2Energy Sciences, National Renewable Energy Laboratory, Golden, Colorado 80401, USA; E-mail: email@example.com
* To whom correspondence should be addressed.
Received April 26, 2007; Revision received June 5, 2007
The F0 fluorescence yield in intact photosystem II (PSII), Ca-depleted PSII (PSII(-Ca/NaCl)), and Mn-depleted PSII membranes was measured before and after dim light treatment (1-2 min), using flash-probe fluorescence and fluorescence induction kinetic measurements. The value of F0 after the light treatment (F0´) was larger than F0 in dark-adapted PSII membranes and depended on the appearance of the slowly relaxing, reduced plastoquinone pool (t1/2 = 4 min) formed during preillumination, which was not totally reoxidized before the F0´ measurement. In PSII(-Ca/NaCl) such a pool also appeared, but the F0´_yield was even higher than in intact PSII membranes. In Mn-depleted PSII membranes, the pool did not form. Interestingly, the yield of F0´ in Ca-depleted PSII membranes prepared using chelators (EGTA and citrate) or containing 5 mM EGTA was significantly lower than in PSII(-Ca/NaCl) samples prepared without chelators. These data indicate that chelators inhibit the reduction of QA and QB and formation of the slowly relaxing plastoquinone pool, or alternatively they increase the rate of its oxidation. Such an effect can be explained by coordination of the chelator molecule to the Mn cluster in PSII(-Ca/NaCl) membranes, rather than different amounts of residual Ca2+ in the membranes (with or without the chelator), since the remaining oxygen-evolving activity (~15%) in PSII(-Ca/NaCl) samples did not depend on the presence of the chelator. Thus, chelators of calcium cations not only have an effect on the EPR properties of the S2 state in PSII(-Ca/NaCl) samples, but can also influence the PSII properties determining the rate of plastoquinone pool reduction and/or oxidation. The effect of some toxic metal cations (Cd, Cu, Hg) on the formation of the slowly relaxing pool in PSII membranes was also studied.
KEY WORDS: photosystem II, oxygen-evolving complex, calcium, plastoquinone QA, plastoquinone QB, fluorescence, fluorescence induction kinetics, F0