## 3 Results and Discussion
### Influence of BTH Treatment on the Physicochemical Parameters of Cabernet Gernischt Grapes
#### 3.1.4 Total Phenolic (TP) and Total Flavonoid (TF) Content  
The TP content of grapes decreased progressively during ripening. The decline was greater at the end-swelling stage (weeks 7-9), while the trend was moderate from end-veraison to ripening (weeks 11-15). These results were similar to reported by [PERSON] et al. [32]. BTH application significantly increased the TP content during specific weeks after veraison (weeks 9, 11, and 15). Notably, at end-veraison (week 11), the TP content of BTH-treated grapes significantly increased by 30.78% compared to the CK (Figure 1G). Flavonoids are significant plant secondary metabolites that serve various functions, including attracting pollinators, providing UV protection, acting as antioxidants, and aiding in the plant defense. The content of TF decreased during the ripening process of grapes. However, ripe grapes treated with BTH showed a significant increase of 17.23% compared to the CK (Figure 1H).  
The main components of the grape TP content are hydroxycinnamic acid derivatives, flavanols (including proanthocyanidins), flavonoids, and anthocyanins. The levels of flavonoids, proanthocyanidins, and hydroxycinnamic acids are higher during the fruit-set stage and decrease significantly during the ripening stage [43]. In this study, the decrease in these compounds may be the primary factor contributing to the decline in the TP and TF content as the grapes matured. The stress response caused by BTH triggers the activation of secondary metabolic pathways, resulting in the accumulation of phenolics. [PERSON] et al. [32] found that BTH effectively increased strawberry phenylalanineaminonalylase activity and elevated the levels of total phenolics, flavonoids, and anthocyanins. Previous studies have also proposed that BTH enhances chalcone synthase activity in the process of polyphenol and anthocyanin biosynthesis [44]. Additionally, it has been found that BTH suppresses the activity and gene expression of phospholipase enzymes, such as phospholipase A\({}_{2}\), phospholipase C (PLC), and phospholipase D (PLD) [45]. Furthermore, the exogenous application of BTH enhances the expression of the MYB-bHLH-WD40 complex, which is related to proanthocyanidins, and increases the content of flavan-3-ol, which leads to the accumulation of proanthocyanidins [46]. Treatment with exogenous SA activated the activity of flavonol synthase (FLS) and stimulated the synthesis of new FLS proteins at a specific stage [47]. SA may regulate the biosynthesis of grape flavonoids by increasing the activity of FLS. The accumulation of flavonoids may also play a role in the formation of SA-mediated acquired resistance [47]. Moreover, SA promoted the accumulation of TP, TF, total flavan-3-ols, and flavan-3-ol monomers in grapevine leaves. This led to an increase in the activities of anthocyanin reductase and leucanthocyanidin reductase, as well as the transcription of genes related to flavan-3-ol biosynthesis (e.g., _VvANR_, _VvLAR1_, _VvLAR2_, _VvMYBBA1_, and _VvMYBBA2_) [48]. Therefore, BTH may enhance the accumulation of proanthocyanidins, flavonoids, and flavanols, as well as increase the overall phenolic content through the regulation of grape chalcone synthase, phospholipase, FLS, anthocyanin reductase, and leucanthocyanidin reductase activities, as well as the expression of related genes [32, 43, 44, 45, 46, 47, 48]. Overall, BTH positively affected the anthocyanin content of Monastrell [25] and Merlot [49] grapes. The activation of enzymes related to anthocyanin metabolism [26], enzymes in the phenylpropanoid pathway [50], and enzymes during polyphenol synthesis [32, 44] by BTH has been widely reported. Based on our research findings, it is easy to conclude that BTH increases the phenolics content mainly by modulating the activity of enzymes involved in metabolic pathways and may not be limited by grape variety.