Observation of two quenchers of chlorophyll fluorescence in chloroplasts at -196 degrees C.

Fluorescence induction at -196 degrees C has been monitored in chloroplasts rapidly frozen after poising at different redox potentials at room temperature. It was found that, as at room temperature, the initial level of fluorescence observed upon shutter opening (Fo), relative to the final level observed after 10 seconds of illumination (Fm) increased as the redox potential of the chloroplasts was lowered. Redox titration and revealed the presence of two quenching components with Em, 7.8 at -70 mV and -275 mV accounting for approx. 75% and 25% of the variable fluorescence (Fv). Parallel observation of fluorescence yield at room temperature similarly gave two components, with Em, 7.8 at -95 mV and -290 mV, also accounting for appox. 75% and 25%. Simultaneous measurement of fluorescence emission at -196 degrees C at 695 nm and 735 nm indicated that both emissions are quenched by the same redox components.