Photoinactivation of Photosynthetic Electron Transport under Anaerobic and Aerobic Conditions in Isolated Thylakoids of Spinach

The effect o f exposure to strong white light on photosynthetic electron transport reactions o f PS I and PS II were investigated in spinach thylakoids in the absence or presence o f oxygen. Irrespective o f the conditions used for photoinactivation, the damage to PS II was always much more than to PS I. Photoinactivation was severe under anaerobic conditions compared to that in air for the same duration. This shows that the presence o f oxygen is required for prevention o f photoinactivation o f thylakoids. The susceptibility o f water-splitting complex in photoinactivation is indicated by our data from experiments with chloride-deficient chloroplast membranes wherein it was observed that the whole chain electron transport from DPC to MV was much less photoinhibited than that from water. The data from the photoinactivation experiments with the Tris-treated thylakoids indicate another photodamage site at or near reaction centre o f PS II. DCM U-protected PS II and oxygen-evolving complex from photo­ inactivation. D C M U protection can also be interpreted in terms o f the stability o f the PS II complex when it is in S2 state.