Singlet oxygen production by photosystem II is caused by misses of the oxygen evolving complex.

Singlet oxygen ( O ) is a harmful species that functions also as a signaling molecule. In chloroplasts, O is produced via charge recombination reactions in photosystem II, but which recombination pathway(s) produce triplet Chl and O remains open. Furthermore, the role of O in photoinhibition is not clear. We compared temperature dependences of O production, photoinhibition, and recombination pathways. O production by pumpkin thylakoids increased from -2 to +35°C, ruling out recombination of the primary charge pair as a main contributor. S Q or S Q recombination pathways, in turn, had too steep temperature dependences. Instead, the temperature dependence of O production matched that of misses (failures of the oxygen (O ) evolving complex to advance an S-state). Photoinhibition in vitro and in vivo (also in Synechocystis), and in the presence or absence of O , had the same temperature dependence, but ultraviolet (UV)-radiation-caused photoinhibition showed a weaker temperature response. We suggest that the miss-associated recombination of P Q is the main producer of O . Our results indicate three parallel photoinhibition mechanisms. The manganese mechanism dominates in UV radiation but also functions in white light. Mechanisms that depend on light absorption by Chls, having O or long-lived P as damaging agents, dominate in red light.