Different types of quenching involved in photosystem II centers.

Abstract Low-temperature fluorescence rise curves are studied in an apparatus which allows fast variation and equilibration of the temperature. 1. 1. Comparison of the effects of low temperature (−40 to −70 °C) and 3-(3,4-dichlorophenyl)-1,1-dimethylurea shows that this inhibitor not only blocks the electron transfer between Q and A, but also removes a fraction of the quencher of Photosystem II centers. 2. 2. Low-temperature fluorescence rises (−40 to −70 °C) depend on the number of oxidizing equivalents stored on the donor side of System II. 3. 3. Three types of quenching can be experimentally distinguished: a quenching QF which is suppressed by a short saturating flash, a quenching QS destroyed under continuous illumination by a low efficiency process, and a quenching QR which cannot be destroyed at low temperature, but is removed by preillumination before cooling the sample. 4. 4. Only the QF quenching seems to be related to the normal electron transfer which leads to O2 formation. 5. 5. It is suggested that destruction of QS is associated with photooxidation of cytochrome 559.