Response of Antioxidant Defense System in Populus tomentosa Cuttings Subjected to Salt Stress

To explore the antioxidant defense system of Populus tomentosa cuttings subjected to salt stress, the cuttings were treated with three different concentrations (85 mmol·L^-1, 170 mmol·L^-1, 260 mmol·L^-1)of NaCl for 3-54 h, and the changes in superoxide dismutase (SOD)and peroxidase (POD)activities, and malondialdehyde (MDA)content in the leaves of cuttings were investigated. The results showed that SOD and POD activities significantly decreased during 3 h at the beginning of all NaCl treatment, and then slightly decreased during a period of 9-54 h, but a lower decrease in SOD activity than in POD activity was observed in all treated cuttings. MDA content distinctly increased and reached a maximum value during 15 h after the beginning of salt stress, and then decreased, but its level was higher than the control.The decreased SOD and POD activities and the increased MDA content affected by the treatment of 260 mmol·L^-1 NaCl were more obvious than that affected by the treatment of 85 mmol·L^-1 or 170 mmol·L^-1 NaCl. It is possible that the decrease in SOD and POD activities under salt stress resulted in the enhancement of membrane lipid peroxidation, which seems to be the key factor that le ads to the increase of MDA content and the occurrence of oxidative damage to cuttings.