## 4 Discussion
### Volatile Metabolite and Gene Transcript Analyses
From these results, it can be proposed that the presence of \(\text{WW}_{\text{r}}\) influenced an entrainment behaviour in \(\text{DS}_{\text{e}}\), resulting in increased isoprene levels compared to the solitary \(\text{DS}_{\text{c}}\). The increase in isoprene emission levels in the \(\text{DS}_{\text{e}}\) treatment could have arisen from isoprene, adsorbed and re-released by the \(\text{DS}_{\text{e}}\) from \(\text{WW}_{\text{r}}\) as a strategy to mitigate drought stress effects. The role of isoprene in improving thermotolerance in drought-tolerant plants under water stress conditions has been well-documented [52; 53; 54]. In a study by [PERSON] et al. [32], the low isoprene emitter map plant has been hypothesised to acquire its drought tolerance from its co-existence in nature with a downy oak, a high isoprene emitter. The researchers speculated that the otherwise susceptible maple adsorbs ambient isoprene produced by the downy oak, hence improving its drought tolerance. Based on the evidence from that previous study, which demonstrated increased drought tolerance in maple after isoprene fumigation, it is probable that the co-located DS emitters in the current work may have taken up isoprene from the non-stressed neighbours as a drought defence mechanism. According to [PERSON] et al. [55], VOC uptake or emission depends on the direction of the concentration gradient between the leaf intercellular air space and the ambient air, and it has also been observed that isoprenoid-emitting species coexisting with non-emitting species can potentially absorb emitted volatiles from the ambient air [31; 56]. It may then be assumed that \(\text{DS}_{\text{e}}\) has provided a sink for isoprene emitted by the \(\text{WW}_{\text{r}}\) receivers [55]. Although the well-documented ecological concept of associational resistance mainly describes resistance achieved from neighbouring heterospecifics against herbivory [57; 58], the results presented herein may suggest a type of associational resistance based on abiotic stress between conspecifics. Contrasting results were observed in this study when noting a synchronised decrease in \(\alpha\)-pinene concentration between \(\mathrm{WW_{r}}\) and \(\mathrm{DS_{e}}\) at PD (Figure 2D). Interestingly, \(\alpha\)-pinene emissions of \(\mathrm{WW_{r}}\) were lower in contrast to those observed in \(\mathrm{WW_{c}}\) on Day 1 (DI). The \(\alpha\)-pinene levels in both co-located treatments remained significantly lower than \(\mathrm{DS_{c}}\) at PD (Day 8) and recovery (Day 9). The general decrease in \(\alpha\)-pinene determined for vines in both drought-stressed treatments and that of isoprene in \(\mathrm{DS_{c}}\) vines during PD may be a strategy used by the vines to mitigate the effects of oxidative stress, using the isoprenoids as precursors to synthesise non-volatile metabolites such as carotenoids, tocopherols, and zeaxanthin [18; 59]. The role of isoprenoids as signalling molecules between plants and other organisms has been well-documented [60], suggesting a possible involvement of these compounds as chemical cues not only within a single plant, but also in interplant communication. \(\alpha\)-Pinene and limonene, as well as several other products, arise due to \(\alpha\)-pinene synthase (VvPNaPin1), which was also considered [61]. Under the experimental conditions used in this study, however, VvPNaPin1 expression was not modulated in response to drought stress, remaining relatively constant between all treatments and time points (Figure 3H).