Caffeine is considered a secondary metabolite. As opposed to primary metabolites, secondary metabolites are not essential for plant growth and development. Rather, they play some useful role, just not a critical one. Caffeine is found in all parts of coffee, from the roots to the seeds and even in the xylem, the upward-elevator organ in plants. A number of hypotheses have been posited for what caffeine can do for the coffee plant. It could be an allelopathic agent, an anti-herbivory agent, a form of nitrogen storage, and/or a pollinator stimulant.
Allelopathy is plant chemical warfare against other plants. Some plants produce chemicals that can harm or kill seeds or plants, typically of other species. These compounds, spread by the decomposition of leaf litter or exudation by roots and seeds, influence the population dynamics of plants within a community; not all allelochemicals kill all plants. Many researchers have demonstrated that caffeine is toxic to a number of different plants. However, nobody has demonstrated caffeine’s efficacy in a natural setting. Thus, just because it can kill some other species, there is no guarantee that it would kill competitor plants in the forests of Ethiopia (where it evolved).
Caffeine is incredibly toxic to some insects and fungi (humans, too, in a high enough concentration). So, it often argued that it is a defense mechanism from critters. This hypothesis is supported by the fact that caffeine is produced in young, developing organs that are more susceptible to insect attack. This is a logical hypothesis but it is incredibly difficult to prove. To prove it inconclusively would require two nearly identical coffee plants, with the only difference being that one produces caffeine while the other one does not. Unfortunately, we are technologically incapable of producing these conditions, so the experiment will have to wait awhile. If caffeine did evolve to protect against insects, it was probably targeted against specific African insects. If it had been successful in defending against them, then they are probably so inconsequential as pests that they haven’t ever caught the attention of researchers.
Since caffeine has been found moving up through a plant and it contains four nitrogen atoms, it is thought that it may simply be a way to store nitrogen until needed for a specific purpose. What little research has been done on this hasn’t successfully demonstrated this function.
Lastly, caffeine may be an incentivizing treat for pollinators, particularly honeybees.
Research has shown that honeybees’ long-term memory is improved after having caffeine.
Presumably, this would help the bees remember the flower they were enjoying and be more likely to return to it in the future, thus helping the plants to cross-pollinate. While this is promising research, it has yet to be tested outside the laboratory. In addition, it wouldn’t explain why caffeine is synthesized in all the organs in the plant.
We will probably never know why coffee first developed caffeine. If we’re lucky, we’ll find out why it has continued to do so. Of course, from the coffee’s perspective, caffeine production has been a huge success. After all, because of that molecule, the human species has spread the seeds of the plant to nearly every place on the planet in which they could thrive!
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