## 4 Discussion
### Characteristics of Fresh and Packed Nettle Leaves
#### 4.4.5 Phenolic Compounds  
Polyphenic compounds are specialized metabolites in plants that possess high antioxidant properties and contribute to the color, flavor, and nutritional value of vegetables [87]. The efficiency of utilization of packaging materials on the preservation of total polyphenols (total polyphenolics, TPC; total flavonoids, TFC; and total nonflavonoids, TNFC) (Figure 7) and individual phenolic compounds (Table 4) during the storage period was also observed within this study. When considering the individual variables tested (Figure 7b), the packaging material (M) had a significant effect on the TPC, TNFC, and TFC content in nettle leaves (\(p\leq 0.0001\)), with the highest values recorded in leaves packaged in LDPE (and for TNFC content in BOPP as well) and the lowest TPC, TNFC, and TFC contents in leaves packaged in PA/PE, and for TFC content also in PLA. Similar results were found in a study conducted by [PERSON] et al. 2011 [88], where fresh-cut spinach packed in LDPE had a higher phenolic content than the one packed in PP. Studies have shown that exposure of plants to high CO\({}_{2}\) levels can increase the production of secondary metabolites and antioxidants [76; 89], as plants can utilize the carbon resources from the available CO\({}_{2}\) surplus for the synthesis of secondary metabolites such as phenolic compounds and flavonoids. For example, under elevated CO\({}_{2}\) levels, lettuce showed a significant increase in total phenolics [89], which is consistent with the present data. Samples packed in LDPE and BOPP with a slightly elevated CO\({}_{2}\) content, had the highest total phenolic levels, compared to the PLA packages with a lower CO\({}_{2}\) content. However, if we take into account the CO\({}_{2}\) content depending on the type of packaging material during the storage period, then we can still see some deviations in the results of this study from the influence of CO\({}_{2}\) already established, namely the fact that a higher CO\({}_{2}\) content utilizes the synthesis of polyphenols. For example, the lowest content of polyphenic compounds was found in PA/PE packaging, although packages had the highest CO\({}_{2}\) content. A possible explanation for this discrepancy could be related to the creation of the internal microenvironment. Therefore, this could have an impact on the metabolic processes in the nettle leaves, as too high CO\({}_{2}\) content can cause side effects and specific stress reactions in the plant material, leading to degradation or inhibition of the synthesis of polyphenic compounds [80; 90]. These results emphasize the complex relationship between CO\({}_{2}\) levels, plant stress responses, and the synthesis of polyphenic compounds, making the management of the internal atmosphere of the packaging crucial for maintaining the nutritional quality of leaf vegetables during storage. When considering the duration of storage (D) as an individual factor influencing the content of polyphenic compounds in nettle leaves, higher levels of TPC, TNFC, and TFC were found after 6 days of storage than after 14 days, regardless of the packaging material. The TPC content was approximately 4% higher after 6 days than after 14 days, the TNFC content was approximately 19% higher, and the TFC content was approximately 14% higher. When looking at the polyphenol content in fresh nettle leaves compared to samples stored for 6 and 14 days, a significant decrease in TPC, TNFC, and TFC content was observed. Thus, the TPC content in fresh nettle leaves was approximately 43% higher compared to 6-day storage and approximately 48% higher compared to 14-day storage. The degradation of total phenols over time during prolonged storage is inevitably due to enzymatic and nonenzymatic activities, and this phenomenon is clearly noticeable in leafy vegetables. Over time, oxidative enzymes such as polyphenol oxidase (PPO) and peroxidase (POD) become more active, leading to the degradation of phenolic compounds [91].