By Liam Elliott
‘My heart aches, and a drowsy numbness pains
My sense, as though of hemlock I had drunk’
The word poisoning conjures up, to many imaginations, images of deadly dinner parties straight from an Agatha Christie novel or a world of cold war espionage. Whilst these depictions are perfectly justifiable, they often relied on the variety of deadly inorganic or synthetic poisons. Look further back into history, however, and there emerges the use of naturally occurring plant poisons, entwined with some of the most classical and romantic of legends. Scientifically, toxic compounds that may be found in plants often originate as secondary metabolites of which over 100,000 are known.
Secondary metabolites are, by definition, generally not considered to be essential for plant life and are derived from the smaller pool of primary metabolites. Some of these compounds we use every day including caffeine and theobromine whilst others, hopefully in less frequent usage, include cocaine and morphine. Some plants produce highly toxic secondary metabolites however and the historical use of some of these to silence an unwanted voice, or as forms of execution, is well documented
Let’s have a look at some of the most notorious poisonous plants and their history.
Atropa belladonna: Deadly Nightshade
Perhaps one of the best known poisonous plants and commonly known as belladonna (literally: beautiful woman), this plant produces a variety of poisonous alkaloids including atropine and hyoscine. The plant is a member of the Solanaceae family which also includes potatoes. Belladonna’s attractive berries are very poisonous and it is therefore somewhat ironic that the plant has a long history of medicinal and cosmetic use. Macbeth of Scotland, immortalized by Shakespeare, is said to have used the plant to poison an invading English army.
Also known as wolf’s bane and devil’s helmet, plants of this genus synthesise toxic aconitine via the terpenoid synthesis pathway. Aconitine is a neurotoxin which targets sodium channels in mammalian neurons. The striking flowers of these plants resemble the hooded clothing of monks and whilst their natural distribution is largely restricted to mountainous regions of the northern hemisphere they are reasonably common features in gardens. Nazi Germany is known to have used bullets coated in aconitum during WWII.
A legume which produces the protein toxin abrin. This is similar to the infamous poison ricin, only around upwards of 70 times as toxic making it perhaps the most potent of plant poisons. The abrin produced is mainly confined to the seeds and the ingestion of a single one may be fatal to an adult human (whilst around 7 ingested berries of the legendary belladonna provide a fatal dose). Traditionally used to make jewelry in areas of South America, aphrodisiacs have also been historically produced from the plant.
Conium maculatum: Poison Hemlock
Native to the Mediterranean, the alkaloids produced by hemlock target neuromuscular junctions and can cause eventual respiratory paralysis and an unpleasant death. The famous Greek philosopher Socrates was sentenced to be executed in 399BC by drinking an infusion of hemlock. It has also been suggested that, in contradiction to traditional beliefs, that the final pharaoh of Egypt, Cleopatra, killed herself by drinking a hemlock-based poison. On the Greek island of Kea, where euthanasia was a societal norm in ancient times, the elderly are said to have drank hemlock infusion once they passed a certain age.
Plant toxins, and their often medicinal potential, give an example of the key place of plant sciences within society. Moreover, a basic level of plant biochemistry and history can give a fascinating insight into the way plants have shaped humanity.