## 3 Results and Discussion
Indeed, this decrease was particularly significant in plants grown in the presence of high concentrations of Mg in the nutrient solution (96-192 mg Mg L\({}^{-1}\)). It is worth pointing out that, while, in these treatments, markedly elevated concentrations of Fe were observed in plant leaves, the corresponding values in the roots were significantly reduced. Similarly, Mg deficiency decreased the concentration of Fe by 45.46% in the roots of sweet orange seedlings [20]. In general, there is an inconsistency in the Mg effect on Fe that is also supported by [PERSON] et al. (2019) [23], experimenting with sweet orange, who suggested that Mg-deficiency either decreased or did not alter Fe concentration in the leaf blades, but it increased its concentration in leaf veins while Fe concentration was decreased in veins of lower leaves. Magnesium deficiency usually leads to the reduction of leaf chlorophyll concentrations, destruction of photosystems, and deceleration of photosynthetic electron transport [23; 49; 50; 51]. The values of the F\({}_{\mathrm{v}}\)/F\({}_{\mathrm{m}}\) ratio, an indicator of photoinhibitory effects on the photosystem II of photosynthesis, varied between 0.76 and 0.82, depending on the concentration of Mg in the nutrient solution (Figure 1a). The corresponding values of the F\({}_{\mathrm{v}}\)/F\({}_{\mathrm{0}}\) ratio were between 3.43 and 4.49 (Figure 1a). This is in contrast to the findings of [PERSON] et al. (2019) [23], where Mg-deficient orange seedlings presented decreased F\({}_{\mathrm{v}}\)/F\({}_{\mathrm{0}}\) in leaves, indicating that Mg deficiency might impair the structure of thylakoids and inhibit the photosynthetic electron transport. Although the concentration of Mg in the plant irrigation solution did not statistically affect the values of the parameters mentioned above (F\({}_{\mathrm{v}}\)/F\({}_{\mathrm{m}}\), F\({}_{\mathrm{v}}\)/F\({}_{\mathrm{0}}\)), their lower values were observed under the influence of the highest concentrations of Mg. In a previous study with orange seedlings [23], Mg deficiency decreased F\({}_{\mathrm{v}}\)/F\({}_{\mathrm{m}}\) in leaves, which, together with other fluorescence parameters, demonstrated that photoinhibition occurred in Mg-deficient leaves [23; 52; 53]. Although Mg, as the central atom, is an essential constituent of chlorophyll, the total chlorophyll concentration (a + b) was not significantly affected by the concentration of Mg in the nutrient solution (Figure 1b). Similarly, [PERSON] et al. (2019) [23] reported that Mg deficiency had no influence on the concentration of Chl\({}_{\mathrm{a+b}}\) in the upper leaves of orange seedlings, while it lowered it in the middle and lower leaves. Moreover, under Mg sufficiency, the concentration did not vary with leaf positions. Depending on the concentration of Mg in the nutrient solution, and the way of expressing the results, the concentration of chlorophyll in the sweet orange leaves of \"Newhall\" ranged between 9.05-9.98 mg g\({}^{-1}\) d.w., whereas, when expressed per unit area, its values ranged between 41.29 mg cm\({}^{-2}\) and 47.23 mg cm\({}^{-2}\) (Figure 1b). It is not the Mg bound to chlorophyll that is restricting its synthesis under Mg-deficient conditions, as evidenced by the fact that the proportion of total Mg bound to chlorophyll varies on Mg status and that its highest values are measured in Mg-deficient plants [2]. The concentration of carbohydrates in the sweet orange leaves ranged from 57.54 to 69.41 umol g\({}^{-1}\) d.w., depending on the concentration of Mg in the nutrient solution (Figure 1c). Their concentration in the plant roots was two- to three-times smaller (24.78-29.96 umol g\({}^{-1}\) d.w.) than those of the leaves (Figure 1c). The concentration of Mg in the nutrient irrigation did not significantly affect the concentration of carbohydrates in the leaves and the roots of the plants (Figure 1c).