Silicon nutrition improves growth of salt-stressed wheat by modulating flows and partitioning of Na+, Cl- and mineral ions

Silicon (Si) is reported to improve salt stress tolerance of cereals, but little is known about the effects of Si on flows and partitioning of sodium (Na+), chloride (Cl-), and essential mineral ions at the tissue and cellular level. Wheat (Triticum aestivum L.) was exposed to 200 mM NaCl for 30 d in hydroponics, with or without 2 mM Si. X-ray microanalysis coupled with scanning electron microscopy (SEM) was used to quantify the cell-specific ion profiles across root and leaf cells, paralleled by measurements of wheat growth and physiological responses. Under salt stress, higher Na+ and Cl- concentrations were detected in root epidermal, cortical and stelar cells, eventually increasing their concentrations in different leaf cells, being highest in the epidermal cells and lowest in the vascular bundle cells. The potassium (K+) and magnesium (Mg2+) profiles were generally opposite to those of Na+ and Cl-. NaCl-dependent deregulation of essential nutrient homeostasis and excessive toxic ions accumulation in leaves was correlated with enhanced electrolyte leakage index (ELI), decreased chlorophyll contents, photosynthesis and other physiological parameters, and ultimately hampered plant growth. Conversely, Si addition improved the growth and physiological performance of salinized wheat by reducing Na+ and Cl- concentration in root epidermal and cortical cells, and it improved root uptake and storage of K+ and Mg2+ ions and their loading into xylem for distribution to shoots. These results suggest that Si-mediated inhibition of Na+ uptake, maintained nutrient homeostasis and improved physiological parameters to contribute to wheat growth improvement under salt stress.