Dioscorea comorensis R.Knuth

Wilkin, Paul, Hladik, Annette, Labat, Jean-Noël & Barthelat, Fabien, 2007, A new edible yam (Dioscorea L.) species endemic to Mayotte, new data on D. comorensis R. Knuth and a key to the yams of the Comoro Archipelago, Adansonia (3) 29 (2), pp. 215-228 : 218-221

Dioscorea comorensis R.Knuth

( Fig. 1 )

Repertorum Specierum Novarum Regni Vegetabilis 22:

347 (1926). — Type: Comoro Isles [ Comoros], Mohilla [Mwali], hills and coast by Fumboni [Foumboni] Town , ♂ fl., 8.IV.1861, Kirk & Meller s.n. (holo-, S; iso-, B, [digital image!], K!) .

SPECIMENS EXAMINED. — Comoros. Grande Comore [Njazidja], Grand Comore-Sud, Ouroveni , ♂ fl., 4.II.1975, Floret 737 (P!). — Moheli [Mwali], Itsamia Ngouni, ♀ fr., 28.V.2006, Labat, Yahaya & Abdou 3717 (CNDRS, K!, MO, P!). — Itsamia Ngouni, ♂ fl., 28.V.2006, Labat, Yahaya & Abdou 3717bis (CNDRS, K!, MO, P!) .

Mauritius. Île de France [Mauritius], no further locality data, ♂ fl., unknown date, Jussieu s.n. (P!) .

DESCRIPTION

Twining vine, upper stems appearing annual, lower stems and tuber unknown but reported to be edible. Indumentum of colourless, caducous (but persistent for longer and in greater density than those of D. mayottensis ) multicellular hairs to 0.7 mm long, usually dense on young vegetative buds and shoots and sparsely persistent around the axil and on the leaf blade lower surface, especially on the veins near the point of petiole insertion, usually appressed but sometimes erect on buds. Stems left-twining, to 3 mm in diameter (base unknown), terete, with shallow longitudinal ridges and grooves on main stems, weakly so or absent in fertile shoots, unarmed, drying mid-brown to grey. Leaves alternate, blade (2.7-)4.6-8.1 × (2.0-) 3.5-6.8 cm, ovate to narrowly so or deltoid-ovate, chartaceous to thinly so, margins entire, base deeply cordate, sinus 3.5-22 mm deep, broad to narrow or subhippocrepiform, apex 6-19 mm long, acuminate to short-acuminate with a 1.5-6 mm long, deltoid to narrowly so, brown forerunner tip, veins 5 to the apex, with a smaller, bifid pair of veins to the base, primary venation prominent on lower leaf blade surfaces; petiole 2.2-6.1 cm long, terete but channeled on upper surface; lateral nodal organs (“stipules” of Burkill 1960) sometimes present as two fleshy, recurved, acute projections to c. 2 mm long on each side of amplexicaul petiole base, some collapsing on drying; bulbils and cataphylls absent (but stem base unknown). Male inflorescences 1-3 per axil, simple, racemose, pendent, primary axis 4.8-21.7 cm long (including 0.7-1.9 cm long peduncle), axis flattened and narrowly winged, axis width 0.5-1.4 mm, nodes quite irregularly spaced and 0.7-5.0(-7.3) mm apart, alternate to suboppposite, each possessing 2-3(-4) flowers in a cymule or a solitary flower plus a cymule where the primary branch of the cymule is very short and concealed by the cymule bract, cymule bracts one per node, 1.3-1.9 mm long, ovate to lanceolate, long-acuminate, membranous, midrib scarcely thickened, cymule primary branch to 1.1 mm long. Female inflorescences to c. 24 cm long in fruit, with a 20-28 mm long peduncle, simple, spicate, pendent, one per axil, flowers c. 20-25 per inflorescence, unevenly spaced 2.4-6.2 mm apart, on all sides of axis. Male flowers pedicellate, pale yellow-green, scent not recorded, pedicels 0.9-3.1 × 0.15-0.6 mm, weakly clavate, ridged and angled; floral bracts at pedicel bases, 0.7-1.3 × 0.2-0.6 mm, narrowly ovate or elliptic to lanceolate, acuminate to obtuse, decreasing acropetally in size and especially width at each node of the cymule, membranous but thinner than cymule bracts; tepals differentiated into 2 whorls of 3 (inner broader), inserted on a shallow, open, bowl-shaped torus 0.2-0.5 × 0.8-1.5 mm, membranous and semitranslucent like tepals except towards centre where filaments are inserted; tepals erect in bud, more or less patent to torus margin at anthesis, membranous, midrib visible but scarcely thickened, apices obtuse to acute, spreading to reflexed; outer tepals 0.9-1.5 × 0.4-1.1 mm, lanceolate to elliptic, elliptic-oblong or narrowly so; inner tepals 0.7-1.6 × 0.8-1.3 mm, elliptic-oblong to ovate or broadly so, stamens 6, inserted in a ring on a thickened area in the torus centre, filaments 0.4-0.8 mm long, erect at base and free or contingent to form a column to 0.3 mm long, above free and apically reflexed in an arching manner such that the anthers are held downwards in dense to lax ring (depending on the age of the flower), not contorted or coiled, anthers 0.15-0.3 × 0.1-0.25 mm, elliptic-oblong, basifixed, pistillode a tiny, fleshy projection in the centre of the ring of filament bases. Female flowers pale yellow-green, scent not recorded, (sub)sessile until capsule development begins; floral bracts 1.6-2.2 × 0.7-1.1 mm, ovate, long-acuminate, membranous, with a thicker midrib, sheathing base of ovary during early development, bracteoles similar but smaller, inserted at 90° to bract, apex less sharply acuminate; ovary 2.6- 3.8 mm long, narrowly elliptic to narrowly oblong in outline, 3-angled, pale green when immature, drying more or less black; floral stipe 0.4-0.8 mm long; tepals differentiated into 2 whorls of 3 (inner broader), spreading at anthesis, later reflexed (especially inner whorl), thickly membranous with a thicker midrib, inserted on the rim of a 0.3-0.7 × 1.0- 1.6 mm torus, shallow, open, bowl-shaped, texture like tepals but thickening gradually towards style bases; outer tepals 1.1-1.7 × 0.5-0.8 mm, elliptic to elliptic-oblong or lanceolate, apex acute or acuminate; inner tepals 1.2-1.8 × (0.6-) 0.8-1.1 mm, broadly elliptic to elliptic-oblong or broadly so, apex obtuse to rounded; staminodia 6, 0.1-0.3 mm long, inserted around the style base, erect, filiform with a capitate, erect apex; styles 0.7-1.0 mm long, fused to form an erect column for 0.2-0.5 mm, apices free, recurved, bifid, stigma branches deltoid, spreading, horn-like, 0.3-0.5 mm long. Capsules 18-23 × 11- 14 mm, ascending at c. 60-45° to infructescence axis on a 2-4 mm long, terete to narrowly conical, 3-angled pedicel, oblong to oblong-elliptic or oblong-obovate in outline, drying glossy light brown with darker matt flecking, margin and axis darker, capsule base rounded to retuse, apex obtuse to acute, with torus and tepals persisting on a short floral stipe until at least submature. Seeds (material examined possibly submature) 5.4-6.2 × 4.2-4.7 mm, lenticularreniform to lenticular-ovoid, dull brown, wing 7.6-9.6 × 4.8-5.8 mm, orientated towards pedicel, oblong, membranous, golden-brown, translucent, apex rounded to acute.

REMARKS

The isotype at B consists of two detached leaves and three mounted male inflorescences, with presumably other loose inflorescences in the capsule. Verifying the collection locality given for the Jussieu s.n. specimen at P is critical in determining whether this species is disjunct in its distribution (or one part of its distribution is anthropogenic) or endemic to Ndjazidja and Mwali. There is unfortunately no duplicate specimen in Jussieu’s herbarium, which might have yielded more label data. Thus this species should be sought urgently in both the Comoro Archipelago and Mauritius. Only the widespread D. alata , D. bulbifera and D. sansibarensis are in the treatment of Dioscoreaceae in the Flore des Mascareignes ( Marais 1978). The Labat et al. collections of D. comorensis in 2006 have allowed the female plant to be described and confirmed that the species is still extant. Unfortunately the female flowers on Labat et al. 3717 are mainly past the period of stigma receptivity.

DISTRIBUTION AND ECOLOGY

This species occurs in the Comoro Archipelago (Ndjazidja and Mwali) and possibly Mauritius in coastal scrub and low forest. It is said to favour dry, bare soils (Msoili pers. comm.). The collections made by Labat et al. were made at c. 90 m in elevation. A second Mwali population was discovered in littoral vegetation on the island of Wenefou (Ouénéfou) South of Nioumachoua in Mwali in March 2007 (Crestey pers. comm.); male and female plants were photographed ( Fig. 1 ) but unfortunately no specimens made.

VERNACULAR NAMES

Chiyazi msirou in Njazidja (“forest yam” according to the label of Floret 737) and M’trou or Mtru (Msoili pers. comm.) in Mwali.

USES

Tuber edible (according to the labels of Floret 737 and Labat et al. 3717).

CONSERVATION STATUS

IUCN Red List category DD ( IUCN 2001). The specimen apparently from Mauritius makes its distribution uncertain (see above) and hence its area of occupancy and extent of occurrence cannot be determined without more information. If it is endemic to Njazidja and Mwali, then it is likely that it should be assessed as EN, given that just one recent collection has been made on either island. Both deforestation and over-utilisation are probable threats.

Figures

FIG. 1. — Dioscorea comorensis R.Knuth: A, female inflorescences with basal ovaries reflexed and enlarging; B, female inflorescence with more ovaries reflexed and enlarging; C, immature infructescence; D, part of male inflorescence showing the flowers in cymules; E, male inflorescences; F, male flowers. Photos Nicole Crestey (A, B, D, E) and Jean-Noël Labat (C, F).