## 4 Discussion
In this study, the nutrient content in the aboveground parts of flowering Chinese cabbage varied depending on the influence of different N sources. Moreover, the nitrogen content determined for the hydroponically grown plants was higher than that of the pot-grown plants. A possible reason for this was sorption of the nutrient in the substrate. [PERSON] and [PERSON] [25] reported that the type of nitrogen fertiliser and the technique of application had little or no effect on the levels of P, K, Mg and Ca in Chinese cabbage cultivation. Others [34] found that leaf N concentration increased with the N rate, but was not affected by the N source or harvest date. In this study, differences in the N content were evident with an increasing N level and varying N sources as well, especially in the hydroponic system. N significantly impacts the amount of RuBisCO and photosynthesis [35; 36]. Since Calvin cycle proteins and thylakoids provide the majority of nitrogen in leaves, there is a relationship between the N content and leaf photosynthesis [36]. The amount of CO\({}_{2}\) fixed per unit of leaf N is defined as the ratio of net photosynthesis to the leaf N content, which varies as a function of the leaf N content [37; 38]. Considering other nutrients, [PERSON] and [PERSON] [34] reported that concentrations of P, Ca and Mg in mature Chinese cabbage wrapper leaves increased with an increasing N rate, while the leaf K concentration decreased. In our study, the effect of nitrogen on nutrients was quite different as a decrease in P and an increase in K, Ca and Mg were observed for substrate cultivation, while for hydroponic cultivation, the contents of P, K and Mg decreased with an increase in the level of N derived from NaNO\({}_{3}\) and Ca(NO\({}_{3}\))\({}_{2}\). In hydroponics, the correlation coefficients between the levels of N and K in plants and between N and Mg were statistically significant. They were as follows: r (N vs. K) = \(-\)0.624 (\(p\) = 0.03) and r (N vs. Mg) = \(-\)0.838 (\(p\) = 0.001). For the pot-cultivated samples, a statistically significant correlation was observed only between N and Mg (r = 0.553, \(p\) = 0.032). According to [PERSON]'s chart [39], N has an negative effect on the K and Cu uptake and a positive effect on Mg. This is partly consistent with our results (for the relationship between N and K in pots and the relationship between N and Mg in hydroponics). In previous studies [4], antagonism between N and Cu was observed for substrate cultivation, which was not the case in this study. In hydroponics, a positive impact of the N dose on Fe, Zn and Cu has been reported [4]. This study observed a significant effect of N on the Fe level in pots (r = 0.551, \(p\) = 0.033) as well as a significant effect of N on Mn (r = 0.693, \(p\) = 0.004). [PERSON] and [PERSON] [40] stated that the application of NH\({}_{4}\)NO\({}_{3}\) led to a significant increase in the content of Fe in red cabbage. [PERSON] et al. [41] found that Cu and Mn concentrations in the aboveground tissues of _Bothriochla ischaemia_ increased significantly with N addition. The determined trends of changes in the Fe, Mn and Cu contents may vary with N nutrition intensity [42] as observed in this study. Generally, the highest macronutrient content in plants grown in the pot cultivation system was determined under treatment with magnesium nitrate, especially at N-70 and N-90 (Figure 4). Simultaneously, when magnesium nitrate was used as a supply of nitrogen, the Mg dose also increased (balancing was performed for N). The beneficial impact of magnesium nutrition on plant production has also been shown in numerous other plant species [15; 43]. Mg is a crucial macroelement for the growth and development of plants. It plays an important role in various metabolic functions, such as photosynthesis and enzyme activation. The application of Mg fertilisers can have a positive impact on the levels of other macro- and microelements in plants, including N, P, K, Fe, Mn, Zn and Cu (according to Mulder's chart). In the study by [PERSON] et al.