PARIS (AFP) - Scientists on Wednesday said they had solved a puzzle over why some wild chilli plants yield red-hot fruit but others have fruit which is mild. The answer lies in exposure to water, they reported in the British journal Proceedings of the Royal Society B: Biological Sciences. The fiery ingredient in chillies is capsaicinoid, which the fruit exudes to protect itself against a nasty damp-loving fungus called Fusarium. Researchers led by David Haak of Indiana University went to Bolivia to check capsaicinoid levels in chillies growing along a 300-kilometre (185-mile) line. In the dryer northeast part of the section, only 15 to 20 percent of the plants had pungent fruit. The pungency level increased along the line to the wetter southwest, where eventually 100 percent of plants produced high-capsaicinoid fruit.