JXB Advance Access published online on April 23, 2009
Journal of Experimental Botany, doi:10.1093/jxb/erp117
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REVIEW-ARTICLE |
Resistances along the CO2 diffusion pathway inside leaves
1Environmental Biology Group, Research School of Biological Sciences, The Australian National University, GPO Box 475, Canberra ACT 2601, Australia
2Department of Applied Plant Sciences, Institute of Botany, Darmstadt University of Technology, D-64287 Darmstadt, Germany
3CEA, CNRS, Université Aix-Marseille, UMR 6191 Biologie Végétale et Microbiologie Environnementale, Laboratoire d'Ecophysiologie Moléculaire des Plantes, CEA Cadarache, 13108 Saint Paul lez Durance, France
4Department of Biological Sciences, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-0033, Japan
* To whom correspondence should be addressed. E-mail: john.evans{at}anu.edu.au
CO2 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, Sc. 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 CO2 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 CO2 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 Sc, mesophyll cell wall thickness, and the permeabilities of the plasma membrane and chloroplast envelope.
Key words: Aquaporin, cell wall, cooporin, internal conductance, mesophyll conductance
Received 10 December 2008; Revised 12 March 2009 Accepted 17 March 2009
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