Pulse-modulated photoacoustic measurements reveal strong gas-uptake component at high CO2-concentrations.

The effect of high CO2-concentration on photoacoustic signals from tobacco leaves is studied by means of a recently developed pulse modulation method which provides simultaneous information on photothermal and photobaric components in the millisecond time domain. High CO2-concentrations are found to induce large gas-uptake signals. Simultaneous measurements of chlorophyll fluorescence suggest that the uptake signals are correlated with energy-dependent fluorescence quenching. Very similar CO2-concentration dependencies are found in the absence and presence of methylviologen, which is known to catalyze O2-reduction, and in the presence of glyceraldehyde, which blocks Calvin cycle and photorespiration. It is suggested that the CO2-enhanced uptake signal is likely to reflect O2-uptake in the Mehler reaction. However, it is not ruled out that also rapid CO2-solubilisation or CO2-binding caused by light-induced stroma alkalisation are involved. Strong uptake is also induced when the CO2-concentration in the closed photoacoustic chamber increases due to dark-respiration. The consequences of these findings with respect to the interpretation of photoacoustic data (e.g., 'low-light effect') and to the regulatory role of O2-dependent electron flow are discussed.