Abstract:The photoreduction of DCIP by PSⅡ pasticles isolated from spinach leaves was inhibited by sulfite and the degree of inhibition was increased with the increase of sulfite concentration. The site of sulfite damage was on the oxidation side. In dark, electron flow from H2O to DCIP and from DPC to DCIP was not affected by sulfite. With certain concentrations of sulfite, the damage to PSⅡ particle varied with time of sulfite treatment and the mechanism of the damage might be related to the discretion of 33 kD polypeptide from thylakoid membrane and the leakage of Mn. Sulfite did not specifically damage the newly prepared thyla- koid, but this was the case with aged thylakoid. The rate of DCIP photoreduction decreased as the aging process was prolonged. Decrease in Mn content correlated with the decrease of DCIP photoreduction. Especially in the presence of EDTA, with the decrease of Mn, the rate of electron transport was severely reduced.