Kinetics of delayed chlorophyll a fluorescence registered in milliseconds time range V. Goltsev 1, *, P. Chernev 1 , I. Zaharieva 1 , P. Lambrev 2 & R.J. Strasser 3 1 Department of Biophysics and Radiobiology, Biological Faculty, St. Kliment Ohridski, University of Sofia, 1164 Sofia, Bulgaria; 2 Institute of Biophysics, Bulgarian Academy of Sciences, Sofia, Bulgaria; 3 Bioenergetics Laboratory, University of Geneva, Switzerland; *Author for correspondence (e-mail: goltsev@ biofac.uni-sofia.bg; fax +359-2-865-66-41) Received 30 September 2004; accepted 17 November 2004 Key words: dark relaxation kinetics, electron transport, millisecond delayed fluorescence, Photosystem II, variable chlorophyll fluorescence Abstract Delayed fluorescence dark decays in the time interval from 0.35 to 5.5 ms are measured during dark to light adaptation in whole barley leaves and isolated thylakoid membranes, using a disc phosphoroscope. The changes in delayed fluorescence features are compared with variable chlorophyll fluorescence simulta- neously registered with the same apparatus as well as in parallel by Handy PEA (Hansatech Instruments Ltd.), and absorbance changes at 820 nm. The registered delayed fluorescence signal is a sum of three components – submillisecond with lifetime of about 0.6 ms, millisecond decayed 2–4 ms and slow com- ponent with lifetime >>5.5 ms. The submillisecond delayed fluorescence component is proposed to be a result of radiative charge recombination in Photosystem II reaction centers in the state Z + PQ A ) Q B ) , and its lifetime is determined by the rate of electron transfer from Q A ) to Q B ) . The millisecond delayed fluorescence component is associated with recombination in Z + PQ A ) Q B = centers with a lifetime deter- mined by the sum of the rate constants of electron transfer from the oxygen-evolving complex to Z + and of the exchange between the reduced and oxidized plastoquinone pool in the Q B -site. On the basis of these assumptions and of the different share of the three components in the integral delayed fluorescence during induction, an attempt has been made to interpret the changes in the delayed fluorescence intensity during the transition of the photosynthetic apparatus from dark to light adapted state. Abbreviations: DF – delayed chlorophyll fluorescence; PF – prompt (variable) chlorophyll fluorescence; PQ – plastoquinone; PS – Photosystem; RC – reaction center Introduction As defined by Lavorel (1975) delayed chlorophyll fluorescence (DF) is a light emission by chloro- phyll molecules secondarily excited as a result of charge recombination in Photosystem (PS) II reaction center (RC). According to the radical pair hypothesis, DF originates from recombination of P 680 + I ) and occurs with a 2–4 ns lifetime (Jursinic 1986). It has been shown that radical pair hypothesis may be valid for explaining DF emitted in the microsec- ond time range (Jursinic et al. 1978). The origin of DF decaying in the seconds-to-minutes time scale (at 30 s, 1 and 2 min after the flash) as well as of thermoluminescence in leaves is ascribed to S 2 Q B ) or S 3 Q B ) recombination (Rutherford et al. 1984). The decay kinetics of DF in a time range from several microseconds to milliseconds after light excitation (decay range that will be investigated in Photosynthesis Research (2005) 84: 209–215 Ó Springer 2005