Dioscorea strydomiana (Strydom's yam)
Adult plant of Dioscorea strydomiana, showing the tuber and shoot habit, and Linda Loffler, a member of the 2008 conservation survey team also seen (Photo: John Burrows).
Dioscorea strydomiana Wilkin
Rated as Critically Endangered (CR) according to IUCN Red List criteria.
Open woodland with a grass-rich understorey on steep, rocky, south-east to south-south-east facing slopes on soils over dolerite (sub-volcanic rock) with quartzite intrusions.
About this species
One of Kew's most striking recent discoveries is Dioscorea strydomiana - a critically endangered yam from South Africa. Only two populations totaling about 200 plants are known in the wild. This species is believed to provide a cure for cancer in the region where it grows, and is consequently under threat from over-exploitation by medicinal plant collectors, who remove parts of the tubers. D. strydomiana was named by Kew botanist Paul Wilkin in honour of the late Gerhard Strydom, who, with Johan Hurter, played a significant role in the discovery of this species when he worked for the Mpumalanga Parks Board.
Geography and distribution
Dioscorea strydomiana is restricted to Mpumalanga in South Africa, where it has been recorded at 1,100 - 1,150 m above sea level. It has been found growing in open acacia woodland with aloes and a grass-rich understory on steep, rocky slopes.
Dioscorea strydomiana does not look like a typical yam (member of the genus Dioscorea) because it is shrub-like with a huge, slow-growing,’lumpy’ wooden tuber which is mostly above ground. The tuber can reach 1 m in height and diameter; multiple shoots sprout from one or more shoot-bearing branches on it each spring, although young tubers have a single shoot-bearing tip. The outer layer of the tuber is ‘corky’ and grooved with numerous vertical furrows. The plant as a whole grows to about 1.5 m tall.
One to several non-twining stems to about 10 mm in diameter grow from each shoot-bearing tip each growing season. The branches spread more or less horizontally, at least at the bases, to give a dense shrub-like habit. The leaves are borne alternately on the stem. The leaf blade is 15-41 × 6-20 mm, thickly papery and stiff at maturity, with 3 or 5 veins running to the tip. The leaves are dull pale green on both surfaces and the petiole (leaf stalk) and main veins are pale whitish-green and often translucent.
There is one simple, racemose inflorescence per axil (the angle between a stem and a leaf) with only a few flowers per inflorescence. The male inflorescences are 16-53 mm long, with a peduncle (flower stalk) 9-14 mm long. The flowers are solitary or in pairs, with bracts present. The female inflorescences are 14-51 mm long, with a peduncle 11-28 mm long and with bracts on the pedicel. The flowers have 6 papery tepals, which are pale cream-yellow with a dark green longitudinal central stripe.
The capsule is 18-20 × 17-20 mm. The seeds are 4-7 × 3.5-6 mm (excluding the wing), dull, matt, mid to dark brown, and smooth to the naked eye but rough when viewed through a microscope. The seeds are winged at the tip or with a narrow wing on the side, and sometimes the base, of the seed. The wing is 6-12 × 6-9.5 mm, membranous and pale to chestnut brown, darker towards the seed.
Threats and conservation
So far, only two populations of Dioscorea strydomiana have been found. A team visits those populations on a regular basis to assess their status. It is estimated that each population comprises about 100 plants. On the last visit, the team noted that one population had suffered significant damage from medicinal plant collectors, who remove parts of the tuber. Dioscorea strydomiana is thought to be slow-growing and therefore slow to recover from such damage. Burning, mining, cattle farming, firewood collection, porcupine activity and removal of plants for the horticultural trade are thought to be further threats to the species. Like almost all Dioscorea species it is dioecious (separate male and female plants) so only the female element of the populations produce seed. It has been rated as Critically Endangered (CR) according to IUCN Red List criteria.
A number of measures have already been taken to protect this species. Careful monitoring by members of the Mpumalanga Tourism and Parks Authority (MTPA), the South African National Biodiversity Institute (SANBI) and the Mpumalanga Plant Specialist Group, has found that its numbers are declining at an alarming rate, so that urgent measures to promote its sustainable use and conservation have been called for. The area in which the species grows is not yet protected, but is monitored by the local tribal authority in collaboration with the MTPA in an attempt to prevent unscrupulous and illegal collection of plant material. An attempt to set up a community-run nursery in the area to provide cultivated plants for the medicinal and horticultural trade has so far not proved successful, but collections of seed-grown individuals have been successfully established in two of SANBI’s National Botanical Gardens (NBGs). Two seed collections have been banked at Kew's Millennium Seed Bank.
Dioscorea strydomiana is used locally with another species of Dioscorea to treat cancer. Its efficacy is unknown. The related species D. elephantipes and D. sylvatica are known to contain high levels of steroidal compounds which can be used to reduce inflammation, for example in the treatment of arthritis or for the promotion of healing.
Millennium Seed Bank: Seed storage
Kew's Millennium Seed Bank Partnership aims to save plant life world wide, focusing on plants under threat and those of most use in the future. Seeds are dried, packaged and stored at a sub-zero temperature in our seed bank vault.
Number of seed collections stored in the Millennium Seed Bank: Two
Germination testing: About 55%
Where to see this at Kew
Pressed and dried specimens of Dioscorea strydomiana are held in the Herbarium, one of the behind-the-scenes areas of Kew. The details of some of these specimens can be seen online in the Herbarium Catalogue.
IUCN (2001). IUCN Red List Categories and Criteria. Version 3.1. Prepared by the IUCN Species Survival Commission, Gland, Switzerland & Cambridge, UK.
Wilkin, P., Burrows, J., Burrows, S., Muasya, A.M., van Wyk, E. (2010). A critically endangered new species of yam (Dioscorea strydomiana Wilkin, Dioscoreaceae) from Mpumalanga, South Africa. Kew Bull.
Kew Science Editor: Paul Wilkin
Kew contributors: Odile Weber, Melanie Thomas, Simon Mayo
Copyediting: Emma Tredwell
Kew would like to thank the following contributors: John Burrows, Sandie Burrows, A. Muthama Muasya, Erich van Wyk.
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