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Delayed Chlorophyll Fluorescence as a Monitor for Physiological State of Photosynthetic Apparatus

ABSTRACT Intact plants emit light quanta called delayed fluorescence (DF). DF is result of radiative deactivation of secondary excited chlorophyll molecules in Photosystem II (PS II) antennae complexes. The excitations are produced by backward electron- transfer reactions both in the donor and acceptor sides of PS II. The poly-exponential dark decay of DF in a time interval of tens of nanoseconds to tens of seconds reflects the kinetics of different forward and backward reactions of the photosynthetic electron transfer. The current work reviews the mechanisms of the DF light quanta generation and the methodical approaches that allow us to obtain quantitative information about the photosynthetic machinery state using the DF signal from native objects. We examine an approach for the simultaneous record of DF and prompt chlorophyll fluorescence during the transition of the photosynthetic machinery from dark-adapted to light-adapted state. A new device (Senior PEA) built by Hansatech (King's Lynn, UK) allows us to measure simultaneously the induction transients of prompt chlorophyll fluorescence, DF decaying in a time range 10 μs—240 ms, and the changes in transmission at 820 nm. The comparative analysis of the three types of signals and the application of a model-based description of the processes and reactions that determine the dynamics of the signals during the light-induced transitions (dynamic models and JIP-test) allow us to obtain, from a single few-seconds-long measurement, quantitative information for: a) energetic fluxes and efficiencies at different steps of energy transformation; b) rate constants of electron transfer in and between the two photosystems; c) energization of the thylakoid membrane. This illustrates that DF in combination with other optical and luminescent measurements is a highly informative method for investigation of the physiological state of the photosynthetic apparatus of plants in vivo and in situ, and is an indispensable tool for the purposes of the biophysical phenomics.