Chlorophyll-a fluorescence induction; Circular dichroism; Conformational changes; Excitonic interaction; Light-harvesting complex II (LHCII); Non-photochemical quenching (NPQ); Photosystem II core complex

The water-splitting function of Photosystem II (PS II) allowed oxygenic photosynthetic organisms to tap into a limitless source of protons and electrons for carbon fixation and, by this means, to colonize the planet. This ability comes with major corollaries – one is the need to synchronize the turnover rate between the two photosystems working in tandem and another is the potential for production of harmful reactive oxygen species by PS II, especially under excess excitation conditions. To cope with this and, in general, to fine-tune the photosynthetic functions upon the often rapidly changing environmental conditions, the photosynthetic machineries evolved multilevel regulatory processes. Key regulatory mechanisms affect the photophysics and photochemistry of PS II; these require well discernible plasticity of the light-harvesting antenna system and the reaction center. Despite intense research in the past decades, our understanding about the underlying physical mechanisms.