| How to efficiently resolve the problem of metabolism disorder of reactive oxygen species (ROS) caused by oxidative stress induced by salt stress is one of mechanisms of halophytes survival under salt conditions, and also indicates the capacity of salt resistance. Ascorbate peroxidase (APX) is one of the important antioxidative enzymes in the antioxidative network, which transforms H2O2 derived from all sorts of sources in cells - especially from the disproportionate of superoxide anion radical((O2) catalyzed by SOD - into H2O , thus prevents cells from the oxidative damage of H2O2 , and therefore prevent the formation of more dangerous ROS such as hydroxyl radical (OH-) which is the most active ROS - derived from Harber- Weiss or Fenton reactions of H2O2 with metal ions such as Cu+ and Fe2+. In cells, APX has several isoenzymes, cytosolic and chloroplastic isoforms are main ones, of which the chloroplastic isoform represents a large portion of APX of the cell. In the present study, halophyte Suaeda salsa treated with salt (0 mmol/L, 200 mmol/L and 400 mmol/L NaCl) and exposed to high photon intensity (1000umol m-2 s-1) was used to examine the relationship between APX activity and metabolism of ROS . At the same time, in order to have a good insight into the relationship between antioxidation of Suaeda salsa and its salt resistance other relatively physiological and biochemical parameters were also measured.First of all, the growth of Suaeda salsa treated with NaCl was consistent with its capacity of adaptation to salt condition. Fresh weight, dry weight and organic dry weight of plant exposed to 200 mmol/L NaCl for seven days were significantly enhanced compared with those of the controls, i.e. up to 198.4%, 153.3% and 120% of those of the controls respectively. Under 400 mmol/L NaCl treatment for seven days, fresh weight and dry weight of plants were still significantly higher than those of the control, up to 154.8% and 150% of those of the controls, and the organic dry weight was not significantly different from that of the controls. In respect of photosynthesis, net photosynthetic rate of plants exposed to 200 mmol/L NaCl for seven days was up to 145.3% that of the controls. Under 400 mmol/L NaCl treatment for seven days, net photosynthetic rate, though slightly higher than the control's,decreased to about the same level as that of controls. At the same time, chlorophyll content was not affected by salt treatment.So, Suaeda salsa grew well under salt condition, which may result from Suaeda salsa having high ability of osmotic adjustment and ionic compartmentazation. On one hand, Suaeda salsa decreased the osmotic potential of cell sap by absorbing a large amount of inorganic ions, such as Na+, Cf etc, and also by synthesizing some compatible organic substances, so that it got water from the salt soil, which was demonstrated by the drastic increment of the content of Na+ and Cl- and significant decrement of osmotic potential of cell sap under 200 and 400 mmol/L NaCl treatments. Water content of plant treated with 200 mmol/L NaCl was significantly higher than that of the controls. However, there were no significant differences in water content between 400 mmol/L NaCl-treated paints and controls. On the other hand, Suaeda salsa transferred a large amount of salt ions into the vacuoles to maintain ion homeostasis and to avoid the toxicity of salt ions.After a short time of salt and high light treatment, the content of malondialdehyde (MDA) increased drastically. But no significant differences in MDA content were observed between the treated plants and controls after long-term treatment. This indicated a rising tendency of ROS content on the whole under salt and high photon intensity, and even a great increment after short time of treatment. So, Suaeda salsa must have a high ability to scavenge ROS in order to grow well under salt stress, otherwise the highly active ROS would induce oxidative damage on the plants and inhibit its growth. Now APX activity changed correspondingly. Under 200 mmol/L a...
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