Abstract
Potassium (K(+)) is often the primary target for research on salinity stress. However, the role of magnesium (Mg(2+)) under salinity stress has not been properly investigated. We aimed to answer the following question: could magnesium (Mg(2+)), often neglected, be the real protector of ionic balance under salt stress? It is known that a deficiency in Mg(2+) increases K(+) uptake. Based on this understanding, we hypothesized that Mg(2+) starvation could worsen salinity defenses compared to K(+) starvation. The nutrient concentration of 0.02 mM Mg(2+) was maintained in the nutrient solution to induce Mg(2+) deficiency in Vicia faba plants. Mg(2+) 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 Mg(2+)-deficient plants, resulting in a major alteration in ionic composition. Mg(2+) deficiency under salt stress resulted in a 14-fold increase in the leaf Na(+) concentration compared to controls. Application of foliar Mg(2+) was very effective in reversing the loss of Mg(2+)-deficient plants, renewing growth, gas exchange, and stomatal activity, and reducing the toxic Na(+)/Mg(2+) ratio (by 90%). Mg(2+) 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 Mg(2+) is a necessary approach for achieving high-quality yields in salt-affected croplands.