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. High-pungency chilis pay a price -- a \"tradeoff\" in evolutionary biology -- for being able to ward off the fungus. They produce half as many seeds as low-pungency chillies, which produce more seeds to boost chances of reproductive success in dry conditions. The study looked at a wild strain of chilli, Capsicum chacoense, which is native to South America and has never been cultivated.