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 Vicia faba 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.