THE EFFECTS OF PHOTOOXIDATIVE STRESS ON PHOTOSYSTEM-I MEASURED IN-VIVO IN CHLAMYDOMONAS

The effects of different photooxidative stresses on the function of ph otosystem I were measured in vivo in Chlamydomonas reinhardtii. Photoo xidative stresses included strong light, light combined with chilling to 0 degrees C, and light combined with several concentrations of meth yl viologen, Photosystem I function was measured in vivo using the abs orbance change at 820 nm associated with P-700 oxidation, Photosystem II function was measured in vivo using chlorophyll fluorescence, Stron g light or light combined with chilling caused inhibition of photosyst em II function earlier than inhibition of photosystem I function, When photosystem I was inhibited, however, it did not recover, Light combi ned with 5 mmol m(-3) methyl viologen caused inhibition of photosystem I function earlier than inhibition of photosystem II, If the methyl v iologen concentration was reduced to 1 mmol m(-3), the damage to PSI w as accelerated by addition of 90 mmol m(-3) chloramphenicol. This effe ct of chloroamphenicol suggests a role for chloroplast-encoded protein s in protecting photosystem I against photooxidative damage caused by methyl viologen.