Bicarbonate protects the donor side of photosystem II against photoinhibition and thermoinactivation

Abstract

The rate of photoinhibition of photosystem II (PSII) activities (photoinduced change of chlorophyll fluorescence yield, ΔF, and photoreduction of 2,6-dichlorophenol-indophenol) in O 2-evolving pea subchloroplast membrane fragments in medium depleted of CO 2 was considerably decreased upon addition of 5 mM NaHCO 3 before the light treatment. A similar effect was observed when 100 μM MnCl 2 was added before the photoinhibition. In PSII membrane fragments depleted of Mn, the pre-illumination led to irreversible loss of the capability of PSII to be reactivated by Mn 2+, and the rate of the photoinhibition was decreased by a factor of 2 or 5 if the pre-illumination was done in the presence of 0.2 μM MnCl 2 (≈4 Mn per PSII reaction center) added alone or in combination with 5 mM NaHCO 3, respectively. A similar protective effect of bicarbonate was also revealed in the dark, during thermoinactivation of O 2-evolving PSII at 40°C: the rate of thermoinactivation of ΔF was decreased by a factor of 3 if 5 mM NaHCO 3 was added to the medium. The results are consistent with the idea that bicarbonate is an essential component of the Mn-containing water-oxidizing complex of PSII, which decreases its susceptibility to photoinhibition and thermoinactivation.