The Shape of the Pump-Probe Fluorescence Induction Curves in Chloroplasts is Determined by the Duration of the Pump Light Flash
It is well established that the fluorescence yield of Photosystem II (PS II) in green plants is a function of the photochemical state of the reaction centers. In dark-adapted chloroplasts, when the reaction centers are all open, the fluorescence yield is low, and is denoted by Fo. When the sample is illuminated under steady-state conditions, the fluorescence rises and ultimately reaches a saturation level Fmax, corresponding to a state of PS II in which all of the reaction centers are closed. The ratio R = Fmax/Fo is generally 3–5 in value, and the shape of the variable fluorescence induction curve is sigmoidal in shape under steady-state illumination (for a review see reference 1). This sigmoidicity is generally accounted for in terms of the model of Joliot and Joliot . In this model, it is assumed that an exciton which encounters a closed reaction center can migrate to another, open PS II reaction center. This propability (p) is in the range of 0.4 – 0.7 . The variable fluorescence yield increases more rapidly than q, the fraction of closed reaction centers, and the steady-state illumination fluorescence induction curves are thus sigmoidal in shape.
KeywordsReaction Center Fluorescence Yield Fluorescence Induction Induction Curve Fluorescence Rise
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