Absorbance changes due to the charge-accumulating species in system 2 of photosynthesis.

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.