Abstract
Salt tolerance mechanism of an extreme halophyte Salvadora persica was assessed by analyzing growth, nutrient uptake, anatomical modifications and alterations in levels of some organic metabolites in seedlings imposed to various levels of salinity (0, 250, 500, and 750 mM NaCl) under hydroponic culture condition. After 21 days of salt treatment, plant height, leaf area, and shoot biomass decreased with increase in salinity whereas the leaf succulence increased significantly with increasing salinity in S. persica. The RWC% of leaf increased progressively in salt-treated seedlings as compared to control. Na+ contents of leaf, stem and root increased in dose-dependent manner whereas there was no significant changes in K+ content. There was significant alterations in leaf, stem, and root anatomy by salinity. The thickness of epidermis and spongy parenchyma of leaf increased in salt treated seedlings as compared to control, whereas palisade parenchyma decreased dramatically in extreme salinity (750 mM NaCl). There was a significant reduction in stomatal density and stomatal pore area of leaf with increasing salinity. Anatomical observations of stem showed that the epidermal cells diameter and thickness of cortex decreased by salinity whereas thickness of hypodermal layer, diameter of hypodermal cell, pith area and pith cell diameter increased by high salinity. The root anatomy showed an increase in epidermal thickness by salinity whereas diameters of epidermal cells and xylem vessels decreased. Total soluble sugar content remained unchanged at all levels of salinity whereas reducing sugar content increased by twofold at high salinity (750 mM NaCl). The starch content of leaf decreased progressively in NaCl treated seedlings as compared to control. Total free amino acid content did not change at low salinity (250 mM), whereas it increased significantly at higher salinity (500 and 750 mM NaCl). The proline content increased in NaCl treated seedlings as compared to control. There was no significant changes in polyphenols level of leaf at all levels of salinity. The results from the present study reveal that seedlings imposed with various levels of salinity experience physiological, biochemical and anatomical modifications in order to circumvent under extreme saline environment. The vital mechanisms of salt tolerance in S. persica are higher accumulation of organic metabolites, increase in leaf succulency, efficient Na+ sequestration in the vacuole, K+ retention in the photosynthetic tissue and increase in WUE by reducing stomatal density. Therefore, S. persica is a potential halophytic species to be cultivated in saline lands to eliminate excess salt and make it favorable for agriculture.
Highlights
Salinization of soil is one of the most serious threat to irrigated crop production in arid and semi-arid regions (Zakery-Asl et al, 2014)
In order to survive in soils with high salinity, plants develop various physiological and biochemical mechanisms that include ion compartmentalization (Shabala and Mackay, 2011); biosynthesis of osmo-protectants or compatible solutes viz. amino acids, sugars, quaternary ammonium compounds, tertiary sulfonium compounds, polyols etc. (Slama et al, 2015); increased activation of antioxidative enzymes viz., superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPX), glutathione S-transferases (GST), ascorbate peroxidase (APX), dehydroascorbate reductase (DHAR), glutathione reductase (GR), and monodehydroascorbate reductase (MDHAR) (Parida and Jha, 2010); synthesis of antioxidant compounds viz., ascorbate, glutathione (GSH), flavonoids, carotenoids and tocopherols (Shabala et al, 2012) and modulations of plant hormones (Gupta and Huang, 2014)
Salinity induced higher accumulation of organic metabolites such as amino acids, reducing sugars and polyphenols suggests their role in osmotic regulation, ROS scavenging and protection of cellular macromolecules in S. persica under high salinity condition
Summary
Salinization of soil is one of the most serious threat to irrigated crop production in arid and semi-arid regions (Zakery-Asl et al, 2014). Sodium is a non- essential element affecting growth and various metabolic processes of plants that eventually cause morphological, physiological and biochemical alterations at higher doses of salinity (Parida and Jha, 2013). Salinity mainly affects the plants by reducing the water potential of the soil leading to deficiency of water availability to plants (Hasegawa et al, 2000; Hasegawa, 2013) This decrease in water availability reduces the photosynthetic rate and the overall growth of the plant. In order to survive in soils with high salinity, plants develop various physiological and biochemical mechanisms that include ion compartmentalization (Shabala and Mackay, 2011); biosynthesis of osmo-protectants or compatible solutes viz. Are induced by salinity (Vijayan et al, 2008) These modifications lead to osmotic and ionic adjustment of the cells under high salinity condition. The increase in leaf succulence and presence of salt glands are some essential components for regulation of leaf salt concentrations in halophytic species
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