Nicotine is the addictive chemical in tobacco products. Plants may use nicotine primarily to ward off predators. Tobacco plants contain high levels of nicotine, but other members of the nightshade family (Solanaceae), which includes vegetables, also produce low levels of nicotine. Nicotine production is not confined to the nightshade family, however. The measured amounts of nicotine vary with the growth stage of the plant, the plant part that is used for extraction and the method of extraction.
Nicotine Content of Tobacco Plants
Tobacco plants belong to the genus Nicotiana and encompass a large number of annual, biennial and perennial species. The commercial tobacco plant (Nicotiana tabacum) grows best in U.S. Department of Agriculture plant hardiness zones 8 through 11. The tobacco plant synthesizes nicotine in its roots and then stores it in its leaves. Nicotine constitutes 0.3 to 0.5 percent of the tobacco plant's dry weight. Tobacco plants belong to the nightshade family of plants, and all tobacco species, including the ornamental ones, contain some nicotine.
Nicotine Content in Vegetable and Fruit Plants
Vegetable plants, belonging to the nightshade family, contain low amounts of nicotine. These plants include potatoes (Solanum tuberosum), tomatoes (Lycopersicon esculentum) and eggplant (Solanum melongena). Eggplant contains one of the highest concentrations of nicotine: 100 nanograms, or 0.1 micrograms, of nicotine per gram of eggplant. Most of the nicotine in potatoes is concentrated in the flesh and not the potato skin. Green tomatoes contain about 10 times more nicotine than ripe tomatoes. Celery (Apium graveolens) and cauliflower (Brassica oleracea var. Botrytis) are vegetables outside the nightshade family that also contain low amounts of nicotine. The papaya (Carica papaya, hardy in USDA zones 9B through 11) plant, another non-nightshade plant, contains nicotine in its plant parts but not its fruits.
Nicotine in Herbs and Weeds
Deadly nightshade (Atropa belladonna), a perennial weed growing in USDA zones 5 through 9, contains nicotine, but its toxic effects are mostly due to its other plant alkaloids, such as atropine. Other weeds, such as milkweed (Asclepias syriaca, hardy in USDA zones 3 through 9) and jimsonweed (Datura stramonium, hardy in USDA zones 3 through 9) also contain low amounts of nicotine. Milkweed contains nicotine only in its sprout seedling, while nicotine is present in all parts of jimsonweed. Field horsetail (Equisetum arvense, hardy in USDA zones 3 through 8) is an herb that contains the alkaloids nicotine and palustrine. Although field horsetail contains a relatively high amount of nicotine, the amount is dwarfed by the nicotine content of the tobacco plant. Field horsetail contains 0.4 parts per million, while tobacco leaves contain 20,000 to 40,000 parts per million of nicotine.
Nature's Use of Nicotine
Plants primarily use nicotine to ward of predators. In studies at the Max Planck Institute in Germany, published in "PLOS Biology" in 2004, when coyote tobacco (Nicotiana attenuata) plants were engineered to lack nicotine, the plants were more readily devoured by insects than tobacco plants that contained nicotine. Early insecticides often used nicotine as one of their constituents. Interestingly, some creatures that munch on nicotine-containing plants also use nicotine to repel their own predators. Hornworms that feed on nicotine-free plants are more likely to be eaten by wolf spiders than hornworms that feed on nicotine-containing tobacco plants. Hornworms on a nicotine plant diet will exhale nicotine from their midgut, repelling the wolf spiders.