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 ( F 695 ) has been observed in chloroplasts and whole leaves of spinach. Photosystem I (PS I) fluorescence emission at 735 nm ( F 735 ) 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 F 735 / F 685 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 ( F 695 ) is discussed with respect to the formation of reduced pheophytin in the PS II reaction center at 77 K.