ABSTRACT
The most important function of photosystem II (PSII) is its action as a water-plastoquinone oxido-reductase. At the expense of light energy, water is split, and oxygen and plastoquinol are formed. In addition to this most important activity, PSII has additional functions, especially in the regulation of (light) energy distribution. The downregulation of PSII during photoinhibition is a protection measure. PSII is an anthropogenic target for many herbicides. There is a unique action of bicarbonate on PSII. Decrease in the activity of PSII is the first effect in a plant under stress; this decreased activity can be most easily measured with fluorescence. PSII is a sensor for stress, and induces regulatory processes with different time scales: photochemical quenching, formation of a proton gradient, state transitions, downregulation by photoinhibition and gene expression.