LIGHT‐INDUCED QUENCHING OF PHOTOSYSTEM II FLUORESCENCE AT 77 K

Abstract— 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.