ROS (Reactive Oxygen Species) Regulation And The Relevance To Salt Tolerence In NaCl-stressed Poplar And Mangrove Trees

Using Populus euphratica Olive, Populus pupularis 35-44' (P. popularis), Kandelia candel and Bruguiera gymnorrhiza, we investigated the genotypic difference in the response to salt stress and the relevance to salt-induced anti-oxidative stress. Results show that the rapid response to salinity in salt-resistant P. euphratica, K. candel and B. gymnorrhiza contributes to the salt tolerance. Initial salinity increased anti-oxidative capacity in P. euphratica and mangrove trees, which is benefit to maintain the ROS(reactive oxygen species) balance in a longer-term of salt stress. However, this mechanism was absent in salt-sensitive P. popularis trees. The main results are summarized as follows:1) A longer-term of increasing salinity (19 days, NaCl increased from 100-400 mmol/L):Na⁺ and Cl^- concentrations in leaves of P.popularis increased evidently after 6 days of salt stress, electrolytic leakage, O₂^- production rate and H₂O₂ concentrations markedly increased at the end of treatment (day 19). These results indicate that salt-induced lipid peroxide and membrane permeability took place in P. popularis leaves. In contrast, the salt increase in P. euphratica tissues was lower than P.popularis under the same saline conditions. O₂^- production rate in P. euphratica leaves significant increased after 6 days of salt stress, correspondingly, activities of the antioxidative enzymes were elevated.2) Extraneous ABA introduction and tungstate treatment:After exposure to tungstate treatment (ABA synthesis inhibitor), salt-induced O₂^-production rate and H₂O₂ concentrations in P. euphratica was lower than that of salttreatment (50mmol/L). Similarly, the salt-induced activity of antioxidative enzymes,SOD(superoxidedismutase),POD(peroxidase),APX(ASAorbateperoxida se), CAT(catalase) and GR(glutathione reductase) was lower in tungstate-treated trees. Extraneous ABA was applied via xylem feeding to mimic the salt-induced elevation of ABA in P. euphratica leaves. ROS and activities of antioxidative enzymes in P. euphratica leaves significantly increased after ABA application (10μmol/L, 4h). The data suggest that NaCl-induced ABA acculation triggers ROS generation, which, in turn, leading to the increased activities of antioxidant enzymes. However, this mechanism was absent in salt-sensitive P. popularis trees. 3) A short-term of increasing salinity (96 hours, NaCl increased from 100-400mmol/L):Na⁺ and Cl^- accumulated in root and shoot tissues of Kandelia candel and Bruguiera gymnorrhiza, but more salt was maintained in roots as compared to hypocotyl. The result showed that the salt exclusion in K. candel and B. gymnorrhiza depends on root-to-shoot Na⁺ transport restriction. After 96 hours of salt treatment, MDA(malondialdehyde) concentration and electrolytic in K. candel and B. gymnorrhiza slightly increased, indicating that the two mangroves had strong capability to resist salt (400mM NaCl). O₂^- production rate and H₂O₂ concentrations in Kandelia candel and Bruguiera gymnorrhiza increased in some degree. A same trend was observed in the activities of SOD, APX, CAT, POD and GR during the period of salt stress. Collectively, we conclude that mangroves are able to increase antioxidation mechanisms to control ROS under salt stress.