Responses of anti-oxidant enzymes and the ascorbate-glutathione cycle to heat, drought, and synergistic stress in Phyllostachys edulis seedlings

To study the physiological responses of 3-year-old Phyllostachys edulis seedlings to drought stress, heat stress, and synergistic stress, a pot experiment was conducted. Changes of reactive oxygen species (ROS), malondialdehyde (MDA), ascorbic acid, and glutathione concentration, as well as changes in enzyme activity of anti-oxidative enzymes and ascorbate-glutathione (AsA-GSH) cycle in the leaves of Ph. edulis were determined using an artificial simulation method by a spectrophotometer with different levels of drought stress (control check(70.0%-80.0% of field water-holding capacity), light stress(60.0%-70.0% FC), medium stress(40.0%-50.0% FC), and heavy stress(20.0%-30.0% FC)), heat stress (25℃ and 40℃), and synergistic stress. Results showed that the contents of superoxide anion (O2·-) and hydrogen peroxide (H2O2) in leaves of Ph. edulis increased (P < 0.05) compared to ck with drought stress. Anti-oxidant enzyme activities and the ρGSH/ρGSSG ratio increased (P < 0.05) compared to ck with an increase in the degree of drought stress. With moderate drought stress, compared to ck, highly significant differences (P < 0.01) were found for activities of superoxide dismutase (SOD) (1.7 times higher), peroxidase (POD) (1.7 times higher), and the ratio of ρGSH/ρGSSG (0.6 times higher). Compared to ck, MDA increases (1.4 times) were highly significant (P < 0.01), and increases in the activity of SOD, POD, and catalase (CAT), along with the ratio ρAsA/ρDHA were highly significant (P < 0.01) with heat stress. After synergistic stress, compared to ck, ROS increased (P < 0.01), and the degree of membrane lipid peroxidation was enhanced (P < 0.01); meanwhile, the stability of CAT activity was strongest (P < 0.01) increasing steadily compared to ck with an increase in synergism. Also, the ρAsA/ρDHA ratio showed a downward trend; whereas, the ρGSH/ρGSSG ratio was not significant. Thus, the GSH cycle showed stronger resistance to synergistic stress than the AsA cycle suggesting that Ph. edulis seedlings could protect themselves effectively from oxidative damage with a degree of heat, drought, and synergistic stress.