Inhibition of photosystem II in Chlorogloeopsis fritschii with shikonin acetate

Shikonin acetate extracted from the roots of the desert plant, Arnebia decumbens, was tested for its effect on the photosynthetic electron transport system of C. fritschii. Photosystem (PS) II‐Hill reaction with water and diphenylcarbazide (DPC) as electron donors and ferricyanide as electron acceptor was inhibited completely. Shikonin acetate has little effect on 2,6‐dichlorophenolindophenol (DCPIP)‐Hill reaction and no effect on PSI with durohydroquinone as electron donor. Inhibition of Mehler reaction from water to methylviologen with whole cells was studied using 3‐(3,4‐dichlorophenyl)‐1, 1‐dimethyl urea (DCMU) and shikonin acetate. About 70% inhibition of cyclic and non‐cyclic photophosphorylation was found with 10−7 M shikonin acetate. These results suggest that shikonin acetate inhibits photosynthetic electron flow at the plastoquinone pool.