O2 photoreduction at acceptor side of Photosystem I provide photoprotection to conifer thylakoids in early spring

Green organisms evolve O2 via photosynthesis and consume by respiration. Net O2 consumption only becomes dominant when photosynthesis is suppressed at night. Here, we show that green thylakoid membranes of Scots pine (Pinus sylvestris L) and Norway spruce (Picea abies) needles demonstrate strong O2 consumption even in the presence of light when extremely low temperatures coincide with high solar irradiation during early spring. This phenomenon deviates from the general finding that photosynthetic organisms evolve O2 upon illumination. By using different electron transport chain inhibitors, we showed that O2 consumption occurred around photosystem (PS) I and correlated with higher abundance of flavodiiron (Flv) A protein in ES thylakoid membranes. Furthermore, by measuring P700 absorption changes, we separated different alternative electron flow pathways and demonstrated that electron scavenging from the acceptor-side of PSI via O2 photoreduction is a major alternative pathway in ES. This photoprotection mechanism in vascular plants indicates that conifers have developed an adaptative evolution trajectory for growing in harsh environments. ### Competing Interest Statement The authors have declared no competing interest.