INHIBITION BY ETHOXYZOLAMIDE OF A PHOTOACOUSTIC UPTAKE SIGNAL IN LEAVES - EVIDENCE FOR CARBONIC-ANHYDRASE CATALYZED CO2-SOLUBILIZATION

A photoacoustic pulse-modulation technique is applied for the study of a CO2-stimulated gas uptake signal in leaves (Reising and Schreiber, Photosynth Res 31: 227-238, 1992). It is shown that this uptake signal can be substantially suppressed by application of the carbonic anhydr ase inhibitor, ethoxyzolamide, to leaf discs. This inhibitor does not affect the O-2-evolution signal in air or the chlorophyll fluorescence induction pattern at high CO2, when nonsaturating light intensities a re 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 anhydra se. Modulated CO2-solubilisation appears likely to be induced by light driven H+-translocation from the stroma into the thylakoid lumen. Com parison of the induction patterns of chlorophyll fluorescence quenchin g and the uptake signal suggests a correlation between membrane energi sation and CO2-uptake. The importance of O-2-dependent electron flow a s a major cause of membrane energisation is discussed. It is proposed that in the absence of CO2 the combination of Mehler- and ascorbate pe roxidase reactions does not result in a photobaric signal, as O-2-upta ke and O-2-evolution components cancel each other. Two main conclusion s, which are of considerable importance for future practical applicati ons of the photoacoustic method, are drawn from these findings: (1) Wh en 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 a ir may prevent the occurrence of an uptake signal, thus allowing a str aight-forward deconvolution into photothermal and O-2-evolution compon ents.