Inhibition by ethoxyzolamide of a photoacoustic uptake signal in leaves: Evidence for carbonic anhydrase catalyzed CO2-solubilisation

A photoacoustic pulse-modulation technique is applied for the study of a CO₂-stimulated gas uptake signal in leaves (Reising and Schreiber, Photosynthe Res 31: 227–238, 1992). It is shown that this uptake signal can be substantially suppressed by application of the carbonic anhydrase inhibitor, ethoxyzolamide, to leaf discs. This inhibitor does not affect the O₂-evolution signal in air or the chlorophyll fluorescence induction pattern at high CO₂, when non-saturating light intensities are used. On the basis of these findings it is concluded that at least a major part of the CO₂-stimulated photoacoustic uptake signal results from light-modulated CO₂-solubilisation catalysed by carbonic anhydrase. Modulated CO₂-solubilisation appears likely to be induced by light driven H⁺-translocation from the stroma into the thylakoid lumen. Comparison of the induction patterns of chlorophyll fluorescence quenching and the uptake signal suggests a correlation between membrane energisation and CO₂-uptake. The importance of O₂-dependent electron flow as a major cause of membrane energisation is discussed. It is proposed that in the absence of CO₂ the combination of Mehler- and ascorbate peroxidase reactions does not result in a photobaric signal, as O₂-uptake and O₂-evolution components cancel each other. Two main conclusions, which are of considerable importance for future practical applications of the photoacoustic method, are drawn from these findings: (1) When high CO₂ is applied to leaves, the photobaric uptake component may provide a unique means of monitoring the function of stromal carbonic anhydrase in vivo. (2) Brief flushing of the photoacoustic cell with air may prevent the occurrence of an uptake signal, thus allowing a straight-forward deconvolution into photothermal and O₂-evolution components.