Light Emission as a Probe of Charge Separation and Recombination in the Photosynthetic Apparatus: Relation of Prompt Fluorescence to Delayed Light Emission and Thermoluminescence

The photosynthetic machinery emits a fraction of absorbed light as a highly informative waste process. The emitted light can be observed as prompt fluorescence during illumination or as delayed luminescence after illumination is turned off. Luminescence is measured in the form of delayed light emission (DLE) as a function of time or in the form of thermoluminescence (TL) as a function of temperature. Both fluorescence and luminescence originate mainly in Photosystem II. The yield of chlorophyll a fluorescence is modulated by the reduction state of the electron acceptor Q_A of Photosystem II, whereas the momentary intensity of luminescence emission reflects the rates of charge recombination reactions between electron acceptors and electron donors of the photosynthetic electron transfer chain II. Both fluorescence and luminescence methods have proved to be invaluable tools in the studies of photosynthetic light reactions. In this chapter, we review the basics and the recent advances in DLE, TL and fluorescence of photosynthetic systems, concentrating particularly on the kinetics of these processes and on the links between the three types of measurement