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Plant and Cell Physiology, 1973, Vol. 14, No. 5 837-850
© 1973

Article

Studies on chlorophyll fluorescence in chloroplasts III. Effect of artificial electron donors for photosystexn 2 on reoxidation of the fluorescence quencher, Q, in spinach chloroplasts

Isamu Ikegami and Sakae Katoh

Department of Biophysics and Biochemistry, Faculty of Science, University of Tokyo Tokyo, Japan

  1. Effects of various reducing reagents including dithionite, which serve as artificial electron donors for photosystem 2, on the recovery of fluorescence induction in the presence of 3-(3,4-dichlorophenyl)-l,l-dimethylurea (DCMU) during the dark incubation of preilluminated chloroplasts were investigated.
  2. The dark recovery of fluorescence induction was not affected by the addition of the p-phenylenediamine-ascorbate couple, the hydroquinone-ascorbate couple or manganese. Incubation of chloroplasts with dithionite caused gradual suppression of the dark recovery.
  3. Preillumination of chloroplasts caused partial inhibition of the recovery of fluorescence induction.
  4. At low intensities of excitation light, the fluorescence yield increased very slowly and continuously, and never reached a steady state. This continuous increase in fluorescence yield under weak light was due to photoinhibition of the dark recovery. A technique was devised to determine the steady state yield of fluorescence, without the intervention of photoinhibition, at weak light intensities. The steady state yield of fluorescence in the presence of DCMU was suppressed at lower excitation intensities. This drop in the fluorescence yield was not altered by the presence of added reducing reagents but was suppressed after long preincubation of chloroplasts with dithionite.
  5. The delayed fluorescence with a decay time of seconds was affected by dithionite but not by other reductants.
  6. Results are discussed in terms of reoxidation of the reduced primary electron acceptor, Q, by the oxidized primary electron donor for photosystem 2. A model of the electron transport associated with photoreaction 2 is proposed to account for the experimental results obtained.

(Received February 27, 1973; )
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