## 4 Discussion  
Sour orange and _Poncirus trifoliata_ \"Flying Dragon\" are considered \"long established\" rootstocks in Florida; however, a brief overview of the three rootstocks that were used in this study is provided here. The C-35 rootstock is a \"citrange\" that is obtained by a cross of 'Ruby' sweet orange (_Citrus sinensis_'Ruby CRC 589') with _P. trifoliata_. Its advantages are tolerance to _Phyophthora_, _Citrus tristeza_ virus (CTV), drought, and nematodes, but it is sensitive to high salinity and is not good in clay soil (source: [[https://crec.ifas.ufl.edu/extension/citrus_rootstock](https://crec.ifas.ufl.edu/extension/citrus_rootstock)]([https://crec.ifas.ufl.edu/extension/citrus_rootstock](https://crec.ifas.ufl.edu/extension/citrus_rootstock)), accessed on 15 January 2018). The US-897 rootstock originated from a cross of 'Cleopatra' mandarin (_Citrus reticulata_\(\times\)_P. trifoliata_). It produces compact, short-lived trees (~17 years) with good yield and quality, and shows tolerance to CTV, _Phyophthora malaria_, and _Diarepes_ root weevil. The sour orange (_Citrus aunrtium_ L. 'Sour') is one of the oldest and most widely used rootstocks for citrus trees. It was commonly planted in Florida until it was found to be susceptible to citrus nematode and CTV, both of which are endemic in Florida [12; 49]. Many growers now choose more CTV-tolerant rootstocks (primarily trifoliates), but sour orange (SO) is still available in many Florida nurseries. SO is somewhat tolerant to salinity, alkalinity, and less than optimal drainage, and it is relatively tolerant to the cold and _Phyophthora_. Graprefruit and orange yields on SO are moderate, with average sized, good quality fruit ([[http://citruspages.free.fr/trifoliates.html#sour](http://citruspages.free.fr/trifoliates.html#sour)]([http://citruspages.free.fr/trifoliates.html#sour](http://citruspages.free.fr/trifoliates.html#sour)), accessed on 15 January 2020). The contributions of rootstocks toward the HLB tolerance of scion remains controversial. In large field-based studies, researchers have found both profound effects of the rootstock on the scion [4] and little effects on the scion [50]. These mixed results may be due to many factors including grove/tree age, scion type, soil type, differences in nutritional regimes, and delays in HLB symptom development. In an extensive study of the effects of HLB on the yield and fruit quality of 'Valencia' sweet orange on multiple rootstocks, [PERSON], [PERSON] and [PERSON] [4] found that US-897 had the lowest _'Ca._ L. asiaticus' titer; however, it also had among the lowest yield and fruit size of the many rootstocks that were measured. On the other hand, scion growth on SO and US-897 ranked near the top compared to other rootstocks, and had fewer foliar HLB symptoms in some trials, but not others. In a similar study, symptom severity varied with season [2]. It should be noted that these excellent, multi-year studies were focused on production parameters and overall tree health, rather than the effects of insect infestation on VOC production and serve here to emphasize the many complex variables at play in the citrus response to _D. citri_ and HLB. Rootstock choice can, however, influence the VOC composition of leaves and fruit. In a study of a Pummelo \(\times\) Graprefruit hybrid on three rootstocks, SO conferred sabinene to the fruit aroma volatiles, while it was absent from fruit that were grown on Volk and _C. macrophulla_ rootstock [51]. Both monoterpenes and sesquiterpenes were affected by the rootstock choice, as \(\beta\)-elemene and \(\beta\)-caryophyllene were significantly higher in fruit grown that were on SO than the other rootstocks [51]. Likewise, the essential oil composition of Persian Lime (_Citrus_\(\times\)_latifolia_) was affected by rootstock choice, but there was no effect on organoleptic traits such as juice content, pH, titratable acidity, or sugars [52]. Raddatz-Motaetal.