PLANT PHYSIOLOGY , Vol 108, Issue 3 1059-1066, Copyright © 1995 by American Society of Plant Biologists
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WHOLE PLANT, ENVIRONMENTAL, AND STRESS PHYSIOLOGY |
Stimulation by Light of Rapid pH Regulation in the Chloroplast Stroma in Vivo as Indicated by CO2 Solubilization in Leaves
M. Hauser, H. Eichelmann, V. Oja, U. Heber and A. Laisk
Julius-von-Sachs-Institute for Biosciences, University of Wurzburg, D-97082 Wurzburg, Germany (M.H., U.H.)
Leaves of Brassica oleracea, Helianthus annuus, and Nicotiana rustica were
exposed for 20 s to high concentrations of CO2. CO2 uptake by the leaf,
which was very fast, was measured as a transient increase in the
concentration of oxygen. Rapid solubilization of CO2 in excess of that
which is physically dissolved in aqueous phases is proposed to be caused by
bicarbonate formation in the stroma of chloroplasts, which contain carbonic
anhydrase. On this basis, pH values and bicarbonate accumulation in the
chloroplast stroma were calculated. Buffer capacities were far higher than
expected on the basis of known concentrations in the chloroplast stroma.
Moreover, apparent buffer capacities increased with the time of exposure to
high CO2, and they were higher when the measurements were performed in the
light than in the dark. During prolonged exposure of leaves to 16% CO2,
calculated bicarbonate concentrations in the chloroplast stroma exceeded 90
mM in the dark and 120 mM in the light. The observations are interpreted as
indicating that under acid stress protons are rapidly exported from the
chloroplasts in exchange for cations, which are imported. The data are
discussed in terms of effective metabolic pH control by ion transport,
first across the chloroplast envelope and, then, across the tonoplast of
leaf mesophyll cells. The direct involvement of the vacuole in the
regulation of the chloroplast pH in leaf cells is suggested.
