## 4 Discussion
[30] compared the bioactive components of watermelon flesh, rind, and seeds and showed that the highest citruline content was detected in watermelon rind (19.3 \(\pm\) 1.1 mg/100 kcal) followed by watermelon flesh (10.0 \(\pm\) 1.0 mg/100 kcal) and watermelon seeds (1.4 \(\pm\) 0.2 mg/100 kcal). These previous studies reported that watermelon rind has higher L-citruline than watermelon flesh. The present study appears to have the opposite results showing the highest citruline content in watermelon flesh, followed by the skin and rind (Figure 1). However, the opposite results can be explained by the word 'rind' used in some previous studies [12; 18; 30; 38; 39], because 'rind' in these previous studies indicated the white rind and green skin, which are all byproduct parts other than flesh. In addition to the citruline concentration, total phenolic content and antioxidant activity of watermelon parts were investigated in the present study. Watermelon skin showed the highest total phenolic content, free-radical scavenging activity, and ferric reduction antioxidant potential followed by watermelon flesh and skin. These results were consistent with previously reported studies [42; 43]. Green rind (skin) showed the highest total phenolic content (0.7 mg GAE/g), followed by the flesh (0.3 mg GAE/g) and the white rind (0.2 mg GAE/g), with a similar pattern in DPPH radical scavenging activity [42]. The green rind (skin) extract treated at 250 \({}^{\circ}\)C showed the maximum ferric reducing power among the watermelon flesh, white rind, and green rind (skin) parts [42]. [PERSON] et al. [43] extracted watermelon rind using ultrasound-assisted extraction, and it contained 15.1 \(\pm\) 0.6 mg GAE/g, showing similar total phenolic content as the present study. Total phenolic content of red- and yellow-fleshed watermelon rind powders extracted using different solvents (water, methanol, ethanol, and acetone) reported values 1.1-2.2 mg GAE/g, which was lower than our result (rind 2.4 mg GAE/g) [44]. However, there were several studies not consistent with the present study [45; 46]. The white part of watermelon peels (rind) contained 63.3 \(\pm\) 1.5 mg TAE (tannic acid equivalent)/g, showing higher total phenolic content than the red flesh (47.3 \(\pm\) 0.9 mg TAE/g) [45]. In [PERSON] et al. [46], watermelon skin showed the highest total phenolic content (0.087 \(\pm\) 0.002 mg GAE/g) and DPPH (55.8 \(\pm\) 2.4 %), but watermelon rind was higher than the flesh in both total phenolic content and DPPH. It is important to investigate intestinal absorption of citruline because the bioavailability of citruline depends on intestinal absorption and different food matrices [24; 27]. There have been several studies on citruline uptake [27; 30]. [PERSON] et al. [27] investigated the mechanisms and kinetics of citruline uptake using a Caco-2 human intestinal epithelial cell model. They found that citruline uptake was pH-independent and that the uptake rate went down without Na\({}^{+}\). [PERSON] et al. [30] investigated the concentration of L-citruline in plasma over 24 h after watermelon flesh, rind, and seed intake. The maximum concentration showed between 1 and 2 h after watermelon flesh, rind, and seed intake, especially watermelon flesh and rind showing significant increase compared to the control (\(p<0.05\)). However, the transport of citruline from different watermelon parts in the intestine is poorly understood. In the present study, the bioavailability of citruline was slightly greater in the watermelon skin than in the watermelon flesh and rind (Figure 3). In addition, the bioavailability of L-citruline standard was similar or lower than the bioavailability of citruline in different watermelon parts. This difference may be because the citruline standard was not contained within the matrix of the watermelon. [PERSON] et al. [31] performed an in vitro intestinal absorption of L-citruline by using Caco-2 cells, with unpasteurized watermelon juice, pasteurized watermelon juice, and L-citruline standard.