Na⁺/Mg²⁺ ratio: a new physiological trait for salt resistance in faba bean (Vicia faba L.)

Magnesium plays a vital role in enhancing plant resilience under salt stress. However, its specific function in maintaining ion homeostasis, particularly in regulating sodium uptake, remains unclear. Recognizing that magnesium deficiency leads to increased potassium uptake and accumulation, and given that sodium and potassium possess the same charge, we hypothesize that salt stress disrupts ion homeostasis to a greater extent in magnesium-deficient plants compared to those deficient in potassium. To test this hypothesis, Vicia faba plants were cultivated hydroponically and subjected to moderate salinity stress (50 mM NaCl) for 2 weeks starting from four weeks after transplanting. The plants were grown under varying levels of magnesium (0.5 mM sufficient; 0.02 mM deficient) and potassium (2 mM sufficient; 0.3 mM deficient), with harvesting occurring two weeks after exposure to salinity. The results indicated that under salinity conditions, magnesium deficiency had a more severe adverse effects on plant growth and gas exchange parameters than potassium deficiency. Stomatal movement was notably restricted in magnesium-deficient plants, potentially due to the over accumulation of soluble sugars and chloride. In magnesium-deficient plants the Na⁺/Mg²⁺ ratio was significantly higher in leaves (17-fold) and in roots (14-fold) relative to Mg²⁺ sufficient plants under salinity stress. Furthermore, the higher K⁺/Mg²⁺ ratio in magnesium-deficient conditions, observed under both saline and non-saline environments, suggests that potassium's antagonistic effect remains unchanged even under stress conditions. Our findings emphasize for the first time that magnesium, rather than potassium, serves a crucial function in regulating the ion homeostasis necessary for normal plant growth and development in saline environments.