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The nightshade family (Solanaceae) includes several species of fruits which are better-known as vegetables to most people. Since they are typically eaten with a main entree or in a salad (and not as a dessert), they are usually called vegetables. Botanically speaking, these "vegetables" are technically botanical fruits because they are seed-bearing structures that develop from the ripened ovaries of flowers. The cultivated tomato (Lycopersicon esculentum) is native to South America, although archaeological evidence suggests that it was domesticated in Mexico and Central America. The Maya called the fruit tomatl, a name that was corrupted by the Spanish into tomate, and by the English into tomato. Because of its flower structure, the tomato is obviously closely related to the genus Solanum which includes the potato (S. tuberosum), eggplant (S. melongena), and a number of species called nightshades. The generic name Lycopersicon "wolf-peach" is derived from the Greek words lyco "wolf" and persicum "peach." It was placed in this genus by Philip Miller in 1768, although Linnaeus had placed it in the genus Solanum in 1753. The scientific and common names for the nightshade family (Solanaceae) are derived from the type genus Solanum. Recent studies using chloroplast DNA corroborate the genetic affinity between nightshades (Solanum) and tomatoes (Lycopersicon). Some species of nightshades contain several toxic alkaloids. In fact, the tomato was once considered to be inedible and poisonous in the United States until the early 1800s. The Old World ornamental Jerusalem cherry (S. pseudocapsicum) produces small tomatolike fruits that contain the poisonous alkaloid solanocapsine. Tomatoes are grown extensively in California's Central Valley for tomato juice, canned tomatoes, tomato paste, catsup, and for the various salsas used in popular Mexican dishes.

Andrew F. Smith discusses the reluctance to eat tomatoes in his book The Tomato in America: Early History, Culture, and Cookery, University of South Carolina Press, 1994: "While some Americans obviously did believe that tomatoes were poisonous, this phenomenon has been blown out of proportion by well-intentioned popular historians." In Chapter 3 he gives some of the reasons why Americans did not eat tomatoes in the 1800s, including taste, smell, appearance, and a general fear of vegetables. Other myths and unfounded beliefs about tomatoes are discussed by David Gentilcore in Pomodoro: A History of the Tomato in Italy (Arts and Traditions of the Table: Perspectives on Culinary History), Columbia University Press, 2010.

The nightshade family (Solanaceae) contains many species with poisonous alkaloids, including tobacco (Nicotiana tabacum), deadly nightshade (Atropa belladonna), mandrake (Mandragora officinarum) and the deadly datura species (Datura and Brugmansia). Some of the toxic alkaloids include nicotine, atropine, hyoscyamine, scopolamine and solanine. In addition, the chile peppers (Capsicum) contain the alkaloid capsaicin which is used to enhance the flavor of foods and in self defense sprays. Although the jimsonweed (Datura wrightii) contains several toxic alkaloids, its foliage provides the natural food supply for several species of large caterpillars called tomato hornworms (Manduca sexta and M. quinquemaculata). These same larvae also are fond of tomatoes, and can literally devour the leaves of tomato plants. The larvae undergo metamorphosis under the ground and change into magnificent night-flying hawkmoths the size of hummingbirds.

Vine-ripened tomatoes have a much better flavor and texture, compared with typical supermarket tomatoes that are picked green. Tomatoes that ripen on the vine are superior because of a natural hormone called ethylene that is produced by the plant. Ethylene is responsible for the rapid maturation of tomatoes, including the color change, softening, increased sugar content and flavor production. Because the ethylene ripening process occurs very rapidly, the shipping, storage and shelf life of vine-ripened tomatoes is greatly reduced. Consequently, the tomatoes are picked green during the handling process, and then later exposed to ethylene gas to hasten ripening; however, they still don't have the natural flavor and texture of vine-ripened tomatoes. The production of ethylene and the softening of the fruit wall requires the production of specific enzymes. For example, pectinase enzymes break down pectin in the middle lamella layer between cells causing the fruit wall tissue to decompose rapidly. By introducing synthetic genes into the tomato nuclear genome, enzyme production can be blocked. The synthetic genes produce complimentary RNA strands that bind to the normal RNA that codes for the fruit ripening genes. In other words, the normal RNA is blocked so that translation and enzyme synthesis at the ribosomes does not occur. Genetically engineered tomato plants have been produced with delayed fruit ripening and softening. The fruits can be picked at the final stages of ripening, but will not get soft and mushy. A later treatment with ethylene causes the final softening and ripening to resume, just in time for the grocer's shelves.

Most popular supermarket tomatoes are parthenocarpic, a term that applies to seedless fruits that develop without fertilization. Parthenocarpy can be induced artificially by the application of dilute hormone sprays (such as auxins) to the flowers. Auxins are growth hormones produced primarily in the apical meristems, buds, and young leaves of plants. The primary plant auxin is indoleacetic acid (IAA), which is synthesized from an amino acid. Auxins and other plant hormones are involved in numerous developmental aspects of plant growth, including floral initiation, sex determination, growth rate, fruit growth and ripening, abscission, rooting, aging, dormancy, and apical dominance.

Ripe tomatoes are particularly rich in the carotenoid lycopene (C₄₀H₅₆). In addition to imparting the bright red color to tomatoes, 40 milligrams of lycopene ingested daily may reduce the risk of atherosclerosis (clogged arteries) and heart disease. Lycopene may also reduce the risk of certain cancers, including cancers of the prostate, lung, bladder, cervix, skin and digestive tract. According to Dr. A.V. Rao (Experimental Biology and Medicine 227 (10): 908-913, 2002), lycopene is an antioxidant that prevents the oxidation of low density lipoproteins (LDLs). High LDL oxidation is associated with increased risk of atherosclerosis and coronary heart disease. According to the Merck Index (1983), one kilogram (2.2 pounds) of fresh tomatoes yields 20 milligrams of lycopene. To obtain 40 milligrams of lycopene, you would need to eat about eight medium-sized tomatoes (or four large tomatoes) every day. Apparently lycopene is absorbed more readily in the processed form, such as tomato paste and juices. Several on-line references recommended drinking two glasses of tomato juice every day. Perhaps it is also wise to eat plenty of tomato soup, stewed tomatoes and tomato sauce on pasta and pizzas.

According to Dr. Leticia Rao, Assistant Professor of Medicine at University of Toronto, 10 milligrams of lycopene daily may prevent bone deterioration known as osteoporosis (Bottom Line Health Vol. 17, September 2003). This amount of lycopene equates to a 12 ounce glass of tomato juice, one 12 ounce can of tomato sauce, or one large tomato.

An interesting tomato relative is called ground cherry or tomatillo (Physalis ixocarpa). As the fruit develops into a fleshy, tomatolike berry, the outer calyx of the flower also enlarges into an inflated, papery, bladderlike sheath that encloses the berry. The unripe fruits are used in salsa verde, a mildly hot chili sauce often served with tacos, enchiladas, tostadas and other Mexican dishes. They are also stewed, fried, baked, and used in dressings and soups. Ripe fruits can be eaten raw, in sandwiches and salads. There are several wild species of Physalis called ground cherries in the mountain and desert regions of southern California, including P. greenei and P. crassifolia. A Peruvian species (P. peruviana) is naturalized in the Hawaiian Islands. It is called "poha" and the berries are eaten fresh and used in jams and jellies.

Another interesting tomato relative is the eggplant (Solanum melongena) native to tropical Asia (India or southern China). Like zucchini gourds, eggplants are typically picked when immature so they usually don't contain mature seeds in the fruit pulp. Some of the cultivars include the large oval fruits of 'Black Beauty' or the elongate, slender fruits of 'Japanese.' Some of the smaller ornamental varieties actually resemble chicken eggs in size and color. Although the potato (S. tuberosum) belongs to the same genus as eggplant, it rarely produces fruits. The leaves, stems, fruits, and the sprouts and green skin of old potato tubers contain the poisonous, bitter alkaloids solanine and solanidine. Sliced eggplant fruits are commonly fried, baked, stewed and grilled. They are the key ingredient for many delicious entrees.

Another semi-woody species of Solanum from lowland tropical America is called nipplefruit (S. mammosum). This prickly shrub is grown as an ornamental because of its bright orange fruit with basal nipplelike projections. The fruits are considered toxic and are usually not eaten.

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