## 3 Results
### Principal Component Analysis  
To discriminate the effects of rootstock and _D. citri_ infestation on the leaf VOC profiles of Sugar Belle, we used principal component analysis (PCA) (Figure 1). qPCR results of citrus plants and infesting _D. citri_ showed that both plants and psyllids that were used in the experiments were 'Ca. L. asiaticus'-free. The compound numbers (1-44) that are found in the loading plots correspond to the numbers and names that are provided in Table 1. The scatter plots represent the VOCs that were present in leaves of the control trees (Figure 1A), _D. citri_-infested trees (Figure 1B), and both the control and infested trees plotted together (Figure 1C). More separation by rootstock is evident in the infested Sugar Belle (Figure 1B) than the non-infested control trees, and when plotted together (Figure 1C), the control trees (closed symbols) clustered together compared to the infested trees (open symbols). Sour orange rootstock grouped in the upper right quadrant of Figure 1A, and the principal components included \(\alpha\)-pinene, asbinene, \(\gamma\)-terpinene, linalool, p-cymene-8-ol, and m-thymol that were found in higher amounts than in the other two rootstocks. The principal components of US-897, which clustered loosely in the lower left quadrant were decanal, thymol methyl ester, and the unknown with _m/z_ 69 (Figure 1A). VOCs contributing to the profile of C-35 included \(\alpha\)-thujene, d-limoence, palmitic acid, methyl linolente, p-cymeneene, \(\beta\)-elemene, \(\beta\)-caryophyllene, and \(\alpha\)-humulene. Performing the PCA on the VOC concentrations after _D. citri_ infestation provided a much clearer definition of the three rootstocks, demonstrating their differential response (Figure 1B). US-897 clustered across the upper left and lower left quadrants, the sour orange clustered across the upper and lower right quadrants, while C-35 remained clustered in the center (Figure 1B). The compounds contributing to the clustering of US-897 were mainly the monoterpenes (11 monoterpenes were higher in infested US-897 compared to C-35 and SO) and linalool. SO rootstock principal components included nearly all of the terpene alcohols, the two aldehydes, and the sesquiterpenes. C-35 remained clustered in the center with clavicol (#29 in the Figure 1B loading plot, Table 1), \(\alpha\)-terpinene, and stearic acid as important principal components. Figure 1C shows that the infested rootstocks separated away from each other (in the upper and lower right quadrants) and away from the controls (mostly left of center), indicating a differential response to the infestation with _D. citri_.  
\begin{table}
\begin{tabular}{c c c c c c} \hline
**Compound No.** & **Leaf VOC** & **VOC Class** & **Compound No.** & **Leaf VOC** & **VOC Class** \\ \hline
**15** & Linalool & Terpene alcohol & **37** & Calamee & Sesquietrpene \\
**16** & _allo_-Ocimene & Monoterpene & **38** & \(\beta\)-Sinessal & Aldehyde \\
**17** & p-Mentha-1,5,8-triene & Monoterpene & **39** & Unk _m/z_ 69/125 & Unknown \\
**18** & Terpen4-ol & Terpene alcohol & **40** & Palmitic acid & Fatty acid \\
**19** & p-Cymen-8-ol & Terpene alcohol & **41** & Palmitic, ME & Fatty acid ester \\
**20** & \(\alpha\)-Terpineol & Terpene alcohol & **42** & Phytol & Terpene alcohol \\
**21** & Decanal & Aldehyde & **43** & Stearic acid & Fatty acid \\
**22** & Carveol & Terpene alcohol & **44** & Linolenic acid, ME & Fatty acid ester \\ \hline \end{tabular}
\end{table}
Table 1: Cont.