## 4 Discussion
Significantly more K was contained in the leaves of plants grown in mineral wool and fertilized with higher EC of nutrient solution, where this content was on average about 42,275.0 mg\(\cdot\)kg\({}^{-1}\) DM. On the other hand, higher EC of the nutrient solution in the case of lignite substrate did not result in higher K accumulation in cucumber leaves, as it was only 34,358.3 mg\(\cdot\)kg\({}^{-1}\) DM (Table S1 Supplementary Materials). However, when using high EC of the medium in the lignite substrate (L/high EC), a significant correlation was found between the physical properties of the medium and the S-SO\({}_{4}\) content in cucumber leaves. As reported by [PERSON] et al. [58], the content of mineral components in the substrate changes during cultivation, while changes in the content of components in the leaves are minimal; therefore, proper fertilization and pH in the root environment are important. Similarly to the content of bioactive compounds in fruits, the content of macro- and microelements in leaves may be influenced by many factors, starting from antagonistic relationships of elements [59] through the correct pH of the solution and irrigation strategies, to the right cultivation and plant care practices [55; 58]. Micronutrients are elements that are used by plants in small quantities, but a lack or excess of these elements can disrupt the life processes of the plant and consequently reduce the yield [60]. The present study showed a significant correlation of water capacity of lignite mat to Fe (L/control EC) and Zn (L/high EC) content in cucumber leaves (Table 6). Lignite mats, in spite of their stable chemical composition, low chlorine content and high humic acid content, have large amounts of micronutrients in their composition, especially Fe and Zn [17; 57]. In the initial stages of cultivation, negative effects on the plant may occur, e.g., excessive accumulation of these elements in the root zone, although this phenomenon was not observed in the conducted studies. In a study by [PERSON] and [PERSON] [55] it was proved that different Fe concentrations in the nutrient solution had no significant effect on the content of this element in young cucumber leaves. On the other hand, in a study on Zn content in tomato and cucumber leaves, nutrient solution containing different concentrations of Zn was found to have little effect on the Zn content in tomato leaves, while Zn significantly increased in cucumber leaves with increasing concentrations in the nutrient solution. Excessive concentrations of Fe, Zn and other micronutrients may affect the availability of other elements or vice versa [61]. Similarly, salinity stress can affect elemental content in the leaves. Excessive ion accumulation in transpiring tissue may differ between younger and older leaves because the latter transpire the longest, so they accumulate ions the longest. Additionally, ion accumulation in older leaves depends on the properties of these ions and their mode of transport [62]. Despite the introduction of high EC in the nutrient solution, no significant correlation was found between the content of other micronutrients and the physical properties of the media. It is possible that the humic acids present in the lignite substrate reduced the excessive accumulation of ions in the root zone and did not lead to their accumulation in the leaves. Humic substances may mitigate the effects of salinity by altering the absorption of macro- and micronutrients, accelerating root growth and reducing damage to cytoplasmic membranes, the researchers say [59; 63]. There was a significant positive correlation between water holding capacity and bulk density and cucumber fruit yield in lignite mats compared to mineral wool, where no such relationships were found.