It's common wisdom that if you're going to try a spicy Latin salsa, you'd better have a tall glass of water (or better, milk) ready and waiting. The hot chilies that grow in Central and South America are quite good at causing physical pain to the animals that try to eat them, and their pungency keeps them safe from pathogens as well. But all that heat comes at a cost, researchers have now found. Just like you, chilies need easy access to a lot of water in order to take the heat.
For many years, ecologist Joshua Tewksbury of the University of Washington, Seattle, has been traveling to Latin America to study chilies, and one of his favorites is the spicy Capsicum chacoense, which grows in Bolivia. This chili—and many like it—is plagued by a fungus called Fusarium, which enters the peppers once bugs have poked holes in them and eats away the seeds. Peppers can fight the fungus by producing a chemical called capsaicin, the chemical that gives chilies their characteristic, painful heat. But not all peppers contain capsaicin. A single plant may produce some peppers with the chemical and some without it growing right next to one another. This means that the hot pepper is protected from the fungus while the cooler pepper is not. "If it's good to be hot, why isn't everything hot?" Tewksbury wondered.
To find out, he and then-graduate student David Haak, now an evolutionary biologist at Indiana University, Bloomington, set out for Bolivia on a wild chili pepper hunt. They collected 330 C. chacoense plants from across a 300-kilometer-long area, which ranges from desert-like conditions in the northeast to very moist conditions in the southwest. The researchers found that only about 15% to 20% of the chili peppers produced in the dry conditions are hot, compared with 100% of the peppers from plants living in wet conditions.
Reasoning that water availability might play a role in spiciness, the researchers brought the plants back to a greenhouse in Seattle and counted how many stomata, or microscopic pores, each plant had per square millimeter of leaf area. As a plant breathes, the stomata open to release water as vapor, so plants with fewer stomata should be better at retaining water. When the researchers measured how pungent each of the pepper plants was, they found that plants which produce only hot chilies tended to have two or three more stomata per square millimeter than the others, suggesting that they would do poorly under desert conditions.
The team then restricted the amount of water that the plants received to reproduce desert-like conditions. The hot peppers produced 50% fewer seeds than their cooler neighbors under these dry conditions, the group reports today in the Proceedings of the Royal Society B. So when faced with a choice, plants make only hot chilies when they don't have to worry about a drought. Otherwise, they take their chances with the fungus. "There is no free lunch," Tewksbury says.
The researchers aren't sure how water tolerance affects capsaicin production, but they are now trying to nail down the genetic mechanisms. Knowing that water is involved, however, "starts to put the other bookend on the story" of why not all chillies are hot, says Tewksbury.
"I think it's an elegant demonstration of a critically important phenomenon: linking of a specific trait to a specific adaptive advantage," says Molly Jahn, a geneticist at the University of Wisconsin, Madison. And she says it's important for plant breeders like herself to keep in mind that the traits they select for in plants—such as heat in peppers—might have disadvantages under certain conditions.