# Watermelon Rind and Flesh Volatile Profiles and Cultivar Difference
## 3 Results
### Rind and Flesh Volatile Comparison with PCA and Heat Map  
Differences between watermelon flesh and rind were observed in the number of volatile compounds recovered, their total peak area, and the types that were most abundant (Table 1). The differences in volatile abundance between samples is partially due to the samples being from different cultivars or being from different parts of the fruit (flesh or rind). There was a cultivar effect and fruit part effect for 66 and 69 of the volatiles, respectively (Table 1). Of the dominant volatiles in the flesh and rind samples, the abundance of hexanal, (\(E\))-2-nonenal (except in Captivation), 6-methyl-5-hepten-2-one decreased in rind samples compared to flesh samples, while (\(Z\))-3-nonen-1-ol, (\(E\),\(Z\))-2,6-nonadienal (except in Excursion), (\(Z\))-6-nonenal (except in Excursion), nonanal (except in Excursion), (\(Z\),\(Z\))-3,6-nonadien-1-ol, and acetaldehyde increased in rind samples. Two-way ANOVA revealed that the interaction of cultivar and fruit part had a significant effect on the volatile abundance of 59 volatiles (Table 1). PCA was conducted to visualize the relationship between volatiles in watermelon rind and flesh samples. PCA was conducted with all 132 volatiles as loading values and eight samples as score values. Then, all less important variables (\(\cos^{2}\) < 0.5) were removed. Less important variables meant those 26 volatiles common occurred in watermelon samples. A total of 106 volatiles were selected (\(\cos^{2}\)\(\geq\) 0.5) and a new PCA was performed. As shown in Figure 2, the first two components accounted for 81.85% of the total variance. The first principal component (PC1) explained 62.09% of the total variance, which was the major component to differentiate samples. Four watermelon flesh samples were separated on the positive side of PC1. These four flesh samples contained high amounts of 54 volatiles, which included 35 major ones (\(\cos^{2}\)\(\geq\) 0.8). These major volatiles were nine aldehydes (V12, V13, V16, V19, V21, V22, V30, V31, V32), eight alcohols (V35, V37, V47, V48, V52, V54, V61, V66), six ketones (V73, V74, V77, V79, V81, V82), nine terpen/terpenoids (V89, V96, V98, V102, V104, V105, V106, V107, V108), and two lactones (V117, V118). Three watermelon rind samples (Captivation, Exclamation, and Excursion) were separated on the negative side of PC1, possessing high amounts of 28 volatiles. The eight major ones (\(\cos^{2}\)\(\geq\) 0.8) were V10 (\(E\)-3-hexenal), V11 (\(Z\)-3-hexenal), V42 (\(Z\)-2-penten-1-ol), V55 (nonanol), V64 (3-ethyl-3-undecanol), V95 (menthol), V126 (benzothiazole), and V131 (naphthalene). Fascination Rind was markedly separated from other samples on the positive side of PC2, which contained a high amount of 21 volatiles. The major five (\(\cos^{2}\)\(\geq\) 0.8) were V34 (methanol), V80 (2-nonanone), V87 (\(\beta\)-myrrence), V110 (ethyl Acetate), and V112 (hexyl acetate). The PCA biplot clearly showed the major difference between watermelon rind and flesh volatile composition. Variety difference within flesh was barely identified in the PCA biplot with the first two PCs. Similarly, no obvious separation of three rind samples (Captivation, Exclamation, and Excursion) was observed. Heat map cluster analysis was used to visualize the differences and commonalities of the relative percentage abundances of volatiles recovered from the flesh and find samples (Figure 3). The samples were grouped into two main clusters, one included all four flesh samples and the other included all four rind samples. Within the flesh cluster, Fascination and Exclamation were grouped together and separated from Excursion and Captivation, which were also separated. Within the rind cluster, Excursion and Exclamation were grouped together and separated from Captivation and Fascination, which were also separated. A total of 55 volatiles were selected to differentiate these samples.