Physiological Basis Studies Of Exogenous Nitric Oxide On Alleviating The Injury To Cucumber Seedlings Under Salt Stress

Salinity is the major environment factor limiting plant growth and productivity. It was reported that there was about 2.3×10⁹hm² irrigation land threatened by salinity in the world, occupied 1/3 of total irrigation area. Salinity of soil in greenhouses is also universal problem in the world. Nitric oxide is a widespread redox signal and toxic molecule in animals and plants, and is also reactive nitrogen species. Studies showed that nitric oxide was involved in various physiological processes of plants, such as respiration, photomorphogenic responses, seed germination, senescence, stress responses, programmed cell death, disease resistance in plants. The relationship between nitric oxide and resistance, especially the relationship between nitric oxide and salt tolerance of plants is a research focus in recent years. To the present, information about the effects of nitric oxide in physiological processes of plants is very limited. There are hardly reports about the relationship between salt tolerance of horticulture plants and nitric oxide.In the present experiment, by using exogenous sodium nitroprusside (SNP), a nitric oxide donor, and methylene blue (MB-1), an NO scavenger , the effects of nitric oxide on NaCl tolerance of cucumber (cucumis sativus L.) cultivar seedlings were investigated under NaCl stress. 'Jinchun NO.2' was chosen as an experimental material. The relationship between nitric oxide and seedling growth, photosynthetic characteristics, nitrogen metabolism, antioxidative system, the effect of calcium in adaptation to NaCl stress was discussed. Main research results were as follows:1.10～400μmol·L^-1 exogenous sodium nitroprusside, especially 100μmol·L^-1 SNP, significantly increased seedling growth of cucumber seedlings under 50mmol·L^-1 NaCl stress, but methylene blue blocked the effect of sodium nitroprusside.2. 100μmol·L^-1 exogenous sodium nitroprusside increased net photosynthetic rate (Pn), stomatal conductance (Gs), transpiration rate (Tr), the content of photosynthetic pigment in leaves and the contents of soluble suger, soluble protein in leaves and roots of cucumber seedlings under 50mmol·L^-1 NaCl stress significantly. Meanwhile, exogenous nitric oxide markedly decreased intercellular CO₂ concentration (Ci) under 50mmol·L^-1 NaCl stress, but methylene blue blocked these effects of sodium nitroprusside in some extent.3. 100μmol·L^-1 exogenous sodium nitroprusside increased markedly the content of free PAs and PAO activity in a short time under NaCl stress, but decreased after a long time, significantly alleviated the content of free Put, Spd, total free PAs and PAO activity under NaCl stress, increased the content of Spm and (Spd+Spm)/Put.4. 100μmol·L^-1 exogenous sodium nitroprusside increased the activity of P5CS and content of proline, meanwhile, decreased the activity of ProDH markedly.5. 100μmol·L^-1 exogenous sodium nitroprusside significantly increased NR activity and the content of NO₃^--N under NaCl stress, at the same time, markedly decreased the content of NH₄⁺-N in roots and leaves.6. 100μmol·L^-1 sodium nitroprusside significantly increased activity of SOD, POD, CAT, APX, the content of ASA and GSH as antioxitant matter in leaves and roots of cucumber seedlings under 50mmol·L^-1 NaCl stress, the GR and DHAR activity in relation to antioxitant metabolism. Methylene blue blocked the effects of nitric oxide on adjusting antioxitant system. However, exogenous nitric oxide markedly decreased membrane permeability, rate of O₂^·- production, the content of MDA and H₂O₂ under 50mmol·L^-1 NaCl stress.7. Plasma membrane Ca²⁺ channel blockers La³⁺ blocked the effects on growth and active oxygen metabolism induced by exogenous nitric oxide. Exogenous nitric oxide or Ca²⁺ significantly alleviated the injury to seedlings of 50mmol·L^-1 NaCl stress, and increased seedling growth, activity of SOD, POD, CAT, decreased rate of O₂^·- production, the content of MDA and H₂O₂, but La³⁺ blocked those effects induced by exogenous nitric oxide under NaCl stress. So all that suggested Ca²⁺ maybe played a crucial role on growth and active oxygen metabolism induced by exogenous nitric oxide. The effects of nitric oxide here depended on the content of Ca²⁺ in cytoplasm.