## 3 Results and Discussion
### Effect of BTH on Linolenic and Linoleic Acids in Cabernet Gernischt Grapes  
Linolenic and linoleic acids are the direct precursors of the LOX pathway. Linolenic and linoleic acids increased and then decreased during grape ripening. Linoleic acid was 1.75-11.02 times greater than linolenic acid, and the difference between the two was most pronounced during the mid-ripening stage at week 13 (Figure 2B). This is consistent with the results reported in the study by [PERSON] et al. [34]. BTH treatment resulted in an increased accumulation of linoleic and linoleic acid before veraison, while the opposite effect was observed after veraison (Figure 2). It can be hypothesized that BTH had a promoting effect on acetyl-CoA before veraison [51], which enhanced the conversion of bound fatty acids to free linolenic and linoleic acids. Fatty acid desaturase activity was found to decrease in BTH-treated grapes after veraison, whereas the conversion of bound to free was reduced [45]. In addition, BTH treatment inhibited the relative expression of phospholipid metabolism PLC and PLD genes, leading to a decrease in PLC and PLD activities [45]. Consequently, this reduction in enzymatic activity results in a decrease in the production of phospholipid metabolites, such as linolenic acid and linoleic acid. Overall, the erratic trend of linolenic and linoleic acid concentrations in BTH-treated grapes before and after veraison may be the result of a combination of the factors mentioned above.  
The content and proportion of unsaturated fatty acids affect cell membrane fluidity, which has an important impact on physiological responses, such as resistance and senescence in grapes [45]. In our study, the unsaturation of fatty acids in BTH-treated grapes increased during grape ripening, which is in line with the results of a previous study that reported that BTH treatment can inhibit the degradation of phospholipid and maintain the fluidity of the cell membranes [45]. Drought stress causes an increase in triphospho-pyridine nucleotide oxidase activity at the plasma membrane and the accumulation of a large number of ROS, which when exceeding the threshold will attack the cell membrane system and cause membrane lipid peroxidation. This process involves breaking the double bonds of unsaturated fatty acids, leading to the formation of lipohydroperoxides, which ultimately degrade into malondialdehydedehyde [52]. However, BTH increases the accumulation of ROS by delaying the scavenging process, increasing enzymatic antioxidants, and accumulating osmotic substances [53]. When considering the climate data of the study site (Figure S1), it was found that the rainfall at mid-ripening (week 13) was 52.02% lower than the previous month, while the temperature remained the same. Therefore, BTH may have strengthened the accumulation of ROS in grapevines under drought stress, which led to the conversion of more linolenic and linoleic acids into malondialdehydedehyde. Consequently, it decreased the content of linolenic and linoleic acids. This may be the main reason for the  
Figure 2: Linolenic acid (**A**) and linoleic acid (**B**) of BTH-treated (BTH) and control (CK) Cabernet Germisch grapes. Data are expressed as means (\(n=3\)). Error bars represent standard deviation. Different uppercase letters indicate significant differences between BTH and CK (\(p<0.05\)), while different lowercase letters indicate significant differences within CK or BTH between different weeks of post-flowering (\(p<0.05\)).  
significant reduction in linoleic and linolenic acid contents at mid-ripening (week 13) of BTH-treated grapes.