effects of photooxidative stress on photosystem I measured in vivo in Chlamydomonas

The effects of different photooxidative stresses on the function of photosystem I were measured in vivo in Chlamydomonas reinhardtii. Photooxidative stresses included strong light, light combined with chilling to 0 degrees C, and light combined with several concentrations of methyl viologen. Photosystem I function was measured in vivo using the absorbance change at 820 nm associated with P700 oxidation. Photosystem II function was measured in vivo using chlorophyll fluorescence. Strong light or light combined with chilling caused inhibition of photosystem II function earlier than inhibition of photosystem I function. When photosystem I was inhibited, however, it did not recover. Light combined with 5 mmol m-3 methyl viologen caused inhibition of photosystem I function earlier than inhibition of photosystem II. If the methyl viologen concentration was reduced to 1 mmol m-3, the damage to PSI was accelerated by addition of 90 mmol m-3 chloramphenicol. This effect of chloramphenicol suggests a role for chloroplast-encoded proteins in protecting photosystem I against photooxidative damage caused by methyl viologen.