Multiple functions of photosystem II

The most important function of photosystem II (PSII) is its action as a wat er-plastoquinone oxido-reductase. At the expense of light energy, water is split, and oxygen and plastoquinol are formed. In addition to this most imp ortant activity, PSII has additional functions, especially in the regulatio n of (light) energy distribution. The downregulation of PSII during photoin hibition is a protection measure. PSII is an anthropogenic target for many herbicides. There is a unique action of bicarbonate on PSII. Decrease in th e activity of PSII is the first effect in a plant under stress; this decrea sed activity can be most easily measured with fluorescence. PSII is a senso r 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.