## 4 Discussion
When nitrogen availability is limiting for growth (organic agriculture), metabolism changes more towards carbon (C) containing compounds (flavonoids) [17]. In the presented experiment, organic herbs contained significantly more flavonoids compared to conventional herbs in both years of the experiment. The organic herbs were characterized by a higher level of quercetin-3-O-glucose, quercetin-3-O-rutinoside and kaempfer-3-O-glucose and a lower level of aglycone quercetin compared to conventional herbs. The oxidation rates of individual flavonoids in plant tissue were in the order: quercetin > kaempfer > quercetin glycoside > kaempfer glycoside [27]. The different ratio of glycone and aglycone form of flavonoids is connected with their oxidation rate. In plants cultivated in organic condition, the oxidation process is more effective [28]. The differences in theflavonoid concentration between herbs species is the result of genetic differences. Particular species have different capacities for the production and accumulation of phenolic compounds. Within one herb family, there may be significant differences in bioactive compound concentration [29]. Three of four herbs studied here (basil, marjoram, and oregano) belong to the same family (Lamiaceae), but the variation in total flavonoids concentration was very high. Oregano contained 1410.5 mg 100 g\({}^{-1}\) in 2017 and 1344.8 mg 100 g\({}^{-1}\) in 2018, while marjoram contained 621.9 mg 100 g\({}^{-1}\) in 2017 and 611.7 mg 100 g\({}^{-1}\) in 2018. Similar results are presented in another experiment [19], where the authors analyzed the concentration of total and individual flavonoids in sage, lemon balm, and peppermint. Those species also belong to the Lamiaceae family. The highest number of total flavonoids found was in sage (153.74 mg 100 g\({}^{-1}\) DW), followed by peppermint (113.82 mg 100 g\({}^{-1}\) DW), and the least in lemon balm (105.87 mg 100 g\({}^{-1}\) DW). It is worth noting that the oregano was obtained from a biodynamic farming system, where only organic fertilization is allowed [30]. In another experiment with leafy vegetables (chicory), biodynamic plants produced much more flavonoids than conventional ones [31]. In the present study, the examined herbs consisted in majority of plant leaves, which contain a large amount of the green pigment, chlorophyll. It is well known that there is a link between chlorophyll concentration and the level of nitrogen fertilization [32]. Organic plants are cultivated only with organic fertilizers and with lower level of nitrogen fertilization compared to conventional herbs. White cabbage cultivated in organic farming contained significantly less total chlorophyll and chlorophyll fractions (a and b) compared to that cultivated by conventional methods [17]. In another experiment, a link was found between a higher level of nitrogen fertilization and chlorophyll concentration in maize leaves [33]. The higher variation between the examined herbs in the case of total chlorophyll concentration is an effect of plant genotype. The situation is similar as with total flavonoids concentration. Despite belonging to the same botanical family, the herbs contained different amounts of chlorophyll (Tables 1 and 2). In the presented experiment, the highest levels of total chlorophyll and its fractions (a and b) were found in bear's garlic leaves in both years. In another experiment, 479.72 mg 100 g\({}^{-1}\) DW of total chlorophyll including chlorophyll a (374.95 mg 100 g\({}^{-1}\)) and chlorophyll b (104.77 mg 100 g\({}^{-1}\)) were found [34]. In foliar plants, green chlorophyll is strictly connected with beta-carotene. Carotenoids are synthesized via the general biosynthetic pathway within the chloroplasts of plants. The effect of UV-B radiation increased carotenoid accumulation and total antioxidant capacity in tobacco (_Nicotiana tabacum_ L.) leaves [35]. Higher levels of active photosynthetic pigments were positively correlated with beta-carotene concentration.