## 4 Discussion
[52] found higher levels of aldehydes (desirable in high quality essential oils) in lime oil from fruit grown on the C-35 rootstock than from SO or _P. trifoliata_, and \(\beta\)-myrcene was found only in lime oil from fruit that was grown on SO and not from C-35, _P. trifoliata_, Volk lemon, or Swingle rootstocks. The current study of the Sugar Belle mandarin scion on three different rootstocks found that the extracted VOC content was slightly different in each rootstock before infestation. After infestation, more than 30 of the 44 VOCs were significantly different in the US-897 and SO rootstocks, whereas C-35 had 25 altered VOCs. The dramatic alteration of leaf VOCs after infestation demonstrates that the rootstocks have the potential to contribute to the overall VOC profile of the scion. Interestingly, C-35 and US-897 are trifoliate hybrids of _P. trifoliata_ and seedlings of US-897 showed moderate tolerance toward HLB in early comparative studies [53]. Several studies have focused on identifying individual plant volatiles that may repel or attract _D. citri_, for use in lures or as other forms of chemical control [54; 55; 56; 57]. Instead, in this study, we extracted the leaf volatiles of the Sugar Belle mandarin before and after infestation with _D. citri_ to identify those that were associated with host response to the infestation. We found that the Sugar Belle scion increased its overall leaf VOC production in response to the infestation regardless of the rootstock onto which it was grafted. The C-35 and SO rootstocks responded similarly, while US-897 gave a stronger and more diverse VOC response to the _D. citri_ infestation. C\({}_{10}\) monoterpenes derive from C\({}_{5}\) isoprene units that are catalyzed by terpene syntheses, of which more than 50 have been found thus far in the genome of the sweet orange, _Citrus sinensis_[58]. C\({}_{10}\) monoterpenes are synthesized from geranyl diphosphate (GPP), and C\({}_{20}\) diterpenes from geranylgeranyl diphosphate (GGPP), both in the methylerythitol phosphate pathway in the plastids [59]. C\({}_{15}\) sesquiterpenes are synthesized in the mevalonic acid pathway from farnesyl diphosphate (FPP), either in the cytosol and/or in peroxisomes [59; 60]. The infestation of Sugar Belle trees with _D. citri_ led to increased percentages of monoterpenes (\(\alpha\)-and \(\gamma\)-terpinene, \(\alpha\)-terpinolene, \(\beta\)-ocimene, and p-cymene), sesquiterpenes (\(\beta\)-elemene and \(\beta\)-caryophyllene), and terpene alcohols (p-cymen-8-ol, thymol, andlinalool) in all three of the rootstocks. In our previous study of _D. citri_-infested 'Valencia' sweet orange, we found that 14 monoterpenes (C\({}_{10}\)) were elevated by the infestation, led by high levels of \(\alpha\)-,\(\beta\)-pinene, sabinene, \(\delta\)-care, \(\beta\)-ocimene, and \(\gamma\)-terpinene [42]. Similar to the current results, total monoterpenes and total sesquiterpenes increased in Tahiti lime fruits that were infested with California red scale, _Aonidiella aurantii_[61]. Individually, the majority of the VOCs increased in concentration with red scale infestation, and five volatiles were produced only by infested fruits (\(\beta\)-thujene, limonene oxide, 1-isopropyl-3-propenylcyclopentane, terpinen-4-ol, and trans-\(\gamma\)-bisabolene [61]. Of these five compounds, we detected only terpin-4-ol in increased amounts in _D. citri_-infested trees, but only significantly in the SO rootstock. These differences were perhaps due to the cultivar and tissue type that were analyzed. In this study, the individual VOCs with the largest induction included the monoterpenes \(\alpha\)- and \(\gamma\)-terpinene, \(\alpha\)-terpinolene, terpen-4-ol, and thymol methyl ether. These particular monoterpenes and their oxygenated derivatives have well-documented anti-feedant and insect deterrent properties, including deterrence against aphids, mosquitos, and pysllids [62; 63; 64; 65].