| Superoxide dismutases (SOD) are metalloproteins that catalyze the dismutation of superoxide radicals to hydrogen peroxide and oxygen, which has three isoforms: Cu/Zn -SOD, Fe-SOD and Mn-SOD. In halophyte species, salt stress often leads to oxidative stress. Therefore, SOD may play an important role in salt-tolerance mechanics of halophytes. Three halophyte species, Suaeda salsa (S. salsa), Atriplex centralasiatia (A.centralasiatia) and Limonium bicolor (L.bicolor) were grown on the medium containing different NaCl concentration (0,100,200,400 mmol/L), in order to study effect of salt stress on the photosynthetic character and on activity of leaf SOD, the key component of antioxidant enzyme. All analyses were performed of seedlings after 7- day salt- treatment. Some of them were done after 1, 3, 5 and Vdays salt-treatment. The main results are as follows.1. Effects of NaCl treatment on growth of three types of halophytes.There were significant differences in the different types of halophyte. Under condition of 1 OOmmol/L NaCl, the fresh weight of whole plant of L. bicolor was drastically higher than the control. Under the condition of 200 and 100 mmol/L NaCl, the fresh weight of S. salsa and A. centralasiatia reached the maximum respectively. Fresh weight of three halophytes was decreased differently with the increasing of NaCl concentration, that of S. salsa, L. bicolor and A. centralasiatia was decreased up to 81.7%, 52.0% and 57.3% of their maxima respectively by 400 mmol/L NaCl. Similar changes with fresh weight were observed in dry weight, inorganic dry weight and water content of whole plant of three types of halophyte, but there was the different degree in their changes. In three types of halophytes, leaf Na+ and Cl" contents were significantly increased with the severity of the stress (0-400 mmol/L), whereas K+ content and osmotic potential of the cell sap of leaf were drastically decreased. The results showed that different halophytes had different salt-tolerance and the similar osmotic adjustment.2. Effects of salt-treatment on the photosynthetic characters of three types of halophytesFv/Fm of three types of halophytes did not show responses to salt treatment. In low NaCl concentration (S.salsa 0-200 mmol/L, A.centralasiatia and Licolor bicolor: 0-100 mmol/L), Pn, OPS II, Gs of A.centralasiatia and L.bicolor did not show significant increase, and in S.salsa , Pn and OPS II increased. While in high NaCl concentration, Pn was drastically reduced while Gs, Ci and O PS II decreased in A.centralasiatia and L.bicolor , Pn, Gs, Ci and OPS II of S.salsa were not markedly affected. In S.salsa, chla+b was not affected by salt stress (0-400 mmol/L), chla/b was significantly increased under conditions of 200 mmol/L NaCl (up to 171.1%), then decreased by 400 mmol/L NaCl (up to 136.2%). In A.centralasiatia, chla, chla+b and chla/b were significantly decreased by 400 mmol/L (up to 68.5%, 75.8% and 68.5% respectively ). In L.bicolor, chla, chlb, chla+b was increased under salt stress,and cha/b had similar changes in chla/b were detected in A.centralasiatia.3. Radical damage to three types of halophytes under NaCl-stress condition Constitutive level of lipid peroxidation in S.salsa was lower than that inA.centralasiatia and Lbicolor. In low NaCl concentration (S.salsa: 0-200 mmol/L; A.centralasiatia and Lbicolor. 0-100 mmol/L), leaf MDA and plasmalemma permeability didn't show distinct increase. Under the condition of 400 mmol/L NaCl, leaf MDA of A.centralasiatia and Lbicolor seedlings was significantly increased (up to 195% and 149%, respectively), and the plasmalemma permeability was increased correspondingly. Under the same condition, leaf MDA increase of S.salsa seedlings was by far lowest in three types halophytes. It suggested that oxidative stress was induced differently by salt stress in different halophytes.4. Leaf SOD isoforms of different types of halophytes and its response to salt-treatment.In S.salsa leaves four isoforms of SOD and in A.centralasiatia and Lbicolor leaves three...
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