Citrulline is a non-proteinogenic amino acid (that is, citrulline is an amino acid that is not coded for in mRNA), and it is an important metabolic intermediate in the Urea Cycle. The Urea Cycle is critical for nitrogen (e.g. ammonia) excretion in humans as well as other mammals.
Citrulline is an interesting amino acid, given its vital function in the Urea Cycle, but also because it has such a high concentration in the watermelon. The name citrulline is actually derived from citrullus vulgaris, which is the Latin word for watermelon.
A notable fact about the watermelon is that the watermelon is the only food known to have such high content of citrulline. There are NO other foods that have such high endogenous levels of this amino acid.
As a physiologist citrulline is very interesting metabolically, but I wonder why the watermelon would have such a high concentration? Why would that occur? Would high citrulline content offer some type of competitive advantage to the watermelon in terms of out-competing other plants? As biologists, we usually think about things from a teleologic perspective in order to make sense of the world around us. With that perspective, is there a logical reason that citrulline would be so enriched in the watermelon? It seems very odd that it would be the only plant with notable quantities.