• Nick Dent

5 things you need to know about Dioscorea (yams)

Learn why this amazing genus of plants deserves more attention.

Dioscorea are an important food for millions of people

Several species of Dioscorea can be eaten and they are commonly called yams. Dioscorea are tuberous plants, a bit like potatoes. This means that both yams and potatoes store nutrients underground in a ‘tuber’ to help them survive when the going gets tough. Humans around the world have found that these tubers can be quite delicious.

We have discovered over 600 species of Dioscorea so far, and we think about 50 of these are edible. There are around 8 species in particular which are of huge economic importance: 150 million people in West Africa alone depend on yams as a main source of sustenance. They are considered a staple food source, with over 70 million tonnes farmed world-wide every year, providing an excellent source of both vitamin C and potassium to those who eat them.

Women pounding yams in Nigeria © John & Penny Hubley.

Lots of things we call yams are not really yams

Yams belong to the genus Dioscorea, although many things we call yams are in fact not actually yams. Sweet Potato (Ipomoea batatas) is perhaps the most famous example. In many places the words yam and sweet potato have become interchangeable, even though they have not shared a common ancestor since dinosaurs roamed the earth. If you just looked at the tubers you might think the only difference was their colour, however if you look at their flowers you can see just how different the two plants are.

Dioscorea might save your life

Yams have been used in traditional medicine around the world for thousands of years. They have been used to treat conditions as wide ranging as diarrhoea to head lice. Fitting then, that they should be named after a physician. Carl Linnaeus, the father of modern taxonomy, named them after Pedanius Dioscorides, ancient Greece’s most revered expert on herbal medicine.

Dioscorea have also been used to make a variety of modern medicines. One chemical found in some Dioscorea tubers is diosgenin: This has been synthesised into progesterone for use in the contraceptive pill. Scientists at Kew have been studying one South African species, D.strydomiana, which local people believe has the ability to cure cancer. So far, these studies have been inconclusive but early analysis suggests that it may have high levels of anti-inflammatory drugs which could be used to treat conditions such as arthritis.

While much research has focused on the tubers, recent research into the often-neglected green parts of Dioscorea showed promising results. D.elephantipes was found to contain chemicals which can be turned into anti-viral medication and could provide doctors with a new renewable source.

One of the drugs that can be produced from Dioscorea sp. ‘Tamiflu’ antiviral drug, England, 2009. © Science Museum London

Dioscorea might kill you

Dioscorea are unusual for a staple food, in that if they are not prepared properly, they can be poisonous. Many young and wild plants can have nasty side effects if eaten, especially raw.

Soaking the tubers in water, or properly roasting them are just two methods to make them safe to eat. Because of these risks, many Dioscorea are only eaten in times of drought or at the times of year when other crops are unavailable.

Hunters in Malaysia are said to have utilized these toxic properties, and juice made from the plant is poured into rivers, poisoning all the fish and making them easy to catch.

Many species of Dioscorea are endangered

Dioscorea is considered an understudied genus, which means we could be losing economically important plants before we have had a chance to study them. Kew's Madagascar Conservation Centre has been encouraging people to grow their own yams to prevent over foraging of their wild relatives.

It is estimated that 20% of all plant species world-wide are threatened with extinction, a worryingly high proportion, but over 40% of the Dioscorea species studied were threatened with extinction, twice the world average.

Dioscorea’s vulnerability could have a huge impact on the food industry. The effects of climate change on our food supply may be hard to predict, and the natural diversity that exists in the wild could prove to be crucial to the future of yam production worldwide.

By Nick Dent

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