The Photoacoustic Method in Photosynthesis — Monitoring and Analysis of Phenomena Which Lead to Pressure Changes Following Light Excitation

Abstract

Following light excitation and relaxation of the excited state, pressure changes are developed in an investigated sample or in the gas phase around the sample. Commonly, these changes result from the conversion ofthe light energy to heat, leading to temperature rise and hence to pressure increase (photothermal effect). Although quite small and rapid, the pressure changes are detectable by suitable fast sensors. These are either piezoelectric transducers, used in a flash photolysis mode with time domain measurement or microphones, used commonly in conjunction with periodically modulated light excitation (yielding periodic pressure perturbations) but also in a flash photolysis mode. Photosynthetic samples are quite unique ia that their photochemical activities give rise to additional mechanisms which contribute appreciably to pressure changes. These are: (1) gas evolution and uptake, most notably oxygen evolution, detectable in leaves and lichens with a sensor put in the gas phase (photobaric effect), (2) molecular volume changes, resulting from photosynthetic charge separation and proton transfer to and from the buffered medium. The methodology which deals with the measurement and analysis of these pressure changes is commonly known as photoacoustics (or optoacoustics) and the resulting signals are referred to as photoacoustic (optoacoustic) signals. Although implied by the name, sound production assuch, is important only at sufficiently high frequencies (or short times) where the sound wavelength is small relative to the sample dimensions. Otherwise, the pressure changes are practically uniform over the entire sample. This chapter outlines methods to measure, separate and analyze each of the contributions to the photoacoustic signals in photosynthetic samples and the value of the information obtained by this analysis. Broadly speaking, the photothermal contribution allows to deduce the magnitude of energy storage with the capability to specifically assign energy and decay kinetics to particular electron transfer, intermediates; the photobaric contribution serves as a sensitive analytical tool for photosystem II activity and electron transport in general, in complex structures such as leaves; molecular volume changes are related to molecular structural changes within the reaction center. The use of periodically modulated light offers various other advantages: high sensitivity, separation from dark processes and possibility of conditioning by back ground (non-modulated) lights without interfering with the measurements. Applications include straight forward analysis of the interaction between the two photosystems, characterization and dynamics of physiological changes and stress conditions, possible detection of cyclic electron flow in-vivo, among others.