The key to growing the hottest chillies may lie in the water content of the soil, new research suggests. Wild chillies develop their trademark pungency, or ‘heat’, as a defence mechanism against a type of fungus that destroys their seeds. But that doesn’t explain why hot, otherwise known as pungent, chilli plants can grow alongside mild (non-pungent) chilli plants. New research suggests that an adaptive trade-off is in play – producing better heat versus reproducing more effectively.
“We found that under well-watered conditions, seed production in both hot and non-pungent chillies was equivalent, but under water-stressed conditions, non-pungent plants produced twice the number of seeds. This suggests that the greater presence of non-pungent plants that produce substantially more seeds is the result of a fitness-based tradeoff,” said David Haak from Indiana University in the U.S., lead author of the paper published in Proceedings of the Royal Society B today.
Biologists have learned in recent years that hot chillies are very effective at preventing the growth of the fungal seed pathogen Fusarium; and that pungency, available moisture and fungal pressure are all intertwined. An increase in available moisture in the soil is associated with an increased presence of the fungus, and subsequently an increase in the pungency of chilli plants.
Now, researchers have explained why not all chillies grown in the same environment are hot, revealing clues into the evolution of this defensive trait. Hot chillies were found to produce the same amount of seeds as mild chillies under wet conditions, but in water-stressed conditions, hot chillies are less efficient at holding water than mild chillies, which were able to produce significantly more seeds due to their ability to use available water resources more efficiently.
“We found that an important trait determining water-use efficiency was stomatal density (the number of water-losing pores in the leaf), which is higher in pungent plants than non-pungent plants. This connection is genetic – we found that plants which carry the pungency trait always have a higher stomatal density,” said Haak. “Thus, we have found that an adaptive tradeoff (producing heat versus reproducing) limits the benefit of pungency in dry environments.”