## 3 Results
### Investigating VOC Signalling Using Volatile Metabolite and Transcript Analyses  
#### 3.2.1 Constitutive VOC Emissions  
The constitutive emissions of selected VOCs, \(\alpha\)-pinene, isoprene, MeJA, MeSA, and (Z)-3-hexen-1-ol, were measured from well-watered control treatment plants. Constitutive emissions of \(\alpha\)-pinene and isoprene were recorded from WW\({}_{\rm c}\) at 21.3 \(\mu\)g L\({}^{-1}\) and 24.4 ng mL\({}^{-1}\), respectively (Figure 2). Lower concentrations of MeJA (0.4 \(\mu\)g L\({}^{-1}\)) and MeSA (0.1 \(\mu\)g L\({}^{-1}\)) were measured, but (\(Z\))-3-hexen-1-ol was not detected in any of the samples (Figure 2). 2.2 Synchronised Emission of MeJA, MeSA, \(\alpha\)-Pinene, and Isoprene in Co-Located Treatment Vines  
At DI, detectable levels of MeJA and MeSA were recorded only for the co-located receiver (WW\({}_{\rm r}\)) and emitter (DS\({}_{\rm e}\)) vines (Figure 3A,B). The concentration of MeJA in WW\({}_{\rm r}\) and DS\({}_{\rm e}\) was similar at 16.0 \(\mu\)g L\({}^{-1}\) and 15.9 \(\mu\)g L\({}^{-1}\), respectively (\(p>0.05\)) (Figure 3A). Likewise, MeSA concentration in DS\({}_{\rm e}\) (8.2 \(\mu\)g L\({}^{-1}\)) and receiver (7.9 \(\mu\)g L\({}^{-1}\)) were not significantly different at DI (Figure 3B). At PD and recovery, MeJA (16.6 \(\mu\)g L\({}^{-1}\)) and MeSA levels (8.0 \(\mu\)g L\({}^{-1}\)) were only detected in DS\({}_{\rm c}\) vines (Figure 3A,C). Expression of the JA biosynthesis-related gene, AOS, was similar across treatments at DI, with a mean relative expression of 1.4 (Figure 3E). At PD, higher relative expression levels of AOS were observed in DS\({}_{\rm e}\) (18.7) from an initial expression level of 2.4 observed at DI. This was higher than its WW neighbour, WW\({}_{\rm r}\) (\(p=0.005\)), whose AOS gene expression level of 1.2 was similar to DI (1.0). AOS expression in DS\({}_{\rm c}\) was much lower than DS\({}_{\rm e}\) (\(p=0.005\)). Both WW treatments (WW\({}_{\rm c}\) and WW\({}_{\rm r}\)) showed no significant differences at PD (\(p>0.999\)). The expression levels of the lipoxygenase gene, LOXD, related to MeJA and (\(Z\))-3-hexen-1-ol biosynthesis, were similar between treatments at all time points (Figure 3G). No detectable levels of (\(Z\))-3-hexen-1-ol were observed at any time point. There were also no differences in the expression levels of the MeSA-related CHORS gene between all the treatments at all timepoints (Figure 3F). Significant differences in the isoprene concentration were noted between the treatments at DI (\(p<0.05\)) (Figure 3C), with WW\({}_{\rm c}\) vines having a higher concentration than WW\({}_{\rm r}\) vines, at 24.4 \(\mu\)g L\({}^{-1}\) and 16.2 \(\mu\)g L\({}^{-1}\), respectively (\(p<0.001\)). Isoprene concentration in DS\({}_{\rm c}\) (19.1 \(\mu\)g L\({}^{-1}\)) was also higher than DS\({}_{\rm e}\) (13.8 \(\mu\)g L\({}^{-1}\)) at DI (\(p=0.005\)). A notable increase in isoprene emission in the DS\({}_{\rm e}\) vines was observed at PD (19.5 \(\mu\)g L\({}^{-1}\)), in contrast  
Figure 2: Constitutive VOC mean concentrations of \(\alpha\)-pinene, isoprene, (\(Z\))-3-hexen-1-ol, methyl SA, and methyl J A emitted from well-watered control treatment plants. All the compounds were emitted constitutively except for (\(Z\))-3- hexenol, which was not detected (N/D) in any of the samples. The error bar represents the standard error of the mean (SEM, \(n=3\)). to a decrease in the DS\({}_{\rm c}\) vines (13.5 \(\rm\upmu g\) L\({}^{-1}\)). Isoprene levels of WW\({}_{\rm r}\) remained relatively constant for the duration of the experiment. Figure 3: VOC metabolite concentrations (**A**–**D**) and transcripts of key VOC genes (**E**–**H**). VOC concentrations of (**A**) MeJA (methyl jasmonate), (**B**) MeSA (methyl salicylate), (**C**) isoprene, and (**D**) \(\alpha\)-pinene in isolated WW control (WW\({}_{\rm cj}\), isolated DS (DS\({}_{\rm c}\)), co-located DS (DS\({}_{\rm e}\)), and well-watered (WW\({}_{\rm r}\)) vines during drought initiation, peak drought, and recovery stages.