## 3 Discussion
### Berry Volatiles and (E)-2-Hexenal QTL  
The VRS-F\({}_{2}\) population varied in volatile compound concentrations across generations and years tested. This study focused on methoxyprazines (IBMP, IPMP) and C\({}_{6}\) aldehyde volatiles, as previous work has shown that these herbaceous and undesirable odorant classes are at higher concentrations in wild _Vitis_ spp. [33]. IBMP was not detected in berries in 2018 and was present in only 27% of genotypes in 2013. The mean IPMP concentrations in _V. riparia_ 'Manitoba 37' pistillate grandparent and VRS-F\({}_{2}\) were greater than those previously reported for _V. riparia_; however, several VRS-F\({}_{2}\) genotypes had concentrations in the range reported for _V. riparia_[33]. In _V. vinifera_, methoxyprazines decrease markedly (-90%) during berry maturation, and the high IPMP values observed in this study may also be because berries were sampled at 30 days post-veraison rather than at post-veraison intervals more typical of commercial wine production (-60 days) [60]. The IPMP content in the VRS-F\({}_{2}\) population varied consistently in 2013 and 2018; however, no QTL was identified for this berry volatile.  
The C\({}_{6}\) aldehydes, hexanal and (E)-2-hexenal, were present in the post-veraison berries of all generations. A novel (E)-2-hexenal QTL located on chromosome 2 was observed in 2013 and 2018. The (E)-2-hexenal QTL were located adjacent to the upper end of the anthocyanin QTL confidence intervals. The (E)-2-hexenal volatile concentrations varied among years and were similar to those in post-veraison 'Cabernet Sauvignon' berries but higher than observed in Chinese wild grape cultivars and 'Seyval blanc' [32; 61; 62]. In 'Cabernet Sauvignon', both hexanal and (E)-2-hexenal were present from fruit set through ripening and concentrations peaked post-veraison with C\({}_{6}\) alcohols peaking during late ripening [32]. Several enzymes and genes have been identified in the lipoxygenase pathway leading to the development of C\({}_{6}\) aldehyde products after crushing [31; 32]. In 'Cabernet Sauvignon', volatile concentrations were suggested to be controlled by a tight regulation of the alcohol dehydrogenase, alcohol acetyl transferase, and enal isomerase enzymes in the lipoxygenase pathway during berry development, with the C\({}_{6}\) aldehyde products being most prevalent in the veraison and post-veraison and alcohols in the late ripening stage [32]. However, none of the genes identified previously for the lipoxygenase pathway were found on chromosome 2 [31]. The examination of the genes underlying the (E)-2-hexenal QTL in this study showed a _PHOSPHOLIIPID HYDROPEROXIDE GLUTATHONE PEROXIDASE_ gene near the peak markers for 2016 and 2018. Phospholipid hydroperoxide glutathione peroxididases enzymes are reactive oxygen species (ROS) scavengers responsible for reducing hydroperoxides generated in the oxidation of fatty acids by lipoxygenase hydroperoxide lyase [63]. It is noted that ROS scavenging enzymes increase at veraison, and the phospholipid hydroperoxide glutathione peroxidase enzyme may play a role in regulating the accumulation of (E)-2-hexenal [64]. In this study, the genotype effect plot for the markers surrounding the _PHOSPHOLIIPID HYDROPEROXIDE GLUTATHONE PEROXIDASE_ gene indicated that the pollen grandparent 'Seyval blanc' contributed to the higher concentration of (E)-2-hexenal. It is possible that a higher activity or other differences in this phospholipid hydroperoxide glutathione peroxidase enzyme in _V. riparia_ could limit the production of C\({}_{6}\) aldehydes in contrast to the mechanism in 'Seyval blanc'. However, further characterization of this enzyme in the grandparents, parent, and high and low concentration F\({}_{2}\) is needed to determine whether phospholipid hydroperoxide glutathione peroxidase activity has a role in modulating the herbaceous volatile in the VRS-F\({}_{2}\) population.