## 3 Results and Discussion
### Polyphenols and Carotenoids Content
They take part in, among others, the processes of seed maturation and dormancy, which are essential for the proper development and survival of cereal crops [57] and act as UV-scavengers, protecting plants from the adverse effects of ultraviolet radiation [59]. They are involved in both biotic and abiotic stress responses [57; 58]. In our study, the applied doses of nitrogen in barley  
Figure 1: Phenolic acid concentrations in grains of four different barley varieties grown with four different N fertilization doses. Data are presented as means \(\pm\) standard errors (SE). Mean values followed by different letters (a–h) are significantly different at \(p\) < 0.05. Capital letters and numbers below bars indicate the name of the cultivar and the dose of fertilizer: Z—Zoom; H—Hobbit; G—Galation; S—Sandra; 60—60 kg N/ha; 90—90 kg N/ha; 120—120 kg N/ha; 150—150 kg N/ha. cultivation significantly affected the concentration of flavonoids in grains. It turned out that significantly more flavonoids were identified in grain grown with higher nitrogen rates (120 and 150 kg N ha\({}^{-1}\)). This result is consistent with the observations of other researchers, who claimed that the dose of 120 kg N ha\({}^{-1}\) generally improves plant growth and nutrient content [51; 52]. Significant differences were also observed among the studied barley varieties. The Hobbit variety was richest in flavonoids (Table 4). Among the flavonoids detected in our study, apigenin and kaempfer were found in the highest concentrations in barley grain, and their content depended on both the variety and the fertilization. Similar apigenin contents were found in grain grown with doses of 90, 120, and 150 kg N ha\({}^{-1}\), and they were significantly higher than at the dose of 60 kg N ha\({}^{-1}\). In the case of kaempfer, higher contents were identified in grain grown with doses of 60, 120, and 150 kg N ha\({}^{-1}\) in comparison to 90 kg N ha\({}^{-1}\). Among the varieties tested, Sandra and Galation contained the least apigenin and kaempfer, respectively. The interactions' means concerning the effect of nitrogen rates and barley variety on flavonoid content are presented in Figure 2. According to the literature, the dominant flavonoids in barley vary depending on the grain color and type, which includes varieties such as purple, black, blue, yellow, and white barley [60]. A comparison of the content of the sum of identified polyphenols in grains cultivated with the different nitrogen doses showed no statistical differences. When comparing the studied barley varieties, Zoom stood out in terms of the concentration of these compounds (Table 5). The Zoom variety also showed a tendency towards a higher lutein content in the grain compared to the other varieties, while barley grain grown with different N-fertil doses did not differ in terms of the content of the identified carotenoids, namely lutein and \(\beta\)-carotene (Table 5). As reported by [PERSON] et al. [61], higher nitrogen rates were shown to reduce carotenoid levels in barley under stress conditions (drought). On the other hand, it is possible that moderate nitrogen doses may be beneficial for maintaining or slightly increasing the concentration of these compounds [51].