A photoprotective role for O2 as an alternative electron sink in photosynthesis?

Photoprotection of the photosynthetic apparatus has two essential elements: first, the thermal dissipation of excess excitation energy in the photosystern II antennae (i.e. non-photochemical quenching), and second, the ability of photosystem II to transfer electrons to acceptors within the chloroplast (i.e. photochemical quenching). Recent studies indicate that the proportion of absorbed photons that are thermally dissipated through the non-photochemical pathway often reaches a maximum well before saturating irradiances are reached. Hence, photochemical quenching is crucial for photoprotection at saturating light intensities. When plants are exposed to environmental stresses and the availability of CO2 within the leaf is restricted, the reduction of oxygen by both the photorespiratory and the Mehler ascorbate peroxidase pathways appears to play a critical photoprotective role, substituting for CO2 in sustaining electron flow. Induction of high activity of the Mehler ascorbate peroxidase pathway may be associated with acclimation to environmental stress.