Light-induced quenching of photosystem II fluorescence at 77 K

Light-induced quenching of the low temperature fluorescence emission from photosystem II (PS II) at 695 nm (F695) has been observed in chloroplasts and whole leaves of spinach. Photosystem I (PS I) fluorescence emission at 735 nm (F735) is quenched to a lesser degree but this quenching is thought to originate from PS II and is manifest in a reduced amount of excitation energy available for spillover to PS I. Differential quenching of these two fluorescence emissions leads to an increase in the F735/F685 ratio on exposure to light at 77 K. Rewarming the sample from -196°C discharges the thermoluminescence Z-band and much of the original unquenched fluorescence is recovered. The relationship between the thermoluminescence Z-band and the quenching of the low temperature fluorescence emission (F695) is discussed with respect to the formation of reduced pheophytin in the PS II reaction center at 77 K.