The 520 nm absorbance changes in Scenedesmus obliquus and its relation to photosystem I

The kinetics (region of seconds) of the light-induced 520 nm absorbance changes and its dark reversal have been studied in detail in the wild type and in some pigment and photosynthetic mutants of Scenedesmus obliquus. The following 5 lines of evidence led us to conclude that the signal is entirely due to the photosystem I reaction modified by electron flow from Photosystem II. Gradual blocking of the electron transport with 3(3,4-dichlorophenyl)-1,1-dimethylurea resulted in diminution and ultimate elimination of the biphasic nature of the signal without reducing the extent of the absorbance change or of the dark kinetics. On the contrary, blocking electron flow at the oxidizing side of plastoquinone with 2,5-dibromo-3-methyl-6-isoprophyl-p-benzoquinone or inactivating the plastocyanin with KCN, prolonged the dark reversal of the absorbance change apart from abolishing the biphasic nature of the signal. Action spectra clearly indicate that the main signal (I) is due to electron flow in Photosystem I and that its modification (Signal II) is due to the action of Photosystem II. Signal I is pH independent, whereas Signal II demonstrates a strong pH dependence, parallel to the O2-evolving capacity of the cells. Chloroplast particles isolated from the wild type Scenedesmus cells demonstrated in the absence of any added artificial electron donor or acceptor and also under non-phosphorylation conditions the 520 nm absorbance change with approximately the same magnitude as whole cells. The dark kinetics of the particles were comparatively slower. Removal of plastocyanin and other electron carriers by washing with Triton X-100 slowed down the kinetics of the dark reversal reaction to a greater extent. A similar positive absorbance change at 520 nm and slow dark reversal was also observed in the Photosystem I particles prepared by the Triton method. Mutant C-6E, which contains neither carotenoids nor chlorophyll b and lacks Photosystem II activity, demonstrates a normal signal I of the 520 nm absorbance change. This latter result contradicts the postulate that carotenoids are the possible cause of the 520 nm absorbance change.