*4.2. Magnesium*

In human health, magnesium (Mg) is essential in maintaining muscle tone and blood pressure. It participates in glycemic control, neuromuscular function, and myocardial contraction. It is also involved in the energy metabolism, besides being a cofactor of many enzymes [57]. The RDA for Mg ranges between 320 and 420 mg day−1. The UL for adults is 420 mg day−<sup>1</sup> [43]. Magnesium is a divalent cation and it is essential in plants because of its ability to interact with strongly nucleophilic ligands, it participates in the processes of enzyme regulation, pH cellular, and cation–anion balance, besides being a key metal in chlorophyll structure [58]. Magnesium is relatively mobile in soils, where its average concentration can vary between 0.5 and 40 g kg−<sup>1</sup> [59], with a worldwide average of 5 g kg−1. In addition to passive diffusion, as it happens with others divalent cations, Mg is actively absorbed by roots through permeable cation channels [60]. Regarding leaf uptake, younger leaves are more likely to absorb Mg than the aged ones [61]. Mg2+ transporters in higher plants are thought to be derived from the CorA transport system, acting as a gate locking it when the Mg concentration in the cytosol is increasing or opening otherwise [44,62]. Concentration of Mg in the metabolic pool of leaves is supposed to be between 2 and 10 mM, while free Mg concentration is expected to be lower (around 0.4 mM). For an optimal growth, plants demand between 1.5 and 3.5 g kg−<sup>1</sup> of Mg in vegetative fractions [44]. Even though toxicity with Mg is rare, concentrations above 20 mM proved to be phytotoxic, causing symptoms like coppery colored leaves, decrease in starch contents, and growth reduction [63]. In contrast to the translocation difficulties observed for Ca, Mg shows a high phloem mobility and the application of Mg fertilizer can efficiently increase its concentration in leaves, tubers, fruits, seeds, and grains [27,44], making Mg agronomic biofortification of vegetables a feasible option to fight cases of malnutrition. As indicated in Table 1, plants of Indian colza (*Brassica rapa* ssp. *trilocularis*) submitted to different Mg biofortification protocols, showed on average a 3.6-fold increase in Mg content of leaves, when compared to untreated plants. In one experiment, after growing Indian colza plants on peat with a low (0.20 g <sup>L</sup>−1) or high (3.04 g <sup>L</sup>−1) Mg chloride (MgCl2) concentration, leaf content increased up to 12 mg Mg kg−<sup>1</sup> DW. However, the increase was 50% lower when plants received simultaneously a high dose of CaCl2 (3.04 g), showing a possible negative interaction between Mg and Ca [54]. Similarly, Blasco et al. [64] submitted *Brassica rapa* plants to different nutrient solutions. When comparing the application of a low (4.86 mg <sup>L</sup>−1) and a high (486.1 mg <sup>L</sup>−1) dose of Mg (as MgCl2) in the nutrient solution they noticed a 12-fold increase in the Mg content of shoots, passing from low to high dose. The same authors tested the interaction with other minerals and concluded that the Mg concentration in shoots increased with high Zn (500 μM) and low Ca (0.4 mM) supplies and decreased at high Ca (40 mM) supply. Another biofortification study of Mg was conducted applying doses of 0, 50, 100, 150, and 200 mg Mg dm–<sup>3</sup> soil (as magnesium sulfate, MgSO4·7H2O) on growth of onion plants (*Allium cepa* L.) [65]. The maximum Mg content in bulbs was obtained at the dose 150 mg dm–3, i.e., almost 2-times higher than the untreated plants. However, this dose negatively affected the crop yield, and also caused a reduction in the uptake of Ca and potassium (K), showing that the antagonism between these minerals should be carefully evaluated. Therefore, the authors sugges<sup>t</sup> using the Mg-100 dose, as it allowed to increase the Mg content of the bulbs (up to 1.4-fold, when compared to control), with a contextual increase in crop yield (up to 38%). There is evidence that fertilization of Mg via foliar spray can act to improve crop yield and quality [66,67]. The few studies on Mg biofortification show that both MgSO4·7H2O and MgCl2 are effective in enhancing the element content in vegetables. However, Mg biofortification should be carefully managed considering its interaction with Ca, since high Ca content can inhibit Mg uptake by plants.
