The plants cultivated for our popular caffeinated products- including Coffee, Tea, Cacao, Maté, Kola and Guarana- tend to produce fairly high levels of caffeine, but many others that you might not expect also produce it. It can be found in flowers of lemon and other citrus, for instance (Duke, 1992). Why do so many plants produce caffeine, especially in important tissues like flowers and fruit?
Some plants offer attractive chemicals in flowers or fruit as a reward to animals that, for instance, disperse their seeds. Researchers in Zurich have found this is not likely to be the primary use a plant has for caffeine, at least for Guarana. The outer part of the guarana fruit, which is eaten by large birds such as toucans, contains lots of sugar but no measurable caffeine. Experiments simulating the acid conditions of a bird’s digestive tract suggest very little caffeine is leeched from the seed before it leaves the bird’s body (Baumann et al, 1995; Goncalves, 1971).
Caffeine is widely listed as a secondary compound in plant chemical profiles, meaning a harmful or unpalatable chemical that discourages grazers. That could account for the caffeine in citrus flowers, too. If they’re not chemically defended, flowers could be eaten by herbivores before they have a chance to get pollinated and produce seeds. On the other hand, flowers also need to attract the attention of pollinators. Citrus pollen and nectar, both of which are harvested and consumed by pollinators, contain significant amounts of caffeine (Kretschmar and Baumann, 1999). The caffeine in the flowers may benefit the plant by discouraging herbivores, but it is equally plausible that it attracts pollinators- assuming they like the buzz.