## 4 Discussion
Plants grown in lignite medium fertilized with standard EC (L/control EC) obtained a 10% higher total yield compared to mineral wool (MW/control EC), while in the L/high EC combination, plants obtained a 14.8% higher marketable yield and 10% lower non-marketable yield, respectively, compared to MW/high EC. [PERSON] et al. [49] found a positive correlation in tomato marketable yield with the content of readily available water in the substrate and a negative correlation with the amount of air in the substrate. Other researchers also confirm that yield correlates with water buffer in the growing medium [64]. Similar results were reported by studying the yield of cucumber grown in perilla, where higher yield was dependent on the availability of water in the substrate [50]. The obtained results confirm the correlation of bulk density (BD) and water holding capacity (WHC) of the lignite substrate with cucumber yield. However, in the studies with mineral wool substrate (MW/control EC), such a correlation of BD and WHC of the substrate with cucumber yield was not proved (Table 8). Physicochemical properties of substrates affect plant yield and growth [65]. However, which parameter affects this and to what extent is difficult to determine. In a study on the yield of cucumber and tomato plants grown on _Miscanthus_ substrate, there were no differences in yield compared to mineral wool [7]. Similar results were reported in tomato plants, where the substrates had no effect on fruit number or weight [54]. According to [PERSON] et al. [66] fruit number and weight depend on the air and water content of the substrate, because insufficient air in the soil reduces root respiration and negatively affects water and nutrient uptake. Fruit quality is a very broad concept, and it is not possible to say unequivocally which trait affects the final yield quality more significantly. Fruit firmness, texture, color and organic content are important [32; 67]. Table S3 in the Supplementary Materials contains the average values of chosen quality traits of cucumber fruits grown hydroponically in the conducted studies. Despite many studies on new substrates for hydroponic cultivation, there is still little information on the correlation of physical characteristics of the substrate with color and bioactive content. In the study, a significant correlation was found between the bulk density of mineral wool substrate and the color intensity (H*) and color index (a*/b*) of cucumber fruit peel in the CIE Lab system (Table 10). Color saturation (C*) of cucumber fruit was strongly correlated with bulk density of lignite substrate (control EC) and mineral wool (high EC) substrates. The obtained results also confirmed a strong correlation between selected physical substrate characteristics and cucumber fruit firmness. Physical characteristics of the substrate such as bulk density and water holding capacity of lignite mats (BD and WHC) were also strongly correlated with color intensity (H*) and -coloration index (a*/b*) of cucumber fruit peel. Many publications indicate that there is no effect from salinity on the color and hardness of cucumber fruit [68; 69]. In the case of tomato, no differences were found in the firmness of fruit from plants grown in different organic media [6]. According to [PERSON] et al. [19], the substrate and its physical characteristics can affect the dry matter content and TSS in cucumber fruits. In the present study, a high correlation coefficient, but not statistically significant, was obtained between bulk density of lignite growing mats (L/high EC) and dry matter of cucumber fruits. Similar results were obtained in an experiment with twice-used lignite substrate and high EC medium in cucumber cultivation [20]. In studies conducted on tomato plants grown on organic substrates, the highest dry matter content was obtained in fruits grown in sheep wool, but no correlation was found between the physical properties of this substrate and fruit quality [6].