INTRODUCTION
Thevetia peruviana is a plant of tropical, a fast-growing small tree and because of its beautiful flowers and slender leaves very decorative effect. Its flowers have different colors, yellow, white and orange. The tree blooms, the leaves are linear-lanceolate, glossy and green almost all year round. They are coated with a waxy layer to reduce water loss. Its considered as an ornamental tree. Thevetia peruviana plants can succeed in rather poor and dry soils. It can tolerate shorter dry periods and moderately saline soils. Its best grows and flowers in full sun or light shade. T. peruviana contains a number of phytoconstituents which reveals its uses for different therapeutic purposes. The individual parts of plant can be used for the treatment of different disorders in human being such as, liver toxicity, diabetes, fungal infection, inflammation, microbial infection, pyrexia and to relieve pain. Furthermore, so much work is required with the thevetia to investigate the mechanism of actions with other therapeutic activities (Singh Kishan et al. 2012 and Ramos-Silva et al. 2017).
Among the abiotic stresses, salinity is one of the main factor that contributes to desertification of arid lands. Soil salinization has become a great challenge for rehabilitation of range lands and plant productivity (Alqarawi et al. 2014). Salinity causes nutritional disorders in plants which lead to deficiencies of several nutrients and drastically increasing Na+ levels in the plant cells (Iqbal and Ashraf, 2013).Thus, salinity stress affects the plant ability to uptake water in the root zone through decreasing the water potential of the soil (Sabir et al., 2009). This deficiency in available water under saline condition raises the potential of cells to be dehydrated which is a result of the osmotic stress caused by salinity. The higher ratios of toxic ions like Na+ and Cl damage the balance between ions through reducing the plant ability to absorb other ions like K+, Ca2+, and Mn2+ (Hasegawa et al., 2000). In this regard, the application of AM fungi as a biological method could improve plant growth and tolerance through helping the plants to mitigate the negative the adverse effects of salinity ( Abdel-Fattah and Asrar, 2012; Asrar et al., 2014).
Plants can overcome salinity by interacting with beneficial soil microorganisms such as arbuscular mycorrhiza fungi (AMF). Mycorrhiza symbiosis is a close association with the roots of most plants, from which both partners benefit: carbon compounds are supplied by the plant to the fungus, which provides the plant with mineral nutrients, mainly phosphate. AM symbiosis is able to increase plant growth under different environmental stresses (Garg and Chandel, 2015). The result is improved growth of the plant, and completion by the fungus of its life cycle. AM-fungi is known to exist in saline soil, and participates in the plant growth and development, and also improves the plant tolerance against biotic and abiotic stress (Abdel-Fattah et al., 2010) by regulating the physiological and biochemical process of plants (Fernanda et al., 2012).
AM fungi acts as growth regulator and mitigate the harmful effects of plants exposed to salt stress. Its play a key role in alleviating the toxicity induced by salt stress thus normalizing the uptake mechanism in plants by supplying the essential nutrients. The plant recovers the water balance machinery, enhancing their tolerance capacity, and thereby enduring the salt stress (Bhosale and Shinde 2011). Heikham et al. (2009) suggested many physiological parameters which could be responsible for alleviation of the harmful effects of salinity on plants upon inoculation with AM : 1) maintenance of a high K/Na ratio, 2) improved acquisition of nutrients such as N, P, Mg and Ca; 3) extended accumulation of proline, GB, polyamines, and carbohydrates, 4) enhanced activation of antioxidant enzymes, 5) increased chlorophyll content and higher rates of photosynthesis, 6) improved integrity and stability of cell membranes, 7) higher hydraulic permeability and improved water status, 8) increased number of nodules and nitrogen fixation by legumes, 9) molecular changes, such as enhanced expression of the plasma membrane intrinsic protein (PIP) gene, expression of two Na+/H+ antiporters, and expression of genes encoding.
Silicon (Si) is a beneficial element as it improves growth, confers rigidity, strength and enhances plant tolerance to various abiotic stresses (Meena et al. 2014;Abbas et al. 2015). It is always combined with other elements, usually forming silicates and oxides (Gunes et al. 2007). Plant roots generally take up Si in the form of silicic acid which then translocate to the shoot, where it is polymerized to form silica gel (SiO2nH2O) (Zhu and Gong 2014). Exogenous application of Si has been reported to induce favorable effects on plant growth under abiotic stresses (Balakhnina et al. 2015). Many studies have indicated the role of Si in alleviating salt-stress have been attributed to reduced uptake and translocation of Na+ to shoots (Al-Aghabary et al. 2004), maintenance of plant–water relations (Gong et al. 2006), which in turn, contributes to salt dilution (Romero-Aranda et al. 2006). Thus the objective of this study was to investigate of K-silicate or AM-fungi mediated salinity tolerance mechanism for developing Thevetia peruviana resistance to salt stress.