Photochemical and Non-Photochemical Quenching of Chlorophyll Fluorescence Induced by Hydrogen Peroxide

Chlorophyll fluorescence quenching induced by H 20 2 in intact spinach chloroplasts was investi­ gated with a modulation fluorometer which allows to distinguish between photochemical and non­ photochemical quenching com ponents by the so-called saturation pulse method. Residual catalase activity was removed by washing and percoll gradient centrifugation. H 20 2 was found to induce pronounced photochemical and non-photochemical quenching, characteristic for the action of a Hill reagent, with a half-maximal rate already observed at 5 x 10~6 m . The saturation characteris­ tics and maximal rate o f H 20 2-reduction were very similar to those of methylviologen reduction. H 20 2-dependent quenching was stimulated by ascorbate and inhibited by cyanide and azide in agreement with previous findings by other researchers that H 20 2-reduction involves the ascorbate peroxidase scavenging system and that the actual “Hill acceptor” is an oxidation product of ascorbate, i.e. monodehydroascorbate or dehydroascorbate. With well-coupled intact chloro­ plasts reducing C 0 2 at 150 (irnol (mg C hl)“ 'h_ l, iodoacetamide stopped CO;-dependent 0 2evolution and consequent addition of 10“3 m H 20 2 produced an 0 2-evolution rate o f 240 (.irnol (mg C hl)“ 'h_ l. It is concluded that light-dependent H20 2 reduction is a very efficient reaction in intact chloroplasts. As H 20 2 formation and consequent reduction also occur in vivo, the corre­ sponding quenching should be considered when assimilatory electron flow is estimated from quenching coefficients. It is suggested that proton flux associated with H20 2-formation and reduc­ tion may be important for the adjustment of appropriate A T P/N A D PH ratios required for C 0 2fixation in vivo. Furthermore, H 20 2-reduction may serve as a valve reaction whenever Calvin cycle activity is limited by factors different from N A D PH supply, thus protecting against photoinhibitory damage.