Sabicea dichrosepala O. Lachenaud, Zemagho & Sonké, 2020

Lachenaud, Olivier, Zemagho, Lise & Sonké, Bonaventure, 2020, A synopsis of the Sabicea floribunda group (Rubiaceae) from Central Africa, including three new species, Candollea 75 (1), pp. 115-143 : 122-124

2. Sabicea dichrosepala O. Lachenaud, Zemagho & Sonké , sp. nov.

( Fig. 1C–D , 5 , 6D–E ).

Holotypus: GABON. Ngounié: km 43 Mouila–Yéno, 21.II.2008, Dessein et al. 2028 ( BR [ BR0000013945923 ]!; isotype: LBV!, WAG!) .

Habitu sarmentoso, ovario biloculare, seminibus grosse reticulatis, foliis utrinque viridibus subtus ad nervis indumento albido valde appresso munitis, corollaque fauce pilis brevibus inflatis munita S. proselyta similis , a quae differt calyce bicolore apice viride et basi rubro-vinaceo (nec omnino viride), inflorescentiis ad anthesim glomerulatis deinde cymosis (nec semper laxe cymosis), pedunculis nullis vel brevissimis usque 0.3 cm longis (nec 0.7–3.8 cm longis), stipulis basi auriculatis et extus sparsissime pubescentibus (nec basi rotundatis et extus ± dense pubescentibus), foliis subtus venulis tertiariis conspicuis sed haud prominentibus, fructuque albissimo (nec albo-griseo) mesocarpio granulis atris nec munito.

Sarmentose liana, 2–4 m high; stems cylindrical, 2–3 mm thick, densely covered with straight whitish appressed hairs c. 1 mm long, intermingled with shorter uncinate hairs. Stipules interpetiolar, pale green, 9–14 × 6–9 mm, erect, ovate and longitudinally plicate (except on buds), slightly auriculate at base, acute or acuminate at apex, minutely and very sparsely appressed-pubescent outside (except the base with indumentum similar to the stems), sparsely villose inside. Leaves opposite, often slightly unequal; petiole 1–5 cm long, with same indumentum as the stems; lamina 8 – 16 × 3.5–9 cm, elliptic to ovate, acute to subcordate at base, gradually acuminate at apex, papyraceous, discolorous; upper side medium green, minutely and sparsely appressed-pubescent to glabrous (except for the midrib which often has longer hairs); lower side pale green with ± orange-tinged secondary veins, minutely and sparsely appressed-pubescent, more densely so on the veins; margin densely ciliate with appressed hairs; secondary veins 13– 19 pairs, weakly ascending, forming inconspicuous loops near the margin; tertiary veins conspicuous but not or hardly raised on lower leaf surface, slightly more prominent than quaternary veins, the latter densely reticulate forming areolae c. 0.5 mm in diameter. Inflorescences axillary and paired at nodes, manyflowered, glomeruliform and 1.2–2.5 cm in diameter at anthesis, becoming laxer and conspicuously cymose, 2.5–3 cm long in the fruiting stage, densely pubescent with stiff appressed whitish hairs 0.5– 0.7 mm long; peduncle 0–0.3 cm long; primary ramifications not or hardly distinct at anthesis, c. 0.5 cm long in the fruiting stage. Bracts green, free, shortly pubescent on both sides, all recaulescent except the basal pair; basal bracts ovate, lobed at base, 7–8.5 × 2.5–4 mm; median bracts elliptic to obovate, 5–6 × 1–2 mm, entire; upper bracts narrowly elliptic to linear, 1.7–4 × 0.25–0.5 mm, entire. Flowers 5-merous; pedicels 0–1 mm long at anthesis, pubescent like the inflorescence. Calyx dark pinkish-red at base and green at apex; tube 0.7–1 mm long,] appressed-pubescent outside and more sparsely so inside; lobes obovate to elliptic, 2.5–4 × 1.5–2.3 mm, somewhat unguiculate at base, obtuse to acute and curved outwards at apex, covered on both sides with short patent hairs, and alternating with minute colleters. Corolla tube wine red, cylindrical, 5 × 1.5 mm, outside glabrous at base and sparsely puberulous in the upper half, inside with a ring of multicellular hairs around the upper third and short, inflated unicellular hairs at the throat, the intervening zone glabrous; lobes cream on both sides, triangular, 1.5 × 1.5 mm, reflexed, sparsely puberulous outside, inside with short inflated unicellular hairs at base and glabrous at apex; flower buds obtuse, slightly enlarged at apex. Stamens included, inserted around upper ¼TH of the corolla tube with their tips just reaching throat; filaments c. 0.4 mm long; anthers cream, elliptic, c. 1.2 × 0.7 mm, glabrous. Ovary c. 1.5 mm long, densely covered with stiff appressed whitish hairs, 2-locular. Disk cylindrical, c. 0.5 mm long, glabrous, shorter than the calyx tube. Style c. 3.5 mm long, included, bifid, with elliptic flattened stigmas c. 1 mm long, these shortly stipitate. Fruits white, ellipsoid, with persistent calyx, 5–7 × 4–6 mm when dry, sparsely pubescent with appressed hairs, on accrescent pedicel 4–6.5 mm long; mesocarp very soft, entirely white (without darker granules). Seeds numerous, dark brown, polyedric, c. 0.5 × 0.3 mm, the surface prominently and rather coarsely reticulate.

Etymology. – The species epithet refers to the bicoloured calyx lobes, which are the plant’s most striking character in the field.

Distribution, ecology and phenology. – Sabicea dichrosepala is apparently endemic to Gabon, where it occurs in the Cristal Mountains in the northwest and the Chaillu massif in the south ( Fig. 4 ), two areas well-known for their endemism and floristic affinity. It might be expected in adjacent parts of Equatorial Guinea and the Republic of Congo. Although probably under-collected, it is clearly an uncommon species. It occurs in forest edges, especially along tracks or forest streams, 610–730 m in elevation.

Flowers have been collected in January-February (buds) and March (open); fruits in September (immature), February and March (mature).

Conservation status. – The extent of occurrence (EOO) of Sabicea dichrosepala is estimated to be 17,016 km ², and its area of occupancy (AOO) to be 28 km ², respectively within the limits for “Vulnerable” and “Endangered” status under subcriteria B1 and B2. The species is endemic to Gabon and is known from seven specimens, representing six subpopulations, one of which occurs in a protected area, the Crystal Mountains National Park. The species occurs in sparsely populated areas, and favours forest edge habitats, so is likely to benefit from small-scale forest clearing, e.g. that induced by selective logging. However, two of its known sites are very close to the Tchimbélé dam, the construction of which has presumably affected the extent and quality of habitat and number of individuals. The six subpopulations represent six locations in the sense of IUCN (2012), and the species qualifies as “Vulnerable” [VU B2ab(iii,v)] using the IUCN Red List Categories and Criteria ( IUCN, 2012).

Notes. – The earliest collection of this species, J.J.F.E. de Wilde et al. 284, was cited as Pseudosabicea proselyta by SOSEF et al. (2006). Sabicea dichrosepala is indeed closely related to S. proselyta (the two share, in particular, a similar seed ornamentation) but differs by the characters mentioned in Table 1 , the most conspicuous of which are the bicoloured calyx, and the inflorescences that are very dense and glomeruliform in the flowering stage. It is not known whether the species is heterostylous; the only open flowers seen ( Fig. 6D ) are short-styled.

Paratypi. – GABON. Estuaire: Mts de Cristal , vers le secteur Mont Séni , Concession BSG , 0°50'52"N 10°24'12"E, 8.III.2019, fl., Bidault et al. 4401 ( BRLU, LBV n.v., MO); forest exploitation Leroy , 20 km NW of Asok, 27.I.1983, buds, J.J.F.E.de Wilde et al. 284 ( BR, LBV, P, WAG); Mts de Cristal, Tchimbélé-Nkol Madouaka , 17.II.2007, fr., Leal et al. 1396 ( BR) . Ngounié: Mouyanama falls, at base of Mt Songou , 23.II.2008, buds, Dessein et al. 2111 ( LBV); Massif du Chaillu au SE localité Moudouma , 21.IX.2001, imm.fr., Mayombo-Nzengue 928 ( LBV) . Woleu-Ntem: 2 km N of Tchimbélé on former road to Asok, 6.II.2008, old fl., Dessein et al. 1741 ( BR, LBV) .

Figures

Fig. 1. – Seeds of Sabicea spp. viewed in electron microscopy. Sabicea couteronii O. Lachenaud, Zemagho & Sonké: A. Entire seed; B. Detail of seed surface. Sabicea dichrosepala O. Lachenaud, Zemagho & Sonké: C. Entire seed; D. Detail of seed surface. Sabicea floribunda var. anisostegia O. Lachenaud, Zemagho & Sonké: E. Entire seed; F. Detail of seed surface. Sabicea nobilis R.D. Good: G. Entire seed; H. Detail of seed surface. [A–B: Sonké & Zemagho 6334; C–D: Dessein et al. 2028; E–F: N. Hallé 2951; G–H: Sosef et al. 2529] [Photos: I. van der Beeten]

Fig. 5. – Sabicea dichrosepala O. Lachenaud, Zemagho & Sonké. A. Fruiting stem; B. Detail of lower leaf surface; C. Node with stipules, lateral view; D. Node with stipules, dorsal view; E. Node with paired inflorescences; F. Lower bract, inside; G. Upper bract, inside; H. Calyx; I. Longitudinal section of young flower bud; J. Fruit; K. Flower, viewed from above; L. Flower, side view; M. Longitudinal section of flower. [A, C, J: Dessein et al. 2028, BR; B, D–I: J.J.F.E. de Wilde et al. 284, BR; K–M: Bidault et al. 4401, BRLU] [Drawing: A–J: H. de Vries; K–M: O. Lachenaud]

Fig. 6. – Sabicea couteronii O. Lachenaud, Zemagho & Sonké: A. Node with inflorescences; B. Fruiting stem; C. Mature infructescence. Sabicea dichrosepala O. Lachenaud, Zemagho & Sonké: D. Inflorescence; E. Mature infructescence. [A: Sonké & Zemagho 6341; B–C: Sonké & Zemagho 6334; D: Bidault et al. 4401; E: Dessein et al. 2028] [Photos: A–C: B. Sonké; D: E. Bidault; E: S. Dessein]

Fig. 4. – Distribution map of Sabicea couteronii O. Lachenaud, Zemagho & Sonké (squares) and S. dichrosepala O. Lachenaud, Zemagho & Sonké (circles).