The effects of wide concentration ranges of NaCl, KCl, and MgCl(2) on ferricyanide reduction and the fluorescence induction curve of isolated spinach (Spinacia oleracea) chloroplasts were investigated. Concentrations of the monovalent salts above 100 mm and MgCl(2) above 25 mm produced a decrease in the rate of ferricyanide reduction by thylakoids uncoupled with 2.5 mm NH(4)Cl which cannot be attributed to changes in the primary photochemical capacity of photosystem II. Salt-induced decreases in the effective concentration of the secondary electron acceptor of photosystem II, plastoquinone, reduce the capacity for secondary photochemistry of photosystem II and this could contribute to the reduction in ferricyanide reduction by uncoupled thylakoids at high salinities. The rate of ferricyanide reduction by coupled thylakoids is little affected by salinity changes, indicating that the rate-limiting phosphorylation mechanism in electron flow from water to ferricyanide in coupled thylakoids is salt-tolerant, whereas the rate-limiting reaction in uncoupled ferricyanide reduction is considerably affected by salinity changes. Salt-induced changes in the fluorescence induction curve are interpreted in terms of changes in the rate constants for excitation decay by radiationless transitions, exciton transfer from photosystem II chlorophylls to other associated chlorophyll species, and photochemistry.