## 3 Discussion
The concentration of Cit varies a lot in literature, which depends on genotype, environmental factors, and analytical methods [25]. Similar to Cit and in accordance with the literature, the concentration of Arg increased along with rind in the samples in this study [25]. The results indicated the potential nutritional value for watermelon rind. Out of nine essential amino acids, seven were identified in watermelon flesh and rind in this study. Rind had a lower content of all essential amino acids, indicating lower nutritional quality compared to flesh. In addition to nutritional value, amino acids have the potential to contribute to tastes such as bitter, sweet, and umami. However, our results indicated that all measured 16 free amino acids had concentrations below their perception thresholds; this suggests that free amino acids in watermelon most likely do not contribute flavor/taste. The negligible taste contribution of watermelon amino acids might explain the absence of this subject in literature. The most critical sensory quality trait of watermelon is sweetness, which relies mainly on mono- and di-saccharides in addition to other soluble solids [26]. The sugar content in watermelon usually reflects the fruit quality and sweetness. This study showed that watermelon rind had significantly less \({}^{\circ}\)Brix compared to flesh, implying that rind should have a significantly less sweet taste compared to flesh. The results were consistent with our previous consumer study that demonstrated that rind incorporation diminished the sweet taste but increased the sourness taste of watermelon juice [27]. A few studies have reported \"Brix in watermelon flesh, which has an average value around 10 [31; 32]. To the best of our knowledge, \({}^{\circ}\)Brix or total soluble solid in watermelon rind has been never been reported. Organic acids, such as malic, citric, and oxalic acids contribute to the sourness taste, but the concentration in watermelon flesh is generally low, \(<\)0.5% [33]. Organic acids in watermelon rind have not been reported in the literature. \({}^{\circ}\)Brix and TA for the samples in this study were consistent with these literature reports. Commonly, \({}^{\circ}\)Brix and TA in plant fruits vary significantly with endogenous factors (genotype, maturity stage, and fruit sampling area) and environmental factors. Aside from the sweet taste, watermelon aroma is the next most important contributor to sensory quality. Approximately 300 volatiles have been identified in watermelon flesh so far [34]; influential factors include watermelon genotype, agro-environmental condition, and the volatile analysis method. Hence, the number of watermelon volatiles obtained by each researcher varies substantially [35; 36; 37; 38; 39; 40; 41]. Their works show the identification of a lesser number of watermelon compounds (41-47) than those found in this study (126 total in rind, flesh, and rind-flesh blends, Table S1). Of the eight grouped volatiles in this study, the common conclusion was that aldehydes, alcohols, and ketones were the most dominant volatiles in the five samples. The results were consistent with findings in the literature that summed aldehydes and alcohols in watermelon flesh, especially nine-carbon linear compounds, can account for up to 77% of the total volatiles [28; 36; 42]. Although the trend for dominant volatiles was similar for flesh and rind, the abundance of the total volatiles varied tremendously between the samples with flesh and without flesh (100% rind). The 10% rind sample had the overall greatest abundance (peak area) of volatiles, followed by 20% rind, 0% rind, and 30% rind samples. It was hard to explain why the total volatile abundance in 10% rind and 20% rind samples was higher than the 0% rind sample. The phenomenon might be caused by the impact of matrix (ratio of flesh and rind causing different volatile binding and release) on SPME fiber absorption. Nevertheless, the 100% rind sample only had around 15% of the total volatiles in the flesh sample.