## 4 Discussion
### Volatile Metabolite and Gene Transcript Analyses  
MeJA and MeSA concentrations displayed a similar trend, with no detectable levels of either compound recorded for the WW\({}_{\rm c}\) and DS\({}_{\rm c}\) controls at drought initiation, as well as for the co-located treatments at peak drought and recovery (Figure 2A,C). However, DS\({}_{\rm c}\) had significantly higher MeJA and MeSA levels under PD conditions as compared to both co-located treatments (WW\({}_{\rm r}\) and DS\({}_{\rm e}\)), which had non-detectable levels. The concentration of MeJA in DS\({}_{\rm c}\) continued to increase until recovery, while MeSA remained constant until the end of the experiment. The significant increases in MeJA and MeSA concentrations at PD were consistent with the results from previous studies that found an increase in emissions of both metabolites in response to drought stress [43; 44]. In a similar VOC-priming study involving drought stress, tea plants were shown to release MeSA in response to drought [21]. Parallel to these findings, herbivore-induced VOCs were also demonstratedto induce an upregulation of a subset of JA-inducible genes [16], as well as an increased accumulation of JA and SA in the leaves of receiver plants [11; 12], suggesting the key role of JA and SA signalling in mediating defence responses. In the current study, however, the expression of CHORS, the gene corresponding to MeSA production, was not consistent with the metabolite analysis results. There were no significant differences in the expression of the salicate synthesis gene across all treatments. The MeJA-related AOS gene was significantly upregulated in \(\text{DS}_{\text{e}}\). Interestingly, this upregulation was not observed in \(\text{DS}_{\text{e}}\) vines, suggesting a probable influence of the co-located \(\text{WW}_{\text{r}}\) on \(\text{DS}_{\text{e}}\). Although not decisive, the results suggested that emissions of MeJA and MeSA occur concurrently in grapevine and are synchronised between the emitter and receiver vines. Thus, while inter-vine interactions cannot be concluded based on these results, it was nevertheless evident that MeJA and MeSA were involved in the drought stress response of the emitter but not the receiver vines. An increase in GLV emissions during drought episodes has been associated with lipid peroxidation as a result of enhanced drought-induced senescence [45]. Although several studies based on drought [46; 47; 48] and various biotic [13; 14; 15] stresses have shown that (\(Z\))-3-hexen-1-ol is emitted in response to stress, no detectable amounts of (\(Z\))-3-hexen-1-ol in response to drought were observed in the present work. The lipoxygenase gene, VvLOXD, which is involved in the synthesis of oxylipins and their derivatives such as JA and GLVs, was not significantly modulated in any of the treatments (Figure 2F). The majority of plant emissions occur through the stomata of leaves and any variations in \(g_{s}\) can differentially impact the rates at which distinct VOCs are emitted from plants under stress [49]. However, isoprene and \(\alpha\)-pinene have been observed in several studies to be emitted independently of stomatal regulation [50; 51] due to their large Henry's Law constant, which entails that they can sustain an elevated intercellular partial pressure at a specific liquid phase concentration. However, this does not concur with the current results, in which the \(\alpha\)-pinene and isoprene concentrations decreased in \(\text{DS}_{\text{e}}\) at Day 8 (PD) (Figure 2C,D) when both \(g_{s}\) and soil moisture content were low (Table 2). Interestingly, \(\alpha\)-pinene concentration decreased in \(\text{DS}_{\text{e}}\) at PD but not isoprene, with the latter increasing despite the decline in \(g_{s}\) at Day 8 (PD) (Figure 2D). The isoprene levels of \(\text{WW}_{\text{r}}\) vines were not affected by the presence of the emitter and remained constant throughout the experiment.