Mechanism of photoinactivation in photosynthetic systems IV. Light-induced changes in the fluorescence transient

1. 1. Light-induced changes in the fluorescence transient (685nm) of spinach chloroplast fragments at room temperature wereinvestigated in an attempt to correlate these changes with photoinactivationin photosystem II.
2. 2. Parallel decreases in the steady-statelevel of fluorescenceand in the variable fraction, observedunder aerobic light-treatment,were not related processes butseparate reactions as indicatedby an anaerobic-interruptionexperiment where the decrease inthe steady-state level occurredonly after the disappearanceof induction.
3. 3. Anaerobic light-treatmentcaused an increase in the initiallevel of fluorescence parallelto photoinactivation in photosystemII, and a more rapid partialdecrease in the teady-state levelof fluorescence.
4. 4. Thesteady-state level of fluorescence showed pronouncedpH dependency,and had an optimum at about pH 6.5, while theinitial levelwas practically independent of environmental pHwithin a neutralrange. Aerobic or anaerobic light-treatmentcompletely eliminatedpH dependency.
5. 5. Effects of electron acceptors, dichlorophenyl-dimethylurea,dithionite, and of electron donors for photosystem II on thefluorescence transient of photoinactivatedchloroplast fragmentswere investigated. Based on the data presented here, it seemsreasonable to assume that photoinactivation in photosystem IIis closely related to the state of reaction centers in the photosystem.

(Received July 8, 1970; )