Studies On Physiological And Biochemical Mechanisms Of Salt Tolerance Improved By Silicon In Cucumber (Cucumis Sativus L.)

Silicon (Si) is generally considered as a beneficial element for the growth of higher plants. Many previous studies have reported the roles -of Si in alleviating both abiotic and biotic stresses in plants. Salinity is a agricultural and eco-environmental problem worldwide, and is becoming one of the most serious factors limiting the productivity of agricultural crops. Therefore, the investigation on the mechanisms of alleviating salt toxicity by Si has been received more attention recently. In this paper, two cucumber (Cucumis sativus L.) cultivars, i.e. Jinyan 4 and Jilu 4, were grown hydroponically to study the effects of exogenous silicon (K₂SiO₃ 1.0 mM) on ion distribution, reactive oxygen species (ROS) metabolism and antioxidative system in chloroplast, mitochondria and apoplast under NaCl (50 mM) stress. The objective was to clarify the possible physiological and biochemical mechanisms of Si on alleviating salt stress in cucumber. The main results are summarized as follows:1. The selectivity ratios of K, Na (S_K/Na) from nutrient solution to roots and S_K/Na from roots to leaves increased, when silicon was added to cucumber nutrient solutions, indicating that silicon increased the ability of sodium selectivity might via the reduction of sodium content. Silicon decreased the Na⁺ and Cl^- concentrations in chloroplast and apoplast, and increased the K⁺, Ca²⁺ and Mg²⁺ concentrations. As a result the ratio of K/Na increased due to the enhanced ion exclusion and the reduced apoplastic pathway to sodium uptake, that may contribute to ion distribution, ion homeostasis in cells and the increase of osmotic adjustment. In the meantime, the injury of chloroplast membrane under salinity stress in cucumber was alleviated and photosynthesis was increased by the decrease of Na⁺ and Cl^- contents in the chloroplast. The stomatal conductance (Gs), stomatal limitation value (Ls) and net photosynthesis rate (Pn) were increased, which was accompanied by the increases of relative water content (RWC), water potential in leaves and osmotic adjustment in the plant. Therefore, the results showed that Si could alleviate the salt stress and improve the growth of cucumber plants by reducing sodium content and improving the ability of ion distribution in cell.2. Application of silicon remarkably enhanced the activities of superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT) in chloroplast, mitochondria and apoplast in the salt-stressed leaves. The activities of ascobate peroxidase (APX), glutathione reductase (GR) and dehydroascorbate reductase (DHAR), which are involved in the ascorbate-glutathione cycle, were also enhanced simultaneously by additional Si undar salt stress. Exogenous silicon also increased the contents of antioxidants such as ascorbic acid (AsA) and glutathione (GSH) in all fractions, and CoQ10 in mitochondrial fraction. Therefore, silicon increased the ability of scavenging ROS in chloroplast and mitochondria, resulting in lower H₂O₂ content and lipid peroxidation, and higher membrane stability under salt stress, which eventually led to the remarkable increase of respiration and photosynthesis.From above results, it could be concluded that silicon nutrition alleviated the salt damage by improving antioxididant responses and physiological activity in the sub-cellular fractions of cucumber, and improving its adaptability to salt stress.3. The effects of exogenous coenzyme Q10 (CoQlO) on the biophysical characteristics, contents of malondialdehyde (MDA) and H2O2, and respiration in mitochondria from the salt-stressed cucumber roots were investigated in vitro. The results showed that the swelling of mitochondria was decreased, and that the membrane fluxion and membrane potential of mitochondria were increased by adding 10 mM CoQlO. The contents of H2O2 and MDA in mitochondria decreased, leading to the improvement of oxidative phosphorilation and membrane integrity in mitochondria, especially under salt stress. It suggested that CoQlO has an alleviative effect on the injury of salt stress by increasing the phosphorilation and membrane integrity of mitochondria. Additional Si for salt-stressed cucumber significantly decreased H2O2 and MDA contents, but increased the membrane fluxion, CoQlO content, and the activities of ATPase and cytochrome oxidase (CCO) in mitochondria of roots. It may be also indicated that the respiration chain in mitochondria could be protected by resumed oxidative phosphorilation and energy metabolism, which leads to the increase of mitochondria respiration.4. The xanthophyll cycle components and photosynthetic pigments in fresh leaves of cucumber plant were analyzed with HPLC. It was shown that the contents of violaxanthin (V), antheraxanthin (A) and zeaxanthin (Z) involving in the xanthophyll cycle decreased under salt stress, companied with the decrease of (A+Z)/(A+Z+V) which stands for the extent of de-epoxidation of xanthophyll cycle, and the contents of lutein (L), chlorphyll a, b (Chla, b), P -carotene (PC) were also decreased. These results suggested that salt stress has a slight effect on the extent of de-epoxidation of xanthophyll cycle, and no significant correlation was observed between non-photochemical quenching (NPQ) and xanthophyll cycle in leaves. Si had a significant effect on the contents of L, Chla, Chlb and P C in the leaves, but had no effect on (A+Z)/(A+Z+V) under salt stress. It may be concluded that Si is beneficial to the synthesis of lutein, chlorophyll and carotene, helping to improve the stability of photosynthetic pigments and increase the photosynthetic rate.