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ENVIRONMENTAL STRESS AND ADAPTATION

Lateral Diffusion of CO₂ in Leaves Is Not Sufficient to Support Photosynthesis^[w]

Department of Biological Sciences, University of Essex, Colchester CO4 3SQ, United Kingdom (J.I.L.M., E.G., T.L., N.R.B.); Laboratoire d'Ecophysiologie Végétale, Université de Paris 11, 91405 Orsay, France (E.G., G.C.); and Laboratoire d'Analyse, Topologie, Probabilités, Université de Provence, 13453 Marseille, France (R.H.)

Lateral diffusion of CO₂ was investigated in photosynthesizing leaves with different anatomy by gas exchange and chlorophyll a fluorescence imaging using grease to block stomata. When one-half of the leaf surface of the heterobaric species Helianthus annuus was covered by 4-mm-diameter patches of grease, the response of net CO₂ assimilation rate (A) to intercellular CO₂ concentration (C_i) indicated that higher ambient CO₂ concentrations (C_a) caused only limited lateral diffusion into the greased areas. When single 4-mm patches were applied to leaves of heterobaric Phaseolus vulgaris and homobaric Commelina communis, chlorophyll a fluorescence images showed dramatic declines in the quantum efficiency of photosystem II electron transport (measured as F_q'/F_m') across the patch, demonstrating that lateral CO₂ diffusion could not support A. The F_q'/F_m' values were used to compute images of C_i across patches, and their dependence on C_a was assessed. At high C_a, the patch effect was less in C. communis than P. vulgaris. A finite-volume porous-medium model for assimilation rate and lateral CO₂ diffusion was developed to analyze the patch images. The model estimated that the effective lateral CO₂ diffusion coefficients inside C. communis and P. vulgaris leaves were 22% and 12% of that for free air, respectively. We conclude that, in the light, lateral CO₂ diffusion cannot support appreciable photosynthesis over distances of more than approximately 0.3 mm in normal leaves, irrespective of the presence or absence of bundle sheath extensions, because of the CO₂ assimilation by cells along the diffusion pathway.

^[w] The online version of this article contains Web-only data.

Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.105.062950.

^* Corresponding author; e-mail morisj{at}essex.ac.uk; fax 44–1206–872592.

Received March 18, 2005; returned for revision May 31, 2005; accepted June 1, 2005.

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