The dynamics of fluorescence decay and charge recombination were, studied in the ether-extracted photosystem I reaction center isolated from spinach with picosecond resolution over a wide time range up to 100 us. At all temperatures from 268 to 77 K, a slow fluorescence decay component with a 30-40 ns lifetime was detected. This component was interpreted as a delayed fluorescence emitted from the singlet excited state of the primary donor P700*, which is repopulated through charge recombination that was increased by the lack of secondary acceptor phylloquinone in the sample. Analysis of the fluorescence kinetics allowed estimation of the standard free-energy difference -Delta G between P700* and the primary radical pair (P700(+)A0(-)) state over a wide temperature range. The values of -Delta G were estimated to be 160/36 meV at 268/77 K, indicating its high sensitivity to temperature. A temperature-dependent -Delta G value was also estimated in. the delayed fluorescence of the isolated photosystern I in which the secondary acceptor quinone was partially prereduced by preillumination in the presence of dithionite. The results revealed that the temperature-dependent -Delta G is a universal phenomenon common with the purple bacterial reaction centers, photosystem II and photosystem I reaction centers.