The influence of cytochrome b559 on the fluorescence yield of chloroplasts at low temperature

The maximum light-induced fluorescence yield, F_M, of spinach chloroplasts at ? 196 °C was less when the chloroplasts were oxidized with ferricyanide prior to freezing; the minimum fluorescence yield, F₀, of the dark-adapted chloroplasts at ? 196 °C was unaffected. The ratio of the fluorescence yields, F_M/F₀, measured at 695 nm at low temperature was 4.5–5.0 for normal chloroplasts and 2.0–2.5 in the presence of ferricyanide. The oxidative titration curve of F_M followed a 1 electron Nernst equation with a midpoint potential of 365 mV and followed closely to the oxidation of cytochrome b₅₅₉. The photoreduction of C?550 at low temperature was the same at all redox potentials over the range of 200–500 mV. It is suggested that a relatively strong oxidant associated with the water-splitting side of Photosystem II, possibly the primary electron donor, can chlorophyll fluorescence of Photosystem II as well as the primary electron acceptor.