Three distinct phases of chlorophyll a fluorescence quenching were observed in green plants by applying O2 pressures of up to 400 atm. These phases are interpreted as indications of three different mechanisms of O2 quenching. The most sensitive phase is dependent on intersystem electron transport. For dark-adapted bean leaves this fluorescence was quenched to half the initial yield with an O2 pressure of about 3 atm. The second mechanism was observed with 3-(3,4)dichlorophenyl)-1,1-dimethylurea (DCMU), namely the quenching of variable fluorescence in leaves, chloroplasts, and green algae cells. This effect of O2 is thought to be closely associated with the photochemical system II reaction centers. Half of the variable fluorescence was quenched with about 40 atm of O2. Finally, the antennae pigments are quenched, as observed by the effect of O2 on the O-level fluorescence yield, when all photochemical system II reaction center traps are presumably open. The O2 pressure required for half-quenching in this case was about 400 atm.The possibility that the quenching of fluorescence occurs with O2 concentrations low enough for endogenous O2 to have an effect on in vivo fluorescence was investigated. We suggest that O2 quenches by competing with photochemical system I for electrons derived from water splitting, and may thus function as a feedback regulator of photosynthesis.
