Oxygen reduction in the Mehler reaction is insufficient to protect photosystems I and II of leaves against photoinactivation

The relative roles of assimilatory and photorespiratory electron flows on one side and of the Mehler-peroxidase pathway on the other side in sustaining electron transport and providing protection against photoinhibition were investigated in leaves of spinach (Spinacia oleracea L.) and sunflower (Helianthus annum L.). After inhibiting photosynthesis and photorespiration of intact leaves by either HCN or glycolaldehyde, light-dependent linear electron transport was decreased by more than 90% at a photon flux density of 800 μmol m^-2 s^-1. Remaining electron transport exhibited characteristics of the Mehler reaction. Nonphotochemical quenching of chlorophyll fluorescence increased after inhibition of CO₂ assimilation and photorespiration indicating effective dissipation of excess excitation energy. Nevertheless, appreciable photoinactivation was observed under these conditions not only of photosystem II but also of photosystem I. This damage was oxygen-dependent. It was much reduced or absent when the oxygen concentration of the atmosphere was reduced from 21 to 1%.