In Inquiry into Biology, the opening story for chapter 4 describes why biting into a chili pepper produces a burning sensation in your mouth. This is because chili peppers contain a compound called capsaicin that activates the pain receptors in your mouth, which is interpreted by the brain as the “burn.” And as every chili-head can tell you, not all peppers are equal. While many may think that the chili peppers are producing capsaicin for our benefit, there is a much more interesting story behind this compound.
Chili peppers, and indeed many plants, are in a constant state of war with the fungi. Being nonphotosynthetic, fungi have to get their nutrition from somewhere, and although many fungi are saprophytes (decomposers), there exist a significant number of species that are parasites and pathogens of plants. The sworn enemy of the chili pepper is the fungus (spores are shown below), a very common fungus that is widely found in soils.
To combat threat, chilis produce capsaicin, which inhibits the ability of the fungus to infect the plant.However the production of capsaicin comes at a fitness cost to the chilis. Fitness in biology refers to the ability to produce viable offspring for the next generation, and it is known that plants which produce large amounts of the compound are not able to produce as many offspring. Why? Because the production of capsaicin reduces the ability of the plant to manage its water resources. And with a reduced water-efficiency comes fewer offspring. But this is where it gets interesting. In general,prefers moist environments (although it can live in hot, arid regions as well). So, it would make sense that chilis that are found in dry environments should produce less capsaicin, and more offspring, than their more arid,-threatened relatives.To test this hypothesis, researchers at the University of Washington first sampled chili plants along a 185 line in Bolivia that naturally varied in water availability. What they found was that even in the dry environments there were chilis with high amounts of capsaicin, but they only accounted for about 20% of the population. However, as the environment became wetter, the percent of high-capsaicin producing plants increased, and at the extreme end of the spectrum, in the moistest environments, all of the plants were capsaicin producers.To test the relationship between water-efficiency and number of offspring, the researchers designed a controlled experiment in which identical populations of chili plants were grown under similar experiments and then exposed to either a normal, or water-stressed, environment. The results confirmed that plants with higher amounts of capsaicin produced fewer offspring. Although the direct link between capsaicin and water-efficiency has not yet been determined, it is evident that production of the compound comes at a fitness cost to the plant.So the next time you bite into a really hot chili, consider that fact that what you are really experiencing is an ancient arms race between a; pepper and a fungi – and that the chili you are eating sacrificed its fitness for your spiceness.
- D. C. Haak, L. A. McGinnis, D. J. Levey, J. J. Tewksbury.Why are not all chilies hot? A trade-off limits pungency. Proceedings of the Royal Society B: Biological Sciences, 2011; DO