Quenching of fluorescence in PS II by pH: mechanism and effects of antimycin

Christine T. Yerkes and Antony R. Crofts
Biophysics Division, University of Illinois, Urbana IL 61801

1. The following working hypothesis explains fluorescence quenching (qE) associated with DpH (1). As the lumenal pH drops on illumination, the reactions of the S-states are progressively inhibited in transitions associated with H+-release, especially S3-S0 (2). These conditions would lead to accumulation of P+ which quenches fluorescence, allowing the plant to dump excess absorbed energy to the environment as heat, and leads to enhanced delayed fluorescence (3), and a lower photochemical yield.

Antimycin inhibits qE at relatively low concentration (4). We have studied the effects of antimycin on the backreaction QA-YZ+ to QAYZ. Our results from fluorescence and EPR studies show that antimycin acts as a donor to YZ+, suggesting a mechanism for this effect in line with the above hypothesis. This ADRY-like effect also accounts for previous observations suggesting a role for cyt b-559 in fluorescence quenching (5)

2. Under conditions in which P+ accumulates, secondary quenching species (Chl+ and Car+) are formed close to the reaction center (6). These species contribute to a new EPR signal, centered at about g=2, and having a linewidth of about 10 G, which is formed to a much greater extent in Tris-washed than in the control materials. Optical spectra of the photo-inhibited samples and difference spectra between the inhibited and control samples reveal the formation of photo-oxized chlorophylls and carotenoids. Relative amplitudes of the control vs. inhibited EPR signals and optical traces suggest that the EPR radical arises from an oxidized chlorophyll. The spectrum is much more narrow than that of the bulk chlorophyll, with a peak at the red edge (maximal at 686 nm) of the bulk spectrum, suggesting a component of the antenna complex in the energy transfer pathway close to the reaction center.

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