Study On The Response Of AsA-GSH Cycle In Hibiscus Hamabo Sieb.et Zucc. To Salt Stress

The production of reactive oxygen species (ROS) will be increased in plant cellswhen they suffer from salt stress. ROS can cause cell injury if they maintain in ahigher level. The Ascorbic acid-Glutathione (AsA-GSH) cycle is a very importantantioxidant defense system in plant, whose duty is cleaning the excess H2O2. In thiscycle, H2O2can be reduced to H2O by the ascorbate peroxidase (APX) with AsA assubstrate. At the same time, AsA is oxidized into DHA. Under the catalysis of DHAR,DHA is reduced to AsA. In this progress, GSH is turned into GSSG, while the GSSGis reduced to GSH under the catalysis of GR. The ROS in plant are eliminated throughthe coordination of each composition in AsA-GSH cycle.Hibiscus hamabo Sieb.et Zucc, one kind of deciduous shrub or small tree thatbelongs to Malvaceae Hibiscus, is a typical halophyte that can grow in high salinitysoil. This dissertation studied mainly from physiological and biochemical perspectiveon the responses of Hibiscus hamabo Sieb.et Zucc.s’ to NaCl stress in differentgrowth periods around the AsA-GSH cycle. The main results are as follows:1. Treated with150mmol·L-1NaCl, the enzyme activities of SOD, APX and GRin Hibiscus hamabo cotyledons are significantly higher than the control; while treatedwith the300mmol·L-1NaCl, all the activities of SOD, APX and GR are declined, butstill significantly higher than the control. Seedlings in the cotyledon stage aresensitive to external environment. Hibiscus hamabo seedlings in the cotyledon stagehave resistance against a range of salt stress to ensure their regular growth.2. As the growth of Hibiscus hamabo, the early seedlings have a more perfectmechanism of salt tolerance than that of the cotyledon period. We can find that theHibiscus hamabo can disproportionate the O2·-into H2O2by means of enhancing theactivity of SOD when treated with the0,150and300mmol·L-1NaCl respectively.Then H2O2can be cleaned away by AsA-GSH cycle, this reflected the activities ofantioxidant enzymes and antioxidants content increased with salt concentrationincreasing. But the content of H2O2and the O2·-production rate in leaves rise alongwith the salt concentration increased, that means the ROS accumulate in early seedlings and the plantlets can only tolerate a range of salt stress.3. When Hibiscus hamabo grows to the period of seedling stage with10~15leaves, the antioxidant defense mechanism gets more mature and improved, thatmakes the Hibiscus hamabo can tolerate a higher and longer salt stress. TreatedHibiscus hamabo with0,200and400mmol·L-1NaCl for0~6days, there’s nosignificant difference on phenotype between the salt-treated and control group; withthe extension of salt stress, the salt-treated group grow slowly, their leaves appearwilting and lacked of green. The indexes determined in the AsA-GSH cycle suggestedthat, at the early stage of salt stress, the seedlings of Hibiscus hamabo remove excessROS by improving the antioxidant enzymes’ activities and antioxidants’ content,making the ROS stay at lower level so as to prevent damage to seedlings. As the stressstage extended, ROS accumulates in seedlings and beyond the tolerance threshold,both the antioxidant system and cells get damaged seriously.All the results above revealed that Hibiscus hamabo has an efficient andcooperative antioxidant defense mechanism in different growth periods. Fromcotyledon to seedling, the antioxidant defense systems get more pefect in Hibiscushamabo. AsA-GSH cycle plays an important role in responsing to NaCl stress ofdifferent growth periods in Hibiscus hamabo, which ensure Hibiscus hamabo cantolerate a range of salt stress and survive.