title: Absorbance changes due to the charge-accumulating species in system 2 of photosynthesis.
abstract: Absorbance changes are reported associated with Photosystem II and showing a periodicity of two and four as a function of flash number. The absorbance changes showing a periodicity of two were found to occur in the presence of artificial electron donors as well and are presumably caused by the secondary electron acceptor R of Photosystem II. The absorbance difference spectra suggest that R is a plastoquinone molecule, which is reduced to its semiquinone anion after an uneven number of flashes. After an even number of flashes, the semiquinone is reoxidized. The absorbance changes showing a periodicity of four are tentatively ascribed to the charge accumulating donor complex of Photosystem II.
mesh_terms: D002736:Chloroplasts / Q000187:drug effects / Q000378:metabolism*; D003624:Darkness; D004237:Diuron / Q000494:pharmacology; D004579:Electron Transport; D006898:Hydroxylamines / Q000494:pharmacology; D007700:Kinetics; D008027:Light; D010785:Photophosphorylation / Q000187:drug effects; D010944:Plants; D013056:Spectrophotometry, Ultraviolet; D013997:Time Factors
chemical_list: D006898:Hydroxylamines; D004237:Diuron