Watermelon is the number two fresh vegetable crop in the world in terms of area harvested and total production (FAOSTAT, 2009). Recently, it has been identified as a healthy food because it is high in the carotenoid, lycopene. Although watermelon is the leading fruit and vegetable source of lycopene, it also contains other antioxidants and amino acids that have health-promoting activities. Fruits and vegetables contain many compounds that work synergistically. Boileau et al. (2003) indicated that lycopene alone is not responsible for reducing prostate cancer associated with increased tomato intake. Because tomatoes contain the glycoalkaloid tomatine, a demonstrated anticarcinogen, as well as several well-known phenolic compounds such as quercitin, this finding is not unexpected and demonstrates the importance of maintaining the other nutritional compounds in fruits and vegetables.
Amino acids have well-established individual roles in disease prevention. Arginine, an essential amino acid, functions as one of the 20 building blocks of proteins and in free form as a physiological amino acid. L-citrulline is a physiological amino acid endogenous to most living systems. These amino acids are directly involved in clearing excess metabolic ammonia from the human body and are indirectly involved in cardiovascular function, immunostimulation, and protein metabolism (Curtis et al., 2005). Ingested arginine is cleared by hepatic cells, but L-citrulline is not and can serve as an arginine source in other parts of the body.
Watermelon is rich in citrulline (Tedesco et al., 1984) but differences among cultivars have not been adequately studied nor have effects of production environments. Fish and Bruton (2010) reported that one cultivar grown at two locations had little variation in flesh concentration of L-citrulline. Tarazona-Díaz et al. (2011) reported values on five lines (four of them triploid seedless cultivars) grown at one location with a mean citrulline concentration in watermelon flesh of 2.33 mg·g−1; they also demonstrated that the seeded cultivar had the least L-citrulline concentration in flesh tissue. In an earlier report, 14 watermelon cultivars had 0.5 to 3.6 mg·g−1 fresh weight of citrulline with an average concentration of 2.4 mg·g−1 (Rimando and Perkins-Veazie, 2005). Those authors reported less L-citrulline in red-fleshed fruit than in yellow or orange, but only a small sample size (three fruit for each variety) was used, and because the fruit tested were grown at multiple locations, it is difficult to assess the genotype and environmental effect on L-citrulline concentration in those fruits. Liu et al. (2010) reported that greenhouse-grown watermelon fruit of nine induced autotriploid hybrids had higher L-citrulline values than their diploid and induced autotetraploid parents. Fish and Bruton (2010) and Liu et al. (2010) reported that the concentration of L-citrulline was highest at peak ripeness.
Humans can effectively absorb L-citrulline from watermelon, which also increases plasma arginine levels (Collins et al., 2007; Mandel et al., 2005). Recently, subjects consuming watermelon or synthetic citrulline as a drink, combined with exercise, had reduced arterial blood pressure compared with a placebo (Figueroa et al., 2011). The heightened importance of watermelon as a source of bioactive compounds such as L-citrulline highlights the need for a better understanding of the genetic control of this amino acid and other phytonutrients in watermelon. The current study expands on a preliminary trial by Davis et al. (2010) and was designed to measure effects of genotype and environment on L-citrulline accumulation in watermelon. A diverse collection of watermelon germplasm was screened to identify cultigens having a high concentration of L-citrulline that was stable over environments.
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