## 4 Discussion  
Phenolic compound and carotenoid synthesisisactivated in response to climatic, humidity, or other stressful impacts on the plant. 2019 was too dry for the development of willlowherb during their vegetative period, so it could have been one of the key factors in monitoring higher levels of polyphenols and carotenoids. Microbial metabolism and enzymes produced during SPF, such as polyphenol oxidase, have a significant effect on the components of fermented willlowherb leaves, by breaking down macromolecular components (proteins, lipids, and polysaccharides) into lower molecular weight substances and secondary products of metabolism [17]. Also, cutting and pressing during SPF could intensify cell wall degradation, thus improving diffusion of bioactive compounds from the inner cell parts, and then initiating better extraction. Thus, it could have been one of the key factors in monitoring higher levels of polyphenols and carotenoids after solid-phase fermentation. In this experiment, we determined that the short-term (24 h) aerobic solid-phase fermentation process increased the level of total polyphenic compounds by 19.66%. The amount of total phenolic acids significantly increased (by 14.53%) after 24 h aerobic fermentation, and total flavonoids increased by 27.92%, compared to control. Contrarily, the highest amounts of total carotenoids were observed after anaerobic fermentation: 50.38 mg 100 g\({}^{-1}\) DW (72 h) and 48.10 mg 100 g\({}^{-1}\) DW (24 h). Ellagitannin dimer oenothein B quantity depends on ellagic and gallic acid quantities. The amount of oenothein B significantly increased after 24 h aerobic SPF, and significantly decreased after anaerobic SPF (Table 3). This can be explained by the higher oxygen amount and the changes in quantity and quality of gallic and ellagic acids during aerobic solid-phase fermentation. Also, microbial metabolism intensifies and the enzyme (polyphenol oxidase) activity disintegrates them into lower molecular weight components [18]. Quercetin and quercetin-3-\(O\)-rutinoside (rutin) are very biochemically and structurally related to each other, thus we can explain the results among flavonoid quantities (Table 3). Quercetin significantly increased after 24 h aerobic fermentation, but the amount of quercetin-3-\(O\)-rutinoside significantly decreased. It can be explained by the disintegration of glycoside links by fermentation bacteria [19]. Myricetin, luteolin, and quercetin are structurally similar. Myricetin can be produced from kaempfer [20]. The levels of myricetin and kaempfer were significantly higher after aerobic fermentation (Table 3). The highest amounts of gallic acid were determined after 24 h aerobic, and anaerobic fermentation after 48 h and 72 h. Gallic acid is found as part of hydrolysable tannins or in free form. Gallic acid and ellagic acid groups are related. The results could be explained, because hydrolysable tannins (gallotannins and ellagitannins) are disintegrating during hydrolysis into gallic acid and glucose [21]. Cinamic acid takes part in the metabolism of chlorogenic and \(p\)-coumaric acids [22]. Thus, we can explain the results of these two phenolic acids. Chlorogenic acid levels decreased significantly in almost all variants of fermentation, but \(p\)-coumaric acid increased significantly only after anaerobic solid-phase fermentation (Table 4). The most common phenolic acid found in willlowherb leaves is ellagic acid. In plants, ellagitannins hydrolyze into ellagic acids [23]. In this case, SPF significantly increased the level of ellagic acid only after 24 h aerobic fermentation, and decreased it in all other variants (Table 4). During SPF, microbial metabolism and enzyme activity rises, and part of the ellagic acid disintegrates to the final acids. Due to this fact, the amounts of ellagic acid that decreased could be clarified. Benzo acid is found in a lot of plants and it mediates the biosynthesis of many secondary metabolites [24].