The Expression of Citrulline and other Members of the Arginine Metabolic Family in Developing Watermelon Fruit

L-citrulline is a naturally occurring amino acid that functions in detoxification of catabolic ammonia (urea production) and is a key element in production of the vasodialator, nitric oxide. Because watermelon flesh and rind possess large quantities of citrulline (1-3 mg/g fresh tissue), watermelon is viewed as a potential natural source for citrulline. Objectives were to measure expression levels of citrulline and other members of the arginine metabolic family in developing watermelon fruit. Physiological amino acids were extracted and quantified from two cultivars of watermelon produced over three growing seasons. Glutamine was the predominant amino acid in flesh and rind the first 14 days of development, but it rapidly fell in concentration while citrulline rapidly rose during the same interval. At 21-28 days postanthesis, citrulline reached its maximum concentration of 100-300 mmoles/kg dry weight and remained at that level. Variability of citrulline levels among individual fruit of a cultivar age group was much greater in the rind than in the flesh. Citrulline levels were markedly influenced by growing conditions.