Evaluation of Effect of Silicon on NaCl Tolerance in Annual Medicago scutellata L.

Introduction Salinity is one of the most important stress resulting depletion of vegetation in large areas of the world including some regions of Iran. Reduction of plant growth due to salinity occurs with a range of mechanisms, including low external water potential, ion toxicity and interfere with the uptake. Silicon (Si) is the second most abundant element in soil and could efficiently mitigate the effects of various biotic and abiotic stresses, such as drought, heavy metal toxicity and salinity on plants. Medicago scutellata is an important leguminous forage crop throughout the world that could increase soil nitrogen content via reduction of atmospheric nitrogen. To our knowledge, no study have examined the interaction of salinity and Si nutrition in Medicago scutellata or how the beneficial effects of Si in salt-stressed M. scutellata plants (if any) are exerted. Accordingly, the aim of the present study was to evaluate the effect of silicon nutrition on salt tolerance of Medicago scutellata. Materials and Methods Seeds of alfalfa (Medicago scutellata L.) were sterilized with a 2.5% sodium hypochlorite solution and were incubated in a moistened paper towel. Then, they germinated in the dark at 255  C for 48 h. Healthy seedlings of uniform sizes were selected for hydroponic culture (Hoagland solution) in a 10×15×15 cm plastic pots. A factorial experiment carried out based on a completely randomized design with two factors. The first factor was salinity, including 0 and 100 mM NaCl and the second was silicon nutrition, including 0, 0.75 and 1.5 m.M sodium silicate. The pH of the nutrient solution was adjusted daily at 6.4  0.2 and nutrient solution was refreshed weekly. During the experiment, maximum and minimum air temperatures were 30ºC and 21ºC respectively, and the mean relative humidity was 67%. Four weeks after exerting the treatments, plants were harvested and used for the assessment of growth parameters and chemical analyses. Results and Discussion Salinity led to a significant reduction in both the fresh and dry weights of the plants. On the contrary, the dry weight of the plants improved significantly under saline conditions when Si was added to the medium, especially 1.5 mM Si. Salt treatment increased the concentration Na+ and decreased the concentration K+ significantly in both shoots and roots; however, Na+ concentration, reduced and K+ concentration increased due to Si application in salt treated plants. Similarly, Fe content decreased in shoot of plants due to salinity, whereas Si nutrition increased Fe content in plants suffered from salinity. The activity of catalase declined and the amount of hydrogen peroxide increased in plants under salinity. Conversely, Si treatments, especially, at 1.5 mM could recover the activity of this enzyme and reduced hydrogen peroxide content. Salinity imposed significant reduction in the contents of chlorophylls, total carotenoids and xanthophylls and soluble proteins. The amount of hydrogen peroxide, lipid peroxidation as well as electrolyte leakage via plant leaves increased due to salt stress. In contrast, the contents of chlorophylls, carotenoids and xanthophylls, soluble proteins increased following Si application. Also, the amount of hydrogen peroxide and rate of lipid peroxidation and electrolyte leakage decreased in salt-treated plants by Si application. Silicon nutrition can recover the chlorophyll content and the amount of soluble proteins on Medicago scutellata plants under salinity, which suggests that it plays a role in the suppression of oxidative stress. Si application also improved the chlorophyll content of tomato and barley under salt stress (Al-aghabary et al., 2004). The data reported in the present study show new aspect of the beneficial effect of Si on plants grown under saline condition. The application of Si prevented Na+ accumulation and enhanced K+ content in Medicago scutellata plants. Reduced Na+ accumulation improves the plant ROS scavenging capacity through increased antioxidant enzyme activity (Al-aghabary et al., 2004 Hashemi et al., 2010), accompanied by reducing lipid peroxidation. Consequently, photosynthetic pigments increased and membranes functionality improved by Si in plants under salinity (Liang et al., 2005). Conclusions The results indicated that 1.5 mM silicon application alleviated harmful effects salinity, probably through declined Na+ enhanced K+ content that increased antioxidant enzyme activity and reduced reducing oxidative stress. Consequently, photosynthetic pigments increased and membranes functionality improved with plants under salinity. In this regards, Si application led to an increased salt tolerance of Medicago scutellata. Further, field experiments are necessary for confirmation of the results and expedience of economic cost