Inhibition of electron transport at the cytochrome b(6)f complex protects photosystem II from photoinhibition.

Photoinhibition of photosystem II (PS II) activity was studied in thylakoid membranes illuminated in the presence of the inhibitor of the cytochrome b(6)f complex 2'iodo-6-isopropyl-3-methyl-2',4, 4'-trinitrodiphenylether (DNP-INT). DNP-INT was found to decrease photoinhibition. In the absence of DNP-INT, anaerobosis, superoxide dismutase and catalase protected against photoinhibition. No effect of these treatments was observed in the presence of DNP-INT. These data demonstrate that photoinhibition under these conditions is caused by reactive oxygen species which are formed most probably by the reduction of oxygen at photosystem I. The results are discussed in terms of the importance of photosynthetic control in protection against photoinhibition in vivo.