Magnesium Leaf Application as a Rapid Tool for Salt Stress Resistance in Faba Beans (Vicia faba L.)

Potassium (K⁺) is often the primary target for research on salinity stress. However, the role of magnesium (Mg²⁺) under salinity stress has not been properly investigated. We aimed to answer the following question: could magnesium (Mg²⁺), often neglected, be the real protector of ionic balance under salt stress? It is known that a deficiency in Mg²⁺ increases K⁺ uptake. Based on this understanding, we hypothesized that Mg²⁺ starvation could worsen salinity defenses compared to K⁺ starvation. The nutrient concentration of 0.02 mM Mg²⁺ was maintained in the nutrient solution to induce Mg²⁺ deficiency in <i>Vicia faba</i> plants. Mg²⁺ foliar application was carried out five times, at an interval of two times a week, over two weeks of induced salinity stress. Harvesting was carried out 45 days after transplanting, i.e., 2 weeks after salinity stress (50 mM NaCl) was initiated at 4 weeks of transplanting. Salinity had the highest impact on Mg²⁺-deficient plants, resulting in a major alteration in ionic composition. Mg²⁺ deficiency under salt stress resulted in a 14-fold increase in the leaf Na⁺ concentration compared to controls. Application of foliar Mg²⁺ was very effective in reversing the loss of Mg²⁺-deficient plants, renewing growth, gas exchange, and stomatal activity, and reducing the toxic Na⁺/Mg²⁺ ratio (by 90%). Mg²⁺ is a unique regulator of ion homeostasis in salinity stress, and its deficiency causes an imbalanced ionic medium that cannot be normalized through K⁺ supplementation alone. Therefore, the strategic method of selective foliar application of Mg²⁺ is a necessary approach for achieving high-quality yields in salt-affected croplands.