Section 9
Chapter 8,608

Energy-dependent quenching of chlorophyll-alpha-fluorescence: The involvement of proton-calcium exchange at photosystem 2

Krieger, A.; Weis, E.

Photosynthetica 27(1-2): 89-98


ISSN/ISBN: 0300-3604
Accession: 008607100

The pH-dependent quenching of chlorophyll (Chl) fluorescence (the high energy quenching) was characterized by stationary Chl-a-fluorescence in the thylakoid membranes and photosystem (PS) 2 preparations. The variable part of fluorescence was quenched, when the pH in the thylakoid lumen decreased below 5.5, i.e., at high DELTA-pH. This quenching was caused by an inhibition of electron donation from the manganese cluster to the reaction centre of PS 2. The pH response of quenching suggested that a 1 H+-transition with an apparent pK of 4.7 was involved. Parallel to quenching at low pH a Ca-2+-release was measured (1 Ca-2+ per 200 Chl). When the DELTA-pH relaxed and the pH on the lumen side increased again, fluorescence recovered provided Ca-2+ was present (K-D = 100 mu-M). Both the quenching at low pH and the reactivation at pH gt 5.5 are light-dependent processes. In the presence of high concentration of external Ca-2+, fluorescence recovered even at low pH. Inhibition of the donor-side of PS 2 directly affected the acceptor-side of PS 2, as seen by a shift of the redox potential of Q-A from -120 mV (pH 7.0) to +40 mV (pH 4.2). We propose that at high DELTA-pH (1) the water splitting side is inactivated by release of Ca from a high affinity binding site, and (2) Q-A is converted to a high-potential form. Excitation energy is then dissipated at the PS 2 reaction centres by a recombination reaction between donor and acceptor side. As a result, Q-A (and the intersystem electron transport chain) remains oxidized, even in the excessive light.

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