, 2008). CO 2 leakage is described also as a diffusion process with a diffusion coefficient in the cytosol of 1.7 3 10 29 m 2 s 21 (Evans et al, 2009). Apoplastic diffusion of CO 2 is not considered because the outer cell wall of the BSCs of NADP-ME plants usually includes a suberin band, which effectively eliminates apoplastic leakage of CO 2 (Dengler and Nelson, 1999).…”

“…Apoplastic diffusion of CO 2 is not considered because the outer cell wall of the BSCs of NADP-ME plants usually includes a suberin band, which effectively eliminates apoplastic leakage of CO 2 (Dengler and Nelson, 1999). CO 2 flux across the BSC chloroplast envelopes is simulated as a simple diffusion process with a permeability coefficient of 2 3 10 25 m s 21 (Uehlein et al, 2008;Evans et al, 2009;Supplemental Fig. S9; Supplemental Data File S4).…”

Enhancing C3 Photosynthesis

“…The cell wall and membranes are the key elements limiting CO 2 permeability (Evans et al, 2009). There seems little scope for changing cell walls to increase mesophyll conductance.…”