Ben‐Hayyim, G.

Light‐induced absorption changes of cytochrome b559 have been studied in isolated chloroplast preparations at room temperature. It was shown that cytochrome b559 could undergo both photooxidation and photoreduction, both these reactions occurring to its high‐potential form. In the presence of FCCP and DCMU, a reversible photoreduction could be detected which was not sensitive to the presence of oxygen and which was probably a result of a cyclic electron flow around photosystem 2. It is suggested that the high‐potential form of cytochrome b559, which is the native form, is mainly photooxidized by photosystem 2. The photoreduction is also mediated by photosystem 2 and can be detected only when electrons cannot flow from photosystem 2 to photosystem 1, i.e. in the presence of DCMU, or when the carriers are fully reduced in the absence of an electron acceptor. Copyright © 1974, Wiley Blackwell. All rights reserved