Zinc-inhibited Electron Transport of Photosynthesis in Isolated Barley Chloroplasts.

TLDR: It is suggested that Zn inhibits electron flow at the oxidizing side of photosystem II at a site prior to the electron donating site(s) of hydroxylamine and diphenylcarbazide, and with concentrations of ZnSO(4) above 5 millimolar, photosystem I activity is partially inactivated.

Abstract: In isolated barley chloroplasts, the presence of 2 millimolar ZnSO4 inhibits the electron transport activity of photosystem II, as measured by photoreduction of dichlorophenolindophenol, O2 evolution, and chlorophyll a fluorescence. The inhibition of photosystem II activity can be restored by the addition of the electron donor hydroxylamine or diphenylcarbazide, but not by benzidine and MnCl2. These observations suggest that Zn inhibits electron flow at the oxidizing side of photosystem II at a site prior to the electron donating site(s) of hydroxylamine and diphenylcarbazide. No inhibition of photosystem I-dependent electron transport by 3 millimolar ZnSO4 is observed. However, with concentrations of ZnSO4 above 5 millimolar, photosystem I activity is partially inactivated. Washing Zn2+-treated chloroplasts partially restores the O2-evolving activity.