Oxygen dependence of photoinhibition at low temperature in intact protoplasts of Valerianella locusta L.

Photoinhibition of photosynthesis in vivo is shown to be considerably promoted by O₂ under circumstances where energy turnover by photorespiration and photosynthetic carbon metabolism are low. Intact protoplasts of Valerianella locusta L. were photoinhibited by 30 min irradiation with 3000 mol photons · m^–2 · s^–1 at 4° C in saturating CO₂] at different oxygen concentrations, corresponding to 2–40% O₂ in air. The photoinhibition of light-limited CO₂-dependent photosynthetic O₂ evolution increased with increasing oxygen concentration. The uncoupled photochemical activity of photosystem II, measured in the presence of the electron acceptor 1,4-benzoquinone, and maximum variable fluorescence, F_v, were strongly affected and this inhibition was closely correlated to the O₂ concentration. The effect of O₂ did not saturate at the highest concentrations applied. An increase in photoinhibitory fluorescence quenching with O₂], although less pronounced than in protoplasts, was also observed with intact leaves irradiated at 4° C in air. Initial fluorescence, F_o, was slightly (about 10%) increased by the inhibitory treatments but not influenced by O₂]. A long-term cold acclimation of the plants did not substantially alter the O₂-sensitivity of the protoplasts under the high-light treatment. From these experiments we conclude that oxygen is involved in the photoinactivation of photosystem II by excess light in vivo.Abbreviations and Symbols Chl chlorophyll - F_o initial fluorescence - F_M maximum fluorescence - F_v maximum variable fluorescence - PCO photorespiratory carbon oxidation - PCR photosynthetic carbon reduction - PFD photon flux density - q_N non-photochemical quenching - q_P photochemical quenching - _S quantum efficiency of electron transport of photosystem II This study was financially supported by the Deutsche Forschungs-gemeinschaft (SFB 189) and the Foundation for Fundamental Biological Research (BION), which is subsidised by the Netherlands Organization for the Advancement of Pure Research (NWO).