Silicon alleviates salt stress and increases antioxidant enzymes activity in leaves of salt-stressed cucumber (Cucumis sativus L.)
Introduction
Silicon (Si) is the second most abundant element on the surface of the earth, yet its role in plant biology has been poorly understood and the attempts to associate Si with metabolic or physiological activities have been inconclusive [1], [2]. Salt stress has been a major obstacle to the successful use of salt-affected soils for crop production. How to exploit saline soils has received worldwide attention. Salt stress has been shown in some investigations to be mitigated by Si. Matoh [3] reported that silicate at 0.89 mM reduces the translocation of Na+ to the shoots and increases dry matter production of salt-stressed rice plants compared to the control. For wheat [4] and barley [5] similar findings have been reported, namely a repression of Na+ transport in plants growing in salinized solutions supplied with Si, with concomitant improvement in their growth. Liang [6] reported that added Si enhances the growth of salt-treated barley and improves the chlorophyll content and photosynthetic activity of leaf cell organelles of barley.
Aerobic organisms face constant risk from reactive oxygen species (ROS), including superoxide radical (O2−), hydroxyl radical (OH), singlet oxygen () and hydrogen peroxide (H2O2) which are inevitably generated naturally via a number of cell metabolic pathways [7], [8], such as photosynthesis, photorespiration [9], fatty acid oxidation and senescence [10]. ROS have the potential to interact non-specifically with many cellular components, triggering peroxidative reactions and causing significant damage to membranes and other essential macro-molecules such as photosynthetic pigments, protein, nucleic acids and lipids [11], [12], [13], [14]. Therefore, their levels must be carefully monitored and controlled in cells. Plants possess antioxidant defense systems, comprised of enzymatic and non-enzymatic components, which normally maintain ROS balance within the cell. For instance, they use a diverse array of enzymes like superoxide dismutases (SOD), catalases (CAT) and peroxidases as well as low molecular mass antioxidants like ascorbate and reduced glutathione (GSH) to scavenge different types of ROS [14]. SOD catalyzes the dismutation of superoxide to hydrogen peroxide and oxygen. However, hydrogen peroxide is also toxic to cells and has to be further detoxified by CAT and/or peroxidases to water and oxygen. An unfortunate consequence of salinity stress in plants is the excessive generation of ROS [14]. The excess production of ROS during salinity stress results from impaired electron transport processes in chloroplast and mitochondria as well as from pathways such as photorespiration. The results of most studies have shown that the resistance to salt stress is usually correlated with a more efficient antioxidant system [12], [13].
It has been reported that added Si to salt treated barley significantly increases SOD activity and decreases malondialdehyde (MDA) concentration in plant leaves [15], and increases SOD, peroxidase, CAT and glutathione reductase (GR) activity and reduces lipid peroxidation (LPO) in barley roots [16]. This paper focuses on the influence of Si addition on antioxidant enzyme activity in cucumber plants under salt stress. The objectives of this study are to investigate whether Si is involved in regulation of antioxidant enzymes and LPO under salt stress and therefore to elucidate the physiological mechanism of salt stress mitigated by Si in cucumber plants.
Section snippets
Plant material
Seeds of two cultivars (Jinlu4 and Jinyan4) of cucumber (Cucumis sativus L.) were rinsed thoroughly with distilled water, and germinated on moist filter paper in an incubator at 28 °C. The germinated seeds were sown in quartz sands in the greenhouse of Zhejiang University. After 10 days, the seedlings were transferred to plastic containers. Each container had six plants with 10 l of continuously aerated full-strength Hoagland nutrient solution. Daily photoperiod was 12 h and the maximum
Plant growth and protein content
As shown in Table 1, the dry weight of both shoot and root of two cultivars was significantly reduced by NaCl stress; however, this inhibition was significantly alleviated by Si supplement. Under salt stress Si significantly increased the dry weight of shoots and roots of both cultivars. Under non-salt stress condition Si increased the dry weight of the shoots of both cultivars and the root of Jinyan4. Shoot dry matter content of both cultivars was significantly increased by salt stress;
Discussion
Improvement of salt tolerance by addition of Si have been reported in barley [5], [6], [15], [16], rice [3], wheat [4], tomato [25] and Prosopis juliflora [26]. The present result was consistent with the previous findings. The growth of salt-stressed cucumber plants was significantly improved by the addition of Si. It has been reported that the alleviation of salt toxicity by Si addition are due to the reduction of Na content in the shoot of rice [3], [27], P. juliflora [26] and barley [15],
Conclusion
The addition of Si significantly increased SOD, GPX, APX, DR and GR activity and decreased ELP, LPO level and H2O2 concentration in salt-stressed cucumber leaves. Si may act to alleviate salt stress in cucumber by decreasing permeability of plasma membranes and membrane lipid peroxidation, and maintaining the membrane integrity and function. A significant increase in antioxidant enzymes of salt-stressed leaves by Si addition suggest that Si may be involved in the metabolic or physiological
Acknowledgements
The project was supported by National Natural Science Foundation of China (No. 30230250).
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