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.