## 3 Results and Discussion
Conversely, the Ca concentrations of the rootstock's stem were found to be significantly higher in the presence of 192 mg Mg L\({}^{-1}\) (Table 1). However, the Ca use efficiency of the sweet orange plants was not significantly affected by Mg nutrition (Table 2). The total amount of K per plant, and its total absorption from the nutrient solution, was significantly reduced in the treatment of 192 mg Mg L\({}^{-1}\), compared to the treatment where the plants were developed in the absence of Mg (Table 1). Plant K concentrations decreased with the supply of Mg in maize and coffee [46; 47]. The antagonistic effect of Mg on K has been reported in _Citrus_[23; 27; 45]. However, the K use efficiency and K concentrations in the leaves, the stems of the scions, and the rootstock were not significantly influenced by the concentration of Mg in the nutrient solution (Tables 1 and 3). The roots of the plants treated with the nutrient solutions 48-192 mg Mg L\({}^{-1}\), particularly in the treatment 192 mg Mg L\({}^{-1}\), contained significantly more K than those of the plants under the treatments 0-24 mg Mg L\({}^{-1}\) (Table 1). Depending on the ratio of K to Mg in the growth medium, and the plant species employed in the studies, the literature claims that Mg has either a moderate synergistic, an antagonistic, or even no influence on K absorption [48]. The results showed that there was no significant effect of Mg on either P concentrations of various plant parts or the P use efficiency (Tables 1 and 3). In contrast, it was found that increasing the concentration of Mg in the nutrient solution caused a gradual decrease in the total amount of P per plant (Table 3). This decrease was particularly significant in the presence of higher concentrations of Mg in the nutrient solution (96-192 mg Mg L\({}^{-1}\)). Significant decrease in Mn concentrations of the roots was measured when the plants were grown at Mg concentrations higher than 24 mg Mg L\({}^{-1}\) (Table 1). Also, the presence of Mg in the nutrient solution, irrespective of its concentration (12-192 mg Mg L\({}^{-1}\)), resulted in a significant reduction of Mn concentrations in the leaves and the stem of the rootstock, as well as of the total amount of Mn being absorbed by the plants (Tables 1 and 3). Moreover, the Mn use efficiency of plants grown under the influence of the higher Mg concentrations in the nutrient solution (96-192 mg Mg L\({}^{-1}\)) was significantly increased compared to the other Mg treatments (0-48 mg Mg L\({}^{-1}\)) (Table 3). This contradicts research on _Citrus_[23] and other species [19; 46], which found that Mg deficit increased the content of Mn in the roots and leaves. The competitive effects of Mg on Mn, however, may differ, depending on the genotype of plant and the nutrient concentrations in the medium [46]. Magnesium concentration of the nutrient solution did not significantly affect the concentration of Zn in the scion or the Zn use efficiency of the sweet orange plants (Tables 1 and 3), indicating that there was not an antagonistic interaction between the two nutrients. However, the total absorption of the Zn per plant, as well as its concentrations in the stem of the rootstock, initially increased with the increase of Mg in the nutrient solution, and then decreased (Tables 1 and 3). Additionally, root Zn concentration was significantly increased in all Mg treatments (12-192 mg Mg L\({}^{-1}\)) (Table 1). This is in contrast to previous studies performed, not only in _Citrus_, where the Mg concentration of the nutrient solution was negatively correlated with leaf and root Zn concentration [19; 46]. There was no significant effect of Mg on Fe concentrations of the stems of the scion and the rootstock, nor on the Fe use efficiency of plants (Tables 1 and 3). In contrast, it was found that increasing the concentration of Mg in the nutrient solution caused a gradual decrease in the total amount of Fe per plant and, therefore, the ability of the plants to absorb Fe from the nutrient solution (Table 3).