## 3 Elicitors and Their Classifications
### Benzothiadiazole (BTH)  
Benzo (1,2,3)-thiadiazole-7-carbothiooic acid S-methyl ester (BTH), as reported in the literature, was isolated from a synthesis project aimed at isolating 2-benzylthio-3-furanylbenzoic acid methyl ester instead of BTH [142; 143; 144]. However, the authors discovered that BTH could trigger defense mechanisms in plants against several infections [142]. Benzothiadiazole, as a chemical elicitor, induces responses similar to the endogenous plant hormone salicylic acid (SA) at the molecular level. Thus, BTH is said to be a functional analog of salicylic acid (SA) that induces the activation of several enzymes, especially PAL, triggering the syntheses of bioactive secondary metabolites [144; 145; 111; 134].  
The application of BTH in viticulture has proven efficient in enhancing the polyphenol contents of grapes and their resulting wines in different grape varieties. BTH application to Monastrell grapevines at different ripening stages increased the accumulation of anthocyanins (in treated grapes) in both studied seasons [140]. Similarly, in the previous work by [PERSON] et al. [137], an increase in the concentrations of anthocyanins and flavonoids was reported in Monastrell BTH-treated grapes and wines compared to their respective control samples. Moreover, the application of BTH to different grape varieties improved the phenolic compounds and chromatic characteristics in the grapes and wines of Syrah and Merlot treated samples relative to their respective control samples [138].  
These positive influences of BTH on polyphenols contents coupled with induced defense mechanisms in grapes are associated with a metabolic cost [21]. According to [PERSON] et al. [21], the application of BTH reduces the concentrations of primary metabolites in treated grapes, especially the total soluble sugars, which serve as signaling molecules that modulate the set of genes involved in defense and metabolic responses. The authors postulated that the common precursor (UDP-glucose) of the sucrose metabolism pathway and phenylpropanol pathway might have been directed towards the biosynthesis of polyphenol compounds while reducing the accumulation of soluble sugars. Consequently, the reduced levels of soluble sugars and other primary metabolites such as amino acids influence the volatile compositions and sensory quality of treated grapes and their resulting wines. The extent of the impact on these quality parameters differs with different varieties. [PERSON] et al. [123], in their study of BTH application to Monastrell grapevines, reported a significant increase in the concentrations of various volatile classes, especially terpenoids and norisoprenoids, in BTH-treated grapes compared to the control grapes. Concerning the impact of BTH on wine volatiles, [PERSON] et al. [126] treated Groppello Gentile grapevines with BTH for two consecutive seasons. Wines volatile profiles from the treated and untreated grapes were analyzed. The authors reported increased acetals and esters concentrations in BTH-treated wine during the first season and increased total esters concentration only in BTH-treated wine during the second vintage. All other volatile classes recorded low concentrations in BTH-treated wine compared to the wine from conventionally treated grapes in both seasons. However, [PERSON] et al. [123], in their study of Monastrell wines from BTH-treated grapes, reported significant increases in the concentrations of alcohols, esters, terpenes, and norisoprenoids in BTH-treated wine compared to the control wine. Differences in the accumulation and concentrations of volatile compounds (Table 2) in these studies could be attributed to the differences in their primary metabolites as affected by BTH application and several other factors shown in Figure 3.