Single photon timing was used to study picosecond chlorophyll a fluorescence
decay kinetics of pH induced non-photochemical quenching in spinach photosystem
2 particles. The characteristics of this quenching are a decrease in chlorophyll a
fluorescence yield as well as a decrease in photochemistry at low pH. Picosecond
kinetics of room temperature fluorescence temporally resolve the individual
components of the steady state fluorescence yield into components that are related
to primary energy conversion processes in photosystem 2. Four components were
resolved for dark adapted (Fo), light saturated (Fm), and chemically reduced (Nadithionite)
photosystem 2 reaction centres. The fastest and slowest components,
indicative of energy transfer to and energy capture by the photosystem 2 reaction
centre and uncoupled ("dead") chlorophyll, respectively, were not affected by
changing pH from 6.5 to 4.0. The two intermediate components, indicative of
electron transfer processes within the reaction centre of photosystem 2, were
affected by the pH change. Results indicate that the decrease in the steady state
fluorescence yield at low pH was primarily due to the decrease in lifetime and
amplitude of the slower of the intermediate components. These results imply that the
decrease in steady state fluorescence yield at low pH is not due to changes in
energy transfer to and energy capture by the photosystem 2 reaction centre, but is
related to changes in charge stabilization and charge recombination in the
photosystem 2 reaction centre
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