Why the hottest chilies grow in the wettest placesS

What makes some chili peppers burn while others are mild? Biologists already knew that hot peppers evolved their fiery flavor to drive away a seed-eating fungus. So why aren't the mild chilies prey to this fungus? A new study suggests that hot versus mild depends on the amount of water where the chilies grow.

The Fusarium fungus, which targets chili pepper seeds, thrives in wet areas. Chilies that grow in drier climes, on the other hand, don't have to worry about their seeds being eaten. To overcome this problem, some chilies in wet environments evolved to have more seeds, increasing their chance of reproducing even if some of their seeds were destroyed. Others kept their seed counts, but evolved to drive away the fungus. They increased their reserves of capsaicin, the chemical that makes peppers spicy, to irritate the fungus.

Biologists reached this conclusion after studying the chili plants of 12 different areas in Bolivia, in regions that ranged from very dry to very wet. At each site, the researchers compared the proportion of mild plants to spicy ones. In the driest region, about 15 to 20 percent of the peppers were hot, and as the plants moved along the line from dry to wet, this percentage increased until it reached 100 in the wettest region, where all the plants were spicy.

If heat is such an effective shield against Fusarium fungus, then why didn't the dry-environment chilies evolve this defense as well? After all, Fusarium may be rarer in dry environments, but it's not non-existent.

The researchers set out to answer this question by collecting seed samples from both hot and mild plants, and then grew them in new conditions. Some of the plants received enough water to thrive, while others were maintained in a dry, stressful environment. The spicy plants did not have the reserves to adapt to their dry situation, while the mild plants still managed to produce extra seeds, even on limited water rations. This suggests that being spicy takes more energy - and and that, in turn, requires more water. Maintaining the heat reduces the peppers' ability to adapt to tough conditions, which puts them at a reproductive disadvantage even as it protects them from the fungal threat.

As the researchers wrote:

"In high moisture environments, pungency is beneficial because capsaicinoids protect fruit from pathogenic fungi, and is not costly because pungent and non-pungent chilies grown in well watered conditions produce equal number of seeds. In low moisture environments, pungency is less beneficial, as risk of fungal infection is lower, and carries a significant cost because under drought stress seed production in pungent chilies is reduced 50% relative to non-pungent plants grown in identical conditions."

So if you want your peppers spicy, cull them from the dampest places.

Via Proceedings of the Royal Society B.

Photo by Pablo H Caridad via Shutterstock