The correlation between the induction kinetics of the photoacoustic signal and chlorophyll fluorescence in barley leaves is governed by changes in the redox state of the photosystem II acceptor side. A study under atmospheric and high CO2 concentrations

The induction kinetics of chlorophyll fluorescence were measured in dark-adapted barley leaves simultaneously with O2 evolution and thermal dissipation using photoacoustic spectroscopy. Significant correlations were revealed between these parameters at low and high (about 5%) CO2 concentrations and different irradiances. In general, four phases were resolved in the induction curves of chlorophyll fluorescence or PA signals modulated at 35 Hz (O2-dependent component) or 400 Hz (thermal dissipation). During the first 10–15 s after onset of actinic light, a high level of reduction of the primary quinone acceptor of photosystem II (QA), a low magnitude of the O2-dependent photoacoustic signal, and a high level of heat production measured as the photoacoustic signal at 400 Hz were maintained. Further illumination led to a partial QA− reoxidation, an increased rate of O2 evolution, and a decline in heat production owing to an increased energy storage at the level of the plastoquinone pool. At low CO2 concentration, the steady-state level of all the above parameters was attained at the end of the first minute of illumination and remained unmodified after prolonged leaf irradiation. In contrast, the development of an additional slow peak followed by its dissipation was found for all parameters examined under high CO2 concentration and strong irradiance. The position of the slow peak in the kinetics of the O2-dependent photoacoustic signal preceded the corresponding peak of chlorophyll fluorescence measured under actinic light but coincided with the position of the peak in the kinetics of photochemical quenching. The position of the slow peak in the kinetics of thermal dissipation strictly corresponded to the one of chlorophyll fluorescence. A linear correlation was found between the magnitudes of the O2-dependent component of the photoacoustic signal and the extent of photochemical quenching. The results indicate that correlations in the kinetics and magnitudes of chlorophyll fluorescence, O2 evolution, and thermal dissipation are due at least in part to the dependence of the above parameters on the redox state of the acceptor side of photosystem II. Key words: photosynthesis, chlorophyll fluorescence, thermal dissipation, CO2, O2 evolution, plastoquinone, photoacoustic.