Hormesis in photosystem II (PSII) that is observed in appropriately planned studies is a dose or -time -response relationship to a disruption of homeostasis illustrated by U-shaped response curves. PSII that uses the light energy to oxidize water into molecular oxygen and delivers electrons and protons is more susceptible than photosystem I (PSI) to photodamage. A hormetic response of PSII is triggered by the non-photochemical fluorescence quenching (NPQ) mechanism that is a strategy to protect the photosynthetic apparatus from photo-oxidative damage by dissipating excess light energy as heat and preventing the destructive reactive oxygen species (ROS) creation. A basal level of ROS is needed for optimal plant growth, while a low increased level of ROS is beneficial for triggering hormetic responses, and a high level of ROS out of the boundaries is considered harmful to plants.