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 Q F which is suppressed by a short saturating flash, a quenching Q S destroyed under continuous illumination by a low efficiency process, and a quenching Q R which cannot be destroyed at low temperature, but is removed by preillumination before cooling the sample. 4. 4. Only the Q F quenching seems to be related to the normal electron transfer which leads to O 2 formation. 5. 5. It is suggested that destruction of Q S is associated with photooxidation of cytochrome 559.