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Plant and Cell Physiology, 1995, Vol. 36, No. 5 839-848
© 1995

The Steady State Chlorophyll a Fluorescence Exhibits in Vivo an Optimum as a Function of Light Intensity which Reflects the Physiological State of the Plant

Alaka Srivastava1, H. Greppin2 and Reto J. Strasser1

1Bioenergetics Laboratory, University of Geneva 1254, Jussy-Geneva, Switzerland
2Laboratory of Plant Physiology and Biochemistry CH-1211, Geneva-4, Switzerland

Modulated (690 and 730 nm), as well as direct chlorophyll (Chl) a fluorescence and changes in the concentration of the oxidized P700 were measured under steady state conditions in leaves of higher plants adapted to different light intensities. All the leaf samples exhibit an optimum curve of steady state fluorescence yield (Fs) versus the light intensity but its position with respect to light intensity varies considerably from one species to another or from one sample to other even in the same plant or within the same leaf sample. However, the optimum level of Fs was always at a moderate light intensity. By using the modulated fluorescence technique, the system with all closed (Flm) or open reaction center (Flo) were measured in steady state conditions. Each experimentally measured fluorescence yield was separated into a fluorescence emission of open (Fopen = Flo,(1—Vs)) and closed (Fclosed = (Flm . Vs)) reaction center (RC) of photosystem II where Vs=(Fs – Flo)/(Flm – Flo) is the function of fraction of closed reaction centers. With increasing light intensity, the fraction of open RC decreased while the fraction of closed RC increased. Maximum quantum efficiency ({Phi}Po) and actual quantum efficiency ({Phi}P) decreased by increasing light intensity. An optimum level of Fs was observed, when the fraction of closed reaction centers Vs of each sample was about 0.2 showing a common quenching mechanism which determines the fluorescence properties under steady state condition. This explains the apparent phenomenological contradiction that the fluorescence yield under steady state conditions can increase or decrease upon an increase of actinic light.

(Received December 31, 1994; Accepted May 1, 1995)
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