Responses of photosynthesis, lipid peroxidation and antioxidant system in leaves of Quercus mongolica to elevated O3

Quercus mongolica Fisch. ex Ledeb. is an economically and ecologically important oak species mainly distributed in East Asia. Our study suggested that it was susceptible to elevated O-3. Using open top chambers, changes in photosynthesis, lipid peroxidation and antioxidant system were studied in Q mongolica leaves fumigated with elevated O-3 (approximate to 80 nmol mol(-1)). During the O-3 exposure, photosynthetic rate (Pn) and stomatal conductance decreased significantly, but the decrease in Pn hardly resulted from the stomatal limitation, as no decrease was observed in the intercellular CO2 concentration. In addition, significant decrease was noted in carboxylation efficiency, indicating that the CO2 fixation process was affected by O-3, and it can be taken as the main factor leading to the decline of Pn, particularly in the initial 45 days of O-3 fumigation. In the initial 45 days of O-3 fumigation, ascorbate (Asc) content and activities of superoxide dismutase and ascorbate peroxidase were enhanced, but the enhanced antioxidant capability did not balance reactive oxygen species production, resulting in significant increase in malondialdehyde content at day 45. In the following O-3 fumigation period, significant decrease in activities of antioxidant enzymes and Asc content were recorded in the leaves. Along with the decline of antioxidant capability, chlorophyll loss, increase in electrolyte leakage and decrease in maximum photochemical efficiency of photosystem II were also noted. According to the biochemical and photosynthetic changes, we suppose that leaf senescence was accelerated by elevated O-3 in the prolonged fumigation period. (C) 2010 Elsevier B.V. All rights reserved.