ABSTRACT
The genus Dioscorea belongs to the monocotyledonous family Dioscoreaceae (Burkill, 1960; Dahlgren and Clifford, 1982; Knuth, 1930). It is a large genus comprising some 500 (Miège, 1982) to 850 (Al-Shehbaz and Schubert, 1989) species, mainly distributed in humid tropical and sub-tropical regions of the world. Most species are herbaceous perennial climbers and some have been cultivated for their starchy tubers, the ‘yams’. World production of yam crops has doubled during the last two decades, and was 30 million metric tons in 1997 (FAO, 1998), which is about onequarter of that of sweet potato, and one-fifth of that of cassava, two other important tropical tuber crops. More than 90% of world production of yams comes from West Africa (FAO, 1998; Hahn et al., 1987) where economic and cultural relationships between yams and people have developed to such an extent that Miège (1954a) and Coursey (1972) described it as the ‘civilization of the yams’. Other major yam-growing regions are Oceania and Southeast Asia. Economically important yam species are: D. rotundata and D. cayenensis in West Africa, D. alata and D. esculenta in the tropical regions of the world, and D. opposita in temperate East Asia.
