Qs people asked me on science and my replies to them - part 192

Q: Why are chillies so hot? Is there any evolutionary benefit for this?

Krishna: The hotness in chilli peppers has always been something of an evolutionary mystery.

A plant creates fruit in order to entice animals to eat and disperse its seeds, so it doesn't make sense for that fruit to be painfully hot. That would be our first reaction when we think about it.

But wait. Research reveals another secret.

Chillies have a very good reason to make themselves hot. It boils down to protection.

Based on research on wild chilli plants in rural Bolivia, the scientists found that the leading cause of seed mortality is a fungus called Fusarium. The fungus invades the fruits through wounds made by insects and destroys the seeds before they can be eaten and dispersed.

Capsaicin, the chemical that makes the peppers hot, drastically slows microbial growth and protects the fruit from Fusarium. And while capsaicin deters local mammals, such as foxes and raccoons, from consuming the chillies, birds don't have the physiological machinery to detect the spicy chemical and continue to eat the peppers and disperse seeds (2).

Researchers were able to arrive at these conclusions because at least three of the approximately 15 species of chillies that grow in the Bolivian wild are polymorphic for pungency, which means that some individuals of those species produce pungent fruit and others produce non-pungent fruit. This provided the researchers with natural experimental conditions under which they could compare Fusarium attack on fruits with and without capsaicin.

Upon studying various chilli pepper plants, the researchers observed a clear correlation between high levels of capsaicin and low seed mortality due to fungal growth.

And the chemical doesn't just help the plants that produce it, either. The consumption of chillies can help protect humans from the dangerous diseases that are so plentiful in tropical climates. 

Chillies are used as spices in several parts of the world. 

The capsaicin in chillies, one of the first plants domesticated in the New World, may have been used to protect human food from microbial attack long before refrigeration or artificial preservatives were available. 

However,  the production of the chemical comes at a steep price for chilli plants. 

The plants that produced hot chillies has seeds with very thin coats - a presumed consequence of sacrificing the production of lignin, a complex molecule that makes up the protective seed coat, in favour of the production of capsaicin.

From the chilli plant’s metabolic perspective, capsaicin wracks up a very high defense budget. The molecule is relatively large and contains lots of precious nitrogen, which is critical for building proteins and DNA. Furthermore, as a byproduct of making capsaicin, the leaves of spicy plants have more stomata. Stomata are “holes” on plant leaves, guarded by a pair of special cells that monitor how much gas passes through the leaf, and having many stomata means that a plant loses more water when it transpires, a necessary step for performing photosynthesis and making sugars. As a result, when the occasional drought occurs, spicy plants don’t perform as well as non-spicy plants [5]. Non-spicy plants have an advantage over spicy plants during drought, producing more seeds, and thus more progeny, than their spicy brethren. When plants receive enough water, the advantage disappears and the spicy and non-spicy chillies make an equal number of seeds again.

For wild chililes, the dueling selective pressures of fungal pathogens and drought result in a polymorphism, a case where some chillies are spicy and some are not at all. 

This phenomenon represents an interesting tradeoff between chemical and physical seed protection and demonstrates the power of natural selection.

At higher elevations, where moisture is high and Fusarium fungus is rampant, the scientists found that 100 percent of the plants produced hot chilies. In the drier lowlands, where fungus is less of a problem, only 40 percent of the plants produced fiery fruits. The remainder spent more resources developing thick seed coats, which protect against the devastating ant populations common to lower areas.

Sources: 

1) Perry, Linda et al. “Starch Fossils and the Domestication and Dispersal of Chili Peppers (Capsicum spp. L.) in the Americas.” Science 315.986 (2007)

2) Tewksbury, Joshua & Gary Nabhan. “Directed Deterrence by Capsaicin in Chilies.” Nature 412 (2001): 403-404.

3) Story, M. Gina & Cruz-Orengo Lillian. “Feel the Burn.” American Scientist 95 (2007): 326-33.

4) Tewksbury, Joshua et al. “Evolutionary Ecology of Pungency in Wild Chilies.” PNAS 105.33 (2008): 11808-11811.

5) Haak, David et al. “Why are not all chilies hot? A trade-off limits pungency.”    Proc. R. Soc B. 10.1098 (2011) 1-6.

6)  http://sitn.hms.harvard.edu/flash/2012/issue131b/

Related Qs....

What’s inside pepper spray?

The active compounds in pepper spray are collectively known as capsaicinoids. They are given the military symbol OC, for “oleoresin capsicum”.

The most important chemical in OC is capsaicin. This is derived from chilli peppers in a chemical process that dissolves and concentrates it into a liquid. Capsaicin is the same compound that makes chillies hot, but in an intense, weaponised form.

Not all capsaicinoids are obtained naturally. One called nonivamide (also known as PAVA or pelargonic acid vanillylamide) is mostly made by humans. PAVA is an intense irritant used in artificial pepper spray.

Is pepper spray a tear gas?

We’ve established pepper spray is a chemical, but is it also a kind of tear gas?

“Tear gas” is an informal term and a bit of a misnomer, because it isn’t a gas. Rather, tear gas refers to any weaponised irritant used to immobilise people.

More specifically, tear gas is often used to describe weapons that disperse their irritants in the air either as liquid aerosol droplets (such as gas canisters), or as a powder (such as pepper balls). This definition distinguishes tear gas from personal self-defence sprays which use foams, gels and liquids.

Tear gas canisters typically contain the irritants 2-chlorobenzalmalononitrile (CS) and phenacyl chloride (CN). Both CS and CN are man-made chemicals discovered in a lab, unlike capsaicin (the traditional ingredient in pepper spray).

But despite capsaicin coming from chilli peppers, pepper spray is still a weaponised irritant that can be delivered as an aerosol or powder. It should unequivocally be considered a type of tear gas.

Pepper spray as a weapon

The chemical irritants OC, CS and CN have military symbols because they are chemical weapons. They are termed “less-lethal” because they are less likely to kill than conventional weapons. Their use, however, can still cause fatalities.

Technically, pepper spray and other tear gases are classified as lachrymatory agents. Lachrymatory agents attack mucous membranes in the eyes and respiratory system.

Pepper spray works almost instantly, forcing the eyes to close and flood with tears. Coupled with coughing fits and difficulty breathing, this means the targeted person is effectively blinded and incapacitated. Because lachrymatory agents work on nerve receptors that help us sense heat, they also induce an intense burning sensation.

The combined effects of pepper spray can last anywhere from 15 minutes to more than an hour.

Lachrymatory agents emerged on the battlefields of World War I. Artillery shells were filled with chemicals such as xylyl bromide and chloroacetone and fired at enemy soldiers. Agents that induce choking, blistering and vomiting were added as the chemical arms race escalated.

In the 1920s, the Geneva Protocol was enacted to ban the use of indiscriminate and often ineffective chemical weapons on the battlefield. Today, the unjustified use of chemical riot control agents threatens to erode the systems that are meant to protect us from the most dangerous weaponised chemicals.

Source: https://theconversation.com/what-makes-pepper-spray-so-intense-and-...