IN-SITU SUSCEPTIBILITIES OF PHOTOSYSTEM-I AND PHOTOSYSTEM-II TO PHOTOSENSITIZED INACTIVATION VIA SINGLET OXYGEN MECHANISM

A comparative study was carried out on the in situ susceptibilities to photoinactivation of the photosystem I (PS I) and II (PS II) complexes of spinach thylakoids treated with efficient type II sensitizers. While the presence of the exogenous sensitizers caused a substantial increase in the extent of photoinactivation of whole chain electron transport, it did not affect PS I activity of thylakoids in light but exerted an enhanced photoinactivating effect only on PS II. The measurements of the action spectrum for the inhibition of PS II activity of the sensitizer-incorporated thylakoids and that for the generation of singlet oxygen (O-1(2)) from them revealed that photosensitized inactivation of PS II is directly related to the photoproduction of O-1(2) in thylakoid membranes. The results obtained in the present work clearly demonstrate an exceptional sensitivity of PS II to O-1(2), providing circumstantial evidence that high light-induced damage to PS II may result from photosensitization reactions mediated by O-1(2), which is not necessarily produced within the PS II complex.