(higher plants, algae, and cyanobacteria) is exposed to a series of single turnover flashes, oxygen evolution is detected with typical period-four damped oscillation with maxima on the third and the seventh flash and with minima on the first and the fifth flash (for review, see). Based on this experiment, Bessel Kok and co-workers introduced a cycle of five flash-induced transitions of the so-called S-states, describing the four redox states of OEC: When four oxidizing equivalents have been stored (at the S4-state), OEC returns to its basic S0-state. In the absence of light, the OEC will "relax" to the S1 state; the S1 state is often described as being "dark-stable". The S1 state is largely considered to consist of manganese ions with oxidation states of Mn3+, Mn3+, Mn4+, Mn4+. Finally, the intermediate S-states were proposed by Jablonsky and Lazar as a regulatory mechanism and link between S-states and tyrosine Z. In 2012, Renger expressed the idea of internal changes of water molecules into typical oxides in different S-states during water splitting. Inhibitors Inhibitors of PSII are used as herbicides. There are two main chemical families, the triazines derived from cyanuric chloride of which atrazine and simazine are the most commonly used and the aryl ureas which include chlortoluron and diuron (DCMU). See also Oxygen evolution P680 Photosynthesis Photosystem Photosystem I Photosystem II light-harvesting protein Reaction Centre Photoinhibition References Photosynthesis Light reactions Manganese enzymes EC 1.10.3