The researchers said the active ingredient in watermelon which is responsible for the effect is citrulline. The phytochemical can be converted in the body into an amino acid known as arginine, which is known to relax blood vessels much like viagra does.

"The more we study watermelons, the more we realize just how amazing a fruit it is in providing natural enhancers to the human body," said Dr. Bhimu Patil, director of Texas A&M's Fruit and Vegetable Improvement Center in College Station.

They said a typical 4-ounce serving of watermelon (about 10 watermelon balls) has about 150 milligrams of citrulline, though without mentioning how much citrulline is needed to have Viagra-like effects. There is more citrulline in the rind than the flesh.

Another beneficial ingredient in watermelon is lycopene, an anti-oxidant that protects the human heart, prostate and skin health. But without much fat in the melon, adsorption is not expected to be good, the researchers noted.