| The enhanced production of ROS during salt stress can pose oxidative stress to cells and lead to the oxidative destruction. High NaCl concentrations seem to impair electron transport and lead to the formation of ROS. Plants have well-developed antioxidant defense systems against ROS, involving both enzymatic and non-enzymatic mechanisms. The chief toxicity of O2 and H2O2 is thought to reside in their ability to initiate cascade reactions that result in the production of the hydroxyl radical and other destruction species such as lipid peroxides. These dangerous cascades are prevented by efficient operation of the cell's antioxidant defenses. SOD converts superoxide radicals (02) to hydrogen peroxide (H2O2). Hydrogen peroxide acts as a precursor of more cytotoxic or highly reactive oxygen derivatives, such as peroxynitrite or HO .It is very important to scavenge the excess H2O2. CAT GR and GST are related to H2O2-scavenging. In the present study, salt-dilution halophyte S. salsa grown under different salt concentrations was used to investigate the activities of H2O2-scavenging enzymes. The main results are as follows.1. Effects of NaCI treatment on growth of S. salsa seedlings.The growth of S. salsa was significantly enhanced by NaCl treatment. The fresh weight increased significantly with the increase of NaCl concentration up to 200 mmol/L and decreased slightly at higher concentrations. However, the fresh weight of the seedlings treated with 400 mmol/L NaCl was still higher than that of controls. Under the condition of 200 mmol/L NaCl, the fresh weight of 5. salsa reached the maximum.S. salsa plants accumulated high Na+ in salt environment. The leaves of S. salsa are the main organs of Na+accumulation. Leaf Na+ and Cl" contents were significantly increased with the increase of NaCl concentration (0-400 mmol/L), whereas K+ content and osmotic potential of the cell sap of the leaves were drastically decreased. Since the growth of S. salsa seedlings was stimulated by salt stress, the decrease of K+ concentration in the cell sap in a certain extent does not affect the growth of S. salsa. S. salsa modulate the osmolarity by absorbing abundant Na+. 2. Effects of salt-treatment on the photosynthetic characters of S. salsa seedlings.Gs and Ci of S. salsa seedlings did not show responses to salt treatment. Pn andPS II increased with the increase of NaC! concentration. Under 200 mmol/L NaCI, Pn and0 PS II of 5. salsa seedlings reached the maximum.In S. salsa seedlings, chla+b was not affected by salt stress (0-400 mmol/L), chla/b was significantly increased under conditions of 200 mmol/L NaCI (up to 171.1%), then decreased by 400 mmol/L NaCI (up to 136.2%).3. Radical damage to 5. salsa seedlings.lipid peroxidation (MDA) was used as indicators of NaCI stress to test the radical damage. In low NaCI concentration (0-200 mmol/L), leaf MDA did not show distinct increase. Under the condition of 400 mmol/L NaCI, leaf MDA was only increased by1 7.8%.4. Leaf CAT and isoforms of S. salsa response to salt treatment.In S. salsa leaves three isoforms of CAT were detected after staining of the native gels. Salt stress did not affect isoform pattern of CAT. The activity of CAT increased with the increase of NaCI concentration. In low NaCI concentration (0-200 mmol/L), activity of CAT did not show distinct increase, while under the condition of 200-400 mmol/L NaCI, CAT activity increased significantly.5. Effects of NaCltreatment on activity of GR GSH and GR isoforms of S.salsaSalt stress did not change isoform pattern of GR, just increased its activity. There was only one GR isoforms in S. salsa seedlings.GR and GSH belong to ascorbate-glutathione cycle. Under NaCI treatment, the activity of GR increased with the increase of NaCI concentration up to 200 mmol/L and decreased slightly at higher concentration. However, GR activity of NaCl-treated seedlings was still significantly higher than that of control plants .The content of GSH exhibited same trend to GR'sUnder salt stress, similar changes were observ...
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