We should be suspicious of news about watermelons timed to coincide with the Fourth of July, when watermelons are traditionally consumed in the US. If you'd made an earth-shattering discovery about watermelons, chances are you wouldn't save it.

This story, however, is not entirely without merit: according to Dr Bhimu Patil, director of Texas A&M University's Fruit and Vegetable Improvement Center (how do you improve a watermelon? Put a hat on it?), watermelon contains a phyto-nutrient called citrulline.

Citrulline is converted by the body into another amino acid, arginine. Arginine, in turn, boosts nitric oxide levels, "which relaxes blood vessels, the same basic effect that Viagra has," says Dr Patil, "to treat erectile dysfunction and maybe even prevent it." This is why, in the spirit of scientific inquiry, I find myself at the supermarket at midday - not in an attempt to treat erectile dysfunction, mind you, but perhaps to prevent it. Oddly, there is only one watermelon left. Has word got out already?

Citrulline has long been sold as a dietary supplement, a sort of general cardiovascular fitness tonic. And watermelons are full of the stuff: it was first isolated from watermelon in the 1930s and takes its name from the Latin for watermelon, citrullus lanatus.

There is more citrulline in the rind than the flesh, but you've got to eat the flesh to get to the rind. I am discovering, as I write this, that while a slice of watermelon is a summertime treat, eating a whole one, even a small one, is a bit of a chore. I wish I could tell you that my virility levels have, er, rocketed as a result, but given that the watermelon's main constituent - 92% by weight - is water, the experiment is having an altogether more predictable effect on my plumbing. I'll have to leave it there.