Journal Article

Effects of HgCl<sub>2</sub> on CO<sub>2</sub> Dependence of Leaf Photosynthesis: Evidence Indicating Involvement of Aquaporins in CO<sub>2</sub> Diffusion across the Plasma Membrane

Ichiro Terashima and Kiyomi Ono

in Plant and Cell Physiology

Published on behalf of Japanese Society of Plant Physiologists

Volume 43, issue 1, pages 70-78
Published in print January 2002 | ISSN: 0032-0781
Published online January 2002 | e-ISSN: 1471-9053 | DOI: http://dx.doi.org/10.1093/pcp/pcf001
Effects of HgCl2 on CO2 Dependence of Leaf Photosynthesis: Evidence Indicating Involvement of Aquaporins in CO2 Diffusion across the Plasma Membrane

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Experiments were conducted to examine whether mercury-sensitive aquaporins facilitate photosynthetic CO2 diffusion across the plasma membrane of leaf mesophyll cells. Discs without abaxial epidermes from Vicia faba leaflets were treated with HgCl2, an inhibitor of aquaporins. Hydraulic conductivity of the plasma membrane of these discs, measured as the weight loss of the discs in the 1 M sorbitol solution, was inhibited by sub-mM concentrations of HgCl2 by 70 to 80%. Photosynthetic CO2 fixation was also inhibited by the HgCl2 treatment in a similar concentration range. When 0.3 mM HgCl2 solution was fed to the V. faba leaflets with intact epidermes via the transpiration stream, the rate of photosynthesis on leaf area basis (A) measured at photosynthetically active photon flux density of 700 µmol m–2 s–1 and at leaf temperature of 25°C, decreased by about 20 to 30% at any CO2 concentration in the intercellular spaces (Ci). However, when CO2 concentration in the chloroplast stroma (Cc) was calculated from fluorescence and gas exchange data and A was plotted against Cc, A at low Cc concentrations did not differ before and after the treatment. The conductance for CO2 diffusion from the intercellular spaces to the chloroplast stroma (gi) decreased to 40 and 30% of the control value, when the leaflets were fed with 0.3 mM and 1.2 mM HgCl2, respectively. Similar results were obtained with leaves of Phaseolus vulgaris. Although effects of HgCl2 were not specific, the present results showed that HgCl2 consistently lowered gi. It is, thus, probable that the photosynthetic CO2 uptake across the plasma membrane of the mesophyll cells is facilitated by mercury-sensitive aquaporins.

Keywords: Key words: Aquaporin — CO2 diffusion — HgCl2 — Mesophyll — Photosynthesis.; Abbreviations: A, rate of net photosynthesis on leaf area basis; Ca, CO2 concentration in the ambient air; Cc, CO2 concentration in the chloroplast stroma; Ci, CO2 concentration in the intercellular spaces; gi, internal conductance for CO2 diffusion from the intercellular spaces to the chloroplast stroma; PPFD, photosynthetically active photon flux density; Rubisco, ribulose 1,5-bisphosphate carboxylase/oxygenase; RuBP, ribulose 1,5-bisphosphate.

Journal Article.  5206 words.  Illustrated.

Subjects: Biochemistry ; Molecular and Cell Biology ; Plant Sciences and Forestry

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