>What is a Nightshade.

Nightshade is a colloquial name and not a technical one and is used to refer to many different plants.

Most of the plants with the colloquial english name "nightshade" are in the family Solanacea, However there are also several important food crops in that family.

Nightshade traditionally refers to plants in the genus Atropa, such as Atropa belladonna. This is the source of the useful drugs atropine and scopolamine. Atropa is a member of the family Solanacea. However large amounts of atropine are deadly and cause cardiac and respiratory arrest. All parts of plants in the genus Atropa are considered deadly poisonous.

Nightshade also refers to many plants in the genus Solanum, also of the Solanacea family.

This genus also includes the potato (Solanum tuberosa), the tomato (Solanum x lycopersicon), eggplant (S. melongena.). There are several wild relatives of tomatoes, native to south and central america that have edible and tasty fruits, but aren't widely grown.

However other members of the family Solanum have fruits that are toxic. For example, S. dulcamera, called bittersweet nightshade.

The word nightshade can be construed to mean plants in Solanum genus which have no edible parts.

For example, all aboveground parts of potatoes become quite toxic and the toxicity is triggered by light exposure. This includes the fruits and seeds of the potato, and cooking does not destroy the toxins. Only the underground tubers can be eaten. If potatoes are exposed to light for several days they may turn green. This is typically accompanied by modest toxicity, thus green-skinned potatoes are unfit for eating.

Europeans were originally quite skeptical of potatoes and eggplants due to their similarly to other members of the genus Solanum, native to europe which were themselves, very toxic including their fruits. They only became widely adopted in the late 1700s and early 1800's. Because potatoes are a very productive and nutritious crop even in relatively poor soils and are very tolerant of adverse weather conditions, they may be considered one of the unsung heroes of the industrial revolution.

Likewise, only the fruit of the tomato plants is fit for eating and other parts are generally mildly toxic.

Other members of the family Solanacea grown as crops include chile peppers (Capsicum fructescens), Physalis fruits such as tomatillo and lantern-fruit, Lycium barbatum also called wolfberry or goji. It also includes one of the most toxic plants in the family outside of the genus Atropa, Nicotiana tabaccum. This produces the highly toxic alkaloid nicotine.