Silicon Alleviates Sodium Toxicity in Sorghum and Sunflower Plants by Enhancing Ionic Homeostasis in Roots and Shoots and Increasing Dry Matter Accumulation

This study evaluated whether supplied Silicon (Si) in different forms alleviates the deleterious effects of NaCl by decreasing the Na+/K+, Na+/Ca2+, and K+/Ca2+ ratios in Si-accumulating Sorghum bicolor L. ‘Moench’ (sorghum), and intermediate-Si-accumulating Helianthus annuus L. (sunflower). The study involved a period of 30 days of hydroponic cultivation in either the absence or the presence of NaCl (0 and 100 mmol L−1) supplemented with Si (provided from stabilized sodium and potassium silicate) as one of four treatments: 0 Si control (no added Si); Si (F), 28.6 mmol L−1 via foliar spraying; Si (S), 2.0 mmol L−1 via nutrient solution to the root, and Si (S + F), combined application of Si via nutrient solution (2.0 mmol L−1) and foliar spraying (28.6 mmol L−1). Tissue concentrations of Na+, potassium (K+), and calcium (Ca2+) were determined and related to Na+/K+ and Na+/Ca2+ ratios. Plants treated with NaCl alone had reduced dry matter production and K+ and Ca2+ concentrations. Si reduced the uptake and translocation of Na+ but increased K+ and Ca2+ concentrations in both Si-accumulating and intermediate-Si-accumulating plants, depending on the application. These results suggested that Si applications via roots for sorghum plants and the combined supplementation via root and foliar spraying of sunflower plants attenuated Na+ toxicity by improving the K+ and Ca2+ uptake over Na+ and, in turn, decreasing Na+/K+ and Na+/Ca2+ ratios, which could be one of the major mechanisms of enhancing plant growth and productivity under salt stress conditions.