Low pH Treatment of Photosystem II Upshifts the Threshold Temperature of S₁-to-S₂ Transition in the Oxygen Evolving System

Abstract

It is now widely accepted that Ca is an indispensable cofactor for photosynthetic O₂ evolution [1–11]. Requirement of Ca easily manifests when cyanobacterial PSII membranes are exposed to Ca-free buffer. This has been interpreted that Ca is essential for the electron transport from Z to P680⁺ in cyanobacterial PSII [6]. In higher plant PSII, on the other hand, requirement of Ca for PSII reactions has been investigated by use of various preparations: EGTA-treated thylakoids, intermittently flashed thylakoids, Tris- or NH₂OH-treated PSII, NaCl-washed PSII, CaCl₂-washed PSII and detergent-treated LHCP-free PSII core complex. As opposed to the observations with cyanobacterial PSII, exogenous Ca added to NaCl-washed higher plant PSII accelerated the decay of EPR Signal II_fast [12]. This suggests that Ca is required for the electron transport from Mn to Z. However, some of these treatments used to induce clear Ca requirement involve side effects such as removal of a part or all of the extrinsic proteins, and this has been making it difficult to interpret all the results under a comprehensive and consistent view.