Photoactivation of the latent water‐oxidizing complex in photosystem II membranes isolated from dark‐grown spruce seedlings

Photosystem II membranes (D‐PSII) were isolated from dark‐grown spruce seedlings. All major PSII proteins except the 17‐ and 23‐kDa extrinsic proteins were present in D‐PSII. O₂ evolution and Mn content in D‐PSII were negligible, while PSII‐donor activity showed a value comparable to that of NH₂OH‐treated PSII membranes (NH₂OH‐L‐PSII) from light‐grown seedlings. Light incubation of D‐PSII with 1 mM MnCl₂, 50 mM CaCl₂ and 100 μM DCIP at pH 5.3 resulted in activation of the latent water‐oxidizing complex. Accomplishment of photoactivation of PSII membranes from dark‐grown spruce seedlings clearly indicates that only ligation of Mn²⁺ to the apo‐water oxidizing complex is required for expression of O₂ evolution, and that protein synthesis is not involved in the photoactivation process. There was no essential difference between ‘photoactivation’ of naturally Mn‐free PSII membranes and ‘photoreactivation’ of artificially Mn‐depleted PSII membranes on kinetics, pH dependence, Mn²⁺‐concentration dependence. However, kinetics and pH dependence of photoactivation were appreciably different in spruce PSII membranes and in PSII membranes of angiosperms such as wheat and spinach.