Stimulatory effect of calcium on photoactivation of the water oxidation system in dark-grown spruce chloroplasts.

The water-oxidation system in chloroplasts from Picea abies leaves grown without light was activated and stabilized upon exposure of the isolated chloroplasts to weak light. The rate of electron transport from diphenylcarbazide to 2,6-dichloroindophenol in the photosystem II remained constant. Ca2+ added to the chloroplast suspension was incorporated into thylakoid membranes during illumination and strongly stimulated the photoactivation of latent water-oxidation system but not electron transport in photosystem II. It is concluded that the site which requires Ca2+ and is activated by light is the water-oxidation system.