Dissipation of Excess Energy in Mehler-Peroxidase Reaction in Rumex Leaves During Salt Shock

By measurement of gas exchange and chlorophyll fluorescence, the effects of salt shock on photosynthesis and the mechanisms to protect photosynthetic machinery against photodamage during salt shock were investigated in leaves of Rumex seedlings. Salt shock induced significant decrease in photosynthesis both in 21 and 2 % O₂. In 21 % O₂, quantum yield of photosystem 2 (PS2) electron transport (_PS2) decreased slightly and q_P remained constant, suggesting that the excitation pressure on PS2 did not increase during salt shock. In 2 % O₂, however, both _PS2 and q_P decreased significantly, suggesting that the excitation pressure on PS2 increased during salt shock. NPQ increased slightly in 21 % O₂ whereas it increased significantly in 2 % O₂. The data demonstrated that during salt shock a considerable electron flow was allocated to oxygen reduction in the Mehler-peroxidase reaction (MPR). Under high irradiance and in the presence of saturating CO₂, the susceptibility of PS2 to photoinhibition in salt-shocked leaves was increased when the electron flow to oxygen in MPR was inhibited in 2 % O₂. Hence, MPR is important in photoprotection of Rumex seedlings during salt shock.