| Silicon (Si) is a beneficial element for higher plants. Although Si has been a disease-resistant material for a long time, its physiological and nutrient roles in plants are apparently minimal and poorly understood, its disease-resistant mechanism having been argued. Recently, it has been reported that the addition of Si increased the salt tolerance in barley (Hordeum vulgare L.), wheat (Triticum aestivum), maize (Zea mays) and rice (Oryza sativd). However, information is still scant regarding the effects of Si on salt tolerance of cucumber (Cucumis sativus L.) and its mechanism. Based on previous works, hydroponic culture experiments were conducted with both resistant and susceptible cucumber cultivars to investigate the effects of soluble silicon (Si) and inoculation with Sphaerotheca fuliginea and another two cucumber cultivars were also grown in a hydroponics culture system for the investigations of Si and salt (Nad) interactions and their possible mechanisms. The objectives are to demonstrate the effects of Si on the growth of disease-resistant and salt-stressed cucumber and to investigate the possible mechanisms by which Si increases the plant resistance to disease and alleviates salt stress in cucumber. The main results were presented as follows:1. Hydroponic culture experiments were conducted with both susceptible and resistant cucumber cultivars to investigate the effects of Si and inoculation with Sphaerotheca Fuliginea on active oxygen metabolism. The results showed that O2- producing rate, H2O2 and MDA content were increased by inoculation, however, were reduced by Si supply in the inoculated plants. The inoculation of Sphaerotheca Fuliginea enhanced the activities of ascorbate peroxidase (APX) and guaiacol peroxidase (GPX) and catalase (CAT) and decreased superoxide dismutase (SOD) activity, while Si supply increased the activities of APX, GPX, SOD and CAT in the leaves of inoculated plants. Dehydroascorbate reductase (DR) activity in the leaves of Si-fed plants were significantly increased in resistant cucumber cultivar, but not in susceptible cultivar. Reduced ascorbate (AsA) and reduced glutathione (GSH) contents in leaves were increased by inoculation. AsA content in the leaves of Si-supplied plants was lower than Si-deprived plants whereas GSH content in the leaves of Si-supplied plants was higher than Si-deprived plants. SiO2 content in the leaves of Si-supplied plants was significantly higher than Si-deprived plants. The disease index was significantly decreased by Si-supplied after inoculation. Si addition improved the ability of resistance to powdery mildew.2. Chlorophyll content, photochemical efficiency of PSII (Fv/Fm) , quantum yield of PSII (PSII), Fv/Fo, photochemical quenching coefficient (qP) and non-photochemical quenching of fluorescence (NPQ) were significantly decreased by inoculation, however, were increased by Si supply in the inoculated plants. When the plants were inoculated with Sphaerotheca Fuliginea, qP in the leaves of Si-fed plants were significantly increased in resistant cucumber cultivar, but not in susceptible cultivar.3. When the plants inoculated, the activity of phenylalanine ammonia- lyase (PAL), polyphenoloxidase (PPO) and the contents of ferulic acid, chlorogenic acid, HRGP, lignin, phenolic compound and flavonoid were significantly increased in both cucumber cultivar, but the activities of PAL and PPO in resistant cucumber were higher in comparison with susceptible cucumber. Accompanied with the increase of PAL and PPO activity, ferulic acid, chlorogenic acid, HRGP, lignin, phenolic compound and flavonoid in cucumber leaves accumulated rapidly. The activity of PAL and PPO in the leaves of Si-fed plants were significantly increased in susceptible cucumber cultivar after inoculation, but not in resistant cultivar.4. Spraying the surface of the 2nd true leaf (leaf 2) of susceptible cucumber ZJ02-09 plant with 20 mmol.L-1 K2SiO3 effectively enhanced the activities of chitinase and B-1 , 3-glucanase, and induced the activitie...
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