Dioscoreaceae: family of flowering plants having great economic, food and medicinal values

Sanjeet
sanjeet.biotech@gmail.com
Ravenshaw University

Dioscoreaceae is a family of monocotyledonous flowering plants, with about 750 species in eight or nine genera. It is small but systematically and economically significant order of monocotyledons. The most diverse and important element of Dioscoreales is the yam genus, Dioscorea L., with over 600 accepted species names worldwide. Many yam species have edible tubers (Coursey 1967).

There are three major cultigens, the winged yam (D. alata L.), the guinea yams (D. cayenensis Lam./ D. rotundata Poir.) and the lesser or Asiatic yam D. esculenta (Lour.) Burkill. Dioscoreales (see below) are found in temperate and tropical regions worldwide, with the highest diversity being in the seasonally dry tropics of Central South America (Knuth 1924, Kirizawa et al. 2010), Mexico (Sosa et al. 1987, Tellez & Schubert 1994), the Caribbean (Raz 2004), South Africa (Knuth 1924), Madagascar (Burkill & Perrier 1950, Randriamboavonjy et al. in prep) and Indochina (Prain & Burkill 1936, 1938, Wilkin & Thapyai 2009) where Dioscorea species numbers are highest. The APG system (1998) and APG II system (2003) both place it in the order Dioscoreales, in the clade monocots. However, the circumscription changed in the APG II system, with the 2003 system expanded to include the plants that in the 1998 system were treated in the families Taccaceae and Trichopodaceae. Dioscoreales are found in temperate and tropical regions worldwide, with the highest diversity being in the seasonally dry tropics of Central South America, Mexico, the Caribbean, South Africa, Madagascar and Indochina where Dioscorea species numbers are highest. The yam allies are either plants of the wet tropics, where the other genera of Dioscoreaceae and Burmanniaceae occur, or plants of temperate and montane tropical habitats Nartheciaceae



Distribution of Dioscoreaceae



Botany

Habit and leaf form: Shrubs, or herbs, or lianas. Plants autotrophic. Rhizomatous, or tuberous (the tubers giving rise to annual stems). Climbing (usully), or self supporting (rarely); mostly stem twiners, or scrambling (or trailing); Tamus twining clockwise. Mesophytic, or xerophytic. Leaves alternate (usually), or opposite (rarely); usually spiral; petiolate; sheathing to non-sheathing; simple (usually), or compound; when compound, palmate (with three to six or more leaflets). Lamina entire (usually), or dissected (occasionally); when incised, palmatifid; basically palmately veined; cross-venulate; often cordate, or sagittate. Leaves stipulate, or exstipulate.

Leaves and Fruits of D.alata L.

General anatomy: Accumulated starch other than exclusively ‘pteridophyte type’.

Leaf anatomy: Extra-floral nectaries present (often), or absent. Stomata present; anomocytic.  The mesophyll usually containing calcium oxalate crystals. The mesophyll crystals raphides (in mucilaginous idioblasts). Minor leaf veins without phloem transfer cells (Dioscorea).

Leaves of Dioscorea species at Simlipal Biosphere Reserve Forest

Stem anatomy: Secondary thickening absent, or anomalous (e.g. Dioscorea, the vascular bundles arranged in two concentric circles); when present, from a single cambial ring. Xylem with vessels. Vessel end-walls scalariform. Sieve-tube plastids P-type; type II.

Root anatomy: Roots with velamen (single layered), or without velamen. Root xylem with vessels; vessel end-walls scalariform.

Reproductive type, pollination: Plants dioecious (usually), or monoecious, or hermaphrodite (Stenomeris). Floral nectaries present. Nectar secretion from the gynoecium (via septal nectaries). Pollination entomophilous.

Dioscorea species with bulbils

Inflorescence, floral, fruit and seed morphology: Flowers aggregated in ‘inflorescences’; in panicles, in racemes, and in spikes. The ultimate inflorescence unit racemose. Inflorescences axillary. Flowers bracteate; bracteolate (one bracteole, rarely two); small (generally inconspicuous); regular; 3 merous. Perigone tube usually present (short).  Perianth of ‘tepals’; 6; joined; 2 whorled; isomerous; sepaloid, or petaloid; similar in the two whorls, or different in the two whorls.  Androecium 6 (usually), or 3 (the inner whorl sometimes missing). Androecial members adnate (to the perianth); free of one another, or coherent; when cohering 1 adelphous (the filaments connate into a tube); 2 whorled (usually), or 1 whorled (by reduction). Androecium exclusively of fertile stamens, or including staminodes (the inner whorl sometimes staminodal or obsolete). Staminodes when present, 3; internal to the fertile stamens. Stamens 6, or 3; isomerous with the perianth, or diplostemonous. Anthers dorsifixed, or adnate; dehiscing via longitudinal slits; extrorse, or introrse; appendaged, or unappendaged. Endothecium developing fibrous thickenings. The endothecial thickenings spiral. Anther epidermis persistent. Microsporogenesis simultaneous. The initial microspore tetrads tetrahedral, or isobilateral. Anther wall of the ‘monocot’ type. Tapetum glandular. Pollen grains aperturate; 1 aperturate, or 2–5 aperturate; sulcate, or foraminate, or sulculate; 2-celled.  Gynoecium 3 carpelled. The pistil 3 celled. Gynoecium syncarpous; synovarious to synstylovarious; inferior. Ovary 3 locular. The ‘odd’ carpel posterior. Gynoecium stylate. Styles 1, or 3; free, or partially joined; apical. Stylar canal present. Stigmas dry type; non-papillate; Group IV type. Placentation axile. Ovules 2 per locule (usually), or 3–50 per locule (i.e., rarely ‘many’); pendulous; superposed; non-arillate; anatropous; bitegmic; crassinucellate. Polar nuclei fusing prior to fertilization. Antipodal cells formed; 3; not proliferating. Synergids hooked. Hypostase present. Endosperm formation nuclear.  Fruit fleshy, or non-fleshy; dehiscent, or indehiscent; a capsule (usually), or a berry (Tamus), or a samara (Rajania). Capsules loculicidal. Seeds endospermic. Endosperm oily. Seeds winged (e.g. Dioscorea), or wingless. Seeds without starch. Embryo well differentiated (small). Cotyledons 1 (usually, more or less lateral, broad and flat), or 2 (the second rudimentary). Embryo achlorophyllous (1/1). Testa without phytomelan; with red to yellowish brown phlobaphene.

Seedling: Hypocotyl internode present, or absent (sometimes developing into a storage organ). Seedling collar not conspicuous. Cotyledon hyperphyll elongated (Dioscorea), or compact; non-assimilatory; of Dioscorea dorsiventrally flattened (but anatomically unifacial). Coleoptile absent. Seedling cataphylls absent. First leaf dorsiventral. Primary root ephemeral.



Bulbils of Dioscorea species



Physiology, biochemistry: Not cyanogenic. Alkaloids present, or absent (mostly). Proanthocyanidins present (commonly in Dioscorea, often abundant), or absent (e.g. Tamus); when present, cyanidin. Flavonols present (commonly and abundantly, in Dioscorea), or absent (Tamus); kaempferol and quercetin. Ellagic acid absent. Saponins/sapogenins commonly present.

Geography, cytology: Holarctic, Paleotropical, Neotropical, Cape, Australian, and Antarctic. Temperate (warm), or sub-tropical to tropical. Widespread, but mainly tropical. X = 9, 10, 12, 14.

Taxonomy: Subclass Monocotyledonae. Dahlgren et al. Superorder Liliiflorae; Dioscoreales. APG 3 core angiosperms; Superorder Lilianae; non-commelinid Monocot; Order Dioscoreales.

Genus: Dioscorea

Dioscorea is a genus of over 600 species of flowering plants in the family Dioscoreaceae, native throughout the tropical and warm temperate regions of the world. The vast majority of the species are tropical, with only a few species extending into temperate climates. It is named after the ancient Greek physician and botanist Dioscorides. They are tuberous herbaceous perennial lianas, growing to 2–12 m or more tall. The leaves are spirally arranged, mostly broad heart-shaped. The flowers are individually inconspicuous, greenish-yellow, with six petals; they are mostly dioecious, with separate male and female plants, though a few species are monoecious, with male and female flowers on the same plant. The fruit is a capsule in most species, a soft berry in a few species. Several species, known as yams, are important agricultural crops in tropical regions, grown for their large tubers. Many of these are toxic when fresh, but can be detoxified and eaten, and are particularly important in parts of Africa, Asia, and Oceania.

Some Dioscorea species

Dioscorea alata L. Dioscorea althaeoides Knuth Dioscorea altissima Lam. Dioscorea aspersa Prain & Burkill Dioscorea balcanica Kosanin Dioscorea banzhuana C.P'ei & C.T.Ting Dioscorea belophylla Voigt ex Haines Dioscorea benthamii Prain & Burkill Dioscorea bicolor Prain & Burkill Dioscorea biformifolia C.P'ei & C.T.Ting Dioscorea birmanica Prain & Burkill Dioscorea bulbifera L. Dioscorea caucasica Lipsky Dioscorea cayenensis Dioscorea chingii Prain & Burkill Dioscorea cirrhosa Lour. Dioscorea collettii Hook.f. Dioscorea communis (L.) Caddick & Wilkin – see Tamus communis Dioscorea cumingii Prain & Burkill Dioscorea decipiens Hook.f. Dioscorea delavayi Franch. Dioscorea deltoidea Wall. ex Kunth Dioscorea dumetorum Dioscorea elephantipes Dioscorea esculenta Dioscorea esquirolii D.hispida Dioscorea exalata Dioscorea floridana Bartlett Dioscorea fordii Dioscorea futschauensis Dioscorea garrettii Dioscorea glabra Dioscorea gracillima Dioscorea hastifolia Dioscorea hemsleyi Dioscorea hispida Dioscorea japonica - Shan yao in Chinese Dioscorea kamoonensis

Dioscorea linearicordata Dioscorea macrostachya Dioscorea martini Dioscorea melanophyma Burkill & Prain Dioscorea menglaensis Dioscorea mexicana Dioscorea nipponica Dioscorea nitens Dioscorea opposita - Shan yao in Chinese Dioscorea orangeana Wilkin. Dioscorea panthaica Dioscorea pentaphylla Dioscorea persimilis Dioscorea poilanei Dioscorea polygonoides Dioscorea polystachya Turczaninow Dioscorea preussii Dioscorea quinqueloba Dioscorea rotundata Dioscorea sansibarensis Pax Dioscorea scortechinii Dioscorea simulans Dioscorea sinoparviflora Dioscorea spongiosa Dioscorea subcalva Dioscorea sylvatica Dioscorea tentaculigera Dioscorea tenuipes Dioscorea tokoro Dioscorea transversa Dioscorea trifida Dioscorea velutipes Dioscorea villosa L. Dioscorea wallichii Dioscorea xizangensis Dioscorea yunnanensis Dioscorea zingiberensis

Common Dioscorea species found at Simlipal Biosphere Reserve Forest

• Dioscorea alata
• Dioscorea hispida
• Dioscorea glabra
• Dioscorea pentaphylla



Simlipal Biosphere Reserve Forest

• Dioscorea wallichii etc.

The closely related genus Tamus is included in Dioscorea by some sources,^[2] but is maintained as distinct by others. For Dioscorea communis (L.) Caddick & Wilkin, see Tamus communis.

Many species of the genus Dioscorea are or have been sources of food and medicine. The tubers of at least 50 species are sources of dietary starch in both subsistence and an economic context (Coursey 1967). They vary widely in their ease of collection, palatability and degree of chemical protection from herbivory. Some are only used in times of famine, such as D. dumetorum (Kunth) Pax in Subsaharan Africa (Wilkin 2001, 2009). Its tubers are near the soil surface, and thus easy to collect, but need to have toxic alkaloids removed before they can be eaten safely. Others are protected by being more deeply buried, for example many of the endemic species of Madagascar or the candle yam in Thailand (D. brevipetiolata Prain & Burkill). Spiny roots protect the tuber in some forms of D. esculenta. In many parts of the tropics wild yams are a vital food source, especially when staple grains run out. Perhaps the best example of this is Madagascar (Jeannoda et al. 2003, 2007; Randriamboavonjy et al., in prep.). Of the ca. 40 species in Madagascar, 32 are endemic, of which at least 24 are edible and 12 or more are eaten regularly in different parts of the country when rice stores have been exhausted.

There are introduced, cultivated taxa but their use remains small-scale in gardens and the endemic wild yams are preferred. Similar edible taxa are encountered in the Caribbean in the radiation formerly separated as the genus Rajania L (Raz 2004). As Wilkin et al. (2005) showed, edible taxa are concentrated in the later branching clades of Dioscorea. In West Africa and New Guinea, where there is extensive field cultivation of the D. cayenensis/D. rotundata complex and D. alata and D. esculenta respectively as staple starch sources (Coursey 1967). However, yams are also taken from the forest in these areas (e.g. Hladik & Dounias 1996, Dounias 2001). In Africa this is often species such as D. praehensilis Benth., D. abyssinica Hochst ex Kunth., D. sagittifolia De Wild. or D. baya De Wild., supposed wild relatives of D. cayenensis/D. rotundata. People also transplant wild forest yams into their gardens (Hildebrand et al. 2002). In New Guinea D. nummularia Lam. appears to be a semi-domesticate (Malapa et al. 2005). Thus there is a continuum among food yams from taxa where human selection has been limited to those which have been extensively modified by it (e.g. Dumont et al. 2006). In economic terms, medicines have perhaps been an even more important product of Dioscorea than food. Many species are rich in steroidal saponins. In particular, the New World species D. mexicana Schiedw., D. composita Hemsl. and D. floribunda M. Martens & Galeotti were used in the 1950s and 60s as the source plants for the contraceptive pill (Coursey 1967, Marks 2001).  Other taxa rich in steroidal compounds include D. deltoidea Wall. ex Griseb., D. sylvatica Eckl. and its relatives (Wilkin et al. 2010) and the genus Tacca. As well as synthetic human hormones, yam steroids have also been used to make corticosteroid drugs such as cortisone. The range of medicinal uses of steroids is vast (Sparg et al. 2004). However, such compounds are today usually synthesised de novo rather than precursors being sourced from plant material. It is the earlier branches of Dioscorea which are the richest in steroidal saponins (Wilkin et al. (2005). Preliminary data from Thailand (Wilkin & Thapyai 209) suggest that 17 of 43 (ca. 40%) Dioscoreaceae species should be given IUCN Red List categories of Near Threatened, Vulnerable, Endangered or Critically Endangered (IUCN 2001). In Madagascar and the Comoros (Randriamboavonjy et al., in prep), this figure is 18/42 (ca. 43%).

Thus there is a significant need for targeted species conservation. D. bako Wilkin. (Wilkin et al. 2008) is a good example, a large yam which remarkably was not discovered by the many botanists of the 19^th and 20^th centuries who worked in Madagacar. It is edible, and preferred by people living in the small area of central Western Madagascar to which it is endemic. They are already reporting that it is harder to find than it used to be. Burmanniaceae and Nartheciaceae also contain significant numbers of threatened taxa. The research on yam conservation, ethnobotany, domestication and sustainable use necessitated by this conservation challenge needs to be underpinned by a stable and universal taxonomy. There have been numerous floristic studies in last 30 years, especially of Dioscorea, but no global treatment of Dioscoreaceae since Knuth (1924). Thus there is a need for a reliable guide to accepted names, descriptions, identification tools and rich supporting information. It is hoped that delivering a global treatment of the family via this scratchpad will help to facilitate closer collaboration between applied yam researchers and systematists. The affinities of Dioscoreales, the taxa which comprise it and their relationships to each other were uncertain for many years, but significant progress has been made since 1995. It has been shown to be sister to Pandanales (e.g. The Angiosperm Phylogeny Group 2009) and to comprise three families, Burmanniaceae, Dioscoreaceae and Nartheciaceae (Caddick et al. 2002a, b). Until the 1990’s, Burmanniaceae were linked to Orchidaceae and Nartheciaceae to Liliaceae respectively. Dioscoreaceae was thought to belong to a group of plant families with reticulate-veined leaves, which were the most closely-related monocots to the dicots.