Sabicea couteronii 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 : 121-122

1. Sabicea couteronii O. Lachenaud, Zemagho & Sonké , sp. nov.

( Fig. 1A–B , 3 , 6A–C ).

Holotypus: CAMEROON. Reg. South: 13 km E Sangmelima, 24.V.2016, fl., Sonké & Zemagho 6341 ( BR!; isotype: BRLU!, K!, MO!, WAG!, YA!).

Habitu sarmentoso, ovario biloculare, foliis valde discoloribus subtus indumento denso lanato et persistente munitis, corolla fauce pilis brevibus inflatis munita S. nobile similis , a quae differt ramis et foliis subtus ad nervis indumento mixto pilis lanatis et rectis (nec pilis omnibus lanatis) munitis, inflorescentiis ad anthesim breviter ramosis (nec capitatis), fructibus rubro-purpureis deinde nigrescentibus (nec albis), seminibus grosse reticulatis (nec striolatis).

Sarmentose liana, c. 3 m high; stems cylindrical, 3.5 – 6 mm thick, covered with a dense buff indumentum of straight ± appressed hairs c. 1.5 mm long, intermingled with shorter crispate hairs. Stipules interpetiolar, pale green, 10 – 15 × 4– 12 mm, erect, ovate and longitudinally plicate (except on buds), truncate at base, gradually acuminate at apex, uniformly pubescent outside with same indumentum as the stems, more sparsely so inside. Leaves opposite, slightly unequal; petiole (1–) 1.8–5 cm long, with same indumentum as the stems; lamina (9–)10.7–19.3 × (3.8–) 5.8–10.3 cm, elliptic to ovate, obtuse to rounded at base, gradually acuminate at apex, slightly coriaceous, discolorous; upper side dark green, densely pubescent on the veins with same indumentum as the stems, very sparsely so between them; margin densely ciliate with appressed hairs; lower side pale buff, with dense woolly hairs covering most of the surface (sparser on old leaves), and long straight hairs on the veins; secondary veins 13–16 pairs, weakly to moderately ascending, eucamptodromous; tertiary veins raised on the lower leaf surface and distinctly more prominent than quaternary veins, the latter densely reticulate but ± hidden by indumentum. Inflorescences axillary, paired at nodes or sometimes solitary, densely cymose, many-flowered, 1.7–5 cm long, densely pubescent with same indumentum as the stems; peduncle 0.5–2.5 cm long; primary ramifications 0.3–1 cm long. Bracts green, free, pubescent on both sides, all recaulescent except the basal pair; basal bracts broadly ovate, 7.5–12 × 5– 6 mm, entire or dentate, acute at apex; median bracts broadly ovate, 6 –14 × 3– 7 mm, entire, acute to obtuse at apex; upper bracts narrowly elliptic, 3– 6 × 0.8– 2 mm, entire, acute at apex. Flowers 5-merous; pedicels 0–1 mm long, densely villose. Calyx pale green; tube c. 1 mm long, densely villose on both sides; lobes elliptic to obovate, 1.5–2.5 × 0.8–1.5 mm, obtuse and curved outwards at apex, villose on both sides, alternating with minute colleters. Corolla white; tube cylindrical or nearly so, 4–5 × 1–1.5 mm, outside mostly glabrous but sparsely appressed-pubescent at apex, inside pubescent from the upper ¼TH to the throat, the latter with short inflated unicellular hairs; lobes triangular, c. 1.3 × 1.3 mm, patent, pubescent with stiff appressed hairs outside, glabrous inside; flower buds obtuse, slightly enlarged at apex. Stamens included, attached around the upper ¼TH of the corolla tube; anthers sessile, elliptic, 0.75–1 × 0.25–0.4 mm, glabrous. Ovary 1.5–2 mm long, densely villose, 2-locular. Disk cylindrical, c. 0.7 mm long, glabrous, shorter than the calyx tube. Style 4–4.5 mm long, just reaching the corolla throat, glabrous, bifid, with shortly stipitate elliptic flattened stigmas c. 1 mm long. Fruits purplish-red when young, blackish when mature, ellipsoid, with persistent calyx, 8– 11 × 7– 9 mm in life, 6–9 × 4.5–8 mm when dry, villose, with pedicel c. 1 mm long; mesocarp very soft, purple, c. 1.5 mm thick, with numerous darker granules. Seeds numerous, dark brown, polyedric, c. 0.6 × 0.4 mm, the surface rather coarsely reticulate.

Etymology. – The species is named after Dr Pierre Couteron (Institut de Recherche pour le Développement, Montpellier, France) in recognition for his constant support to the second and third authors.

Distribution, ecology and phenology. – Sabicea couteronii has an apparently very scattered distribution ( Fig. 4 ), being known from one locality in south Cameroon (near Sangmelima), one in central Gabon (Waka National Park) and two in southwestern Republic of Congo (Mayombe and Chaillu massifs). It occurs in secondary forest, edges and regrowth, c. 685 m in elevation (probably also lower).

Flowers have been collected in May-June, fruits in November and January (immature) and June (mature).

Conservation status. – The extent of occurrence (EOO) of S. couteronii is calculated to be 73,728 km ², exceeding the limit for “Vulnerable” status under subcriterion B1, while its area of occupancy (AOO) is estimated to be 16 km ², within the limit for “Endangered” status under subcriterion B2. The species occurs in Atlantic Central Africa ( Cameroon, Gabon and Republic of Congo) and is known from six specimens, representing four subpopulations. Two of them occur in protected areas, Waka National Park in Gabon and Dimonika Biosphere Reserve in Congo. The other two subpopulations occur along roads, and the Cameroonian one at least is threatened by habitat destruction due to agricultural activities. A decline in habitat extent and quality and number of individuals is therefore expected. The four subpopulations represent four locations in the sense of ICUN (2012), and the species qualifies for “Endangered” [EN B2ab(iii,v)] using the IUCN Red List Categories and Criteria ( IUCN, 2012).

Notes. – Sabicea couteronii resembles both S. nobilis and S. proselyta , which both occur in its range, and at first sight might be suspected to be a hybrid between them; this is however unlikely, as it differs from both by the colour of its fruits, which are purplish-red when young and blackish when mature (vs. white or greyish-white). It is further separated from S. proselyta by its strongly discolorous leaves with dense woolly indumentum below ( Table 1 ), and from S. nobilis by its (shortly) branched inflorescences, reticulate seed surface, and the presence of long silky hairs (in addition to woolly ones) on the stems, petioles and underside of the leaf veins.

The specimens from Gabon and Congo are either vegetative (Cusset 875) or with immature fruits (Boussengui-Nongo et al. 309, Kimpouni 600) but match very well the Cameroonian material, differing only in the longer peduncles (probably not a very significant character), and, in the case of the Gabonese material, sparser indumentum of the lower leaf surface (probably because the leaves are older). Therefore, we regard them as conspecific. Kimpouni 600 was previously cited as Pseudosabicea proselyta ( LACHENAUD, 2009) of which it was the only record from the Republic of Congo; however, authentic Congolese specimens of S. proselyta have since been found (see that species).

It is not known if S. couteronii is heterostylous; only the type specimen bears flowers, which are of the long-styled form.

Paratypi. – CAMEROON. Reg. South: 13 km E Sangmelima, 28.VI.2015, fr., Sonké 6248 ( BR, BRLU, K, MO, WAG, YA); ibid. loco, 16.V.2016, fr., Sonké & Zemagho 6334 ( BR, BRLU, K, MO, WAG, YA).

GABON. Ngounié: Parc national de Waka, 1°10'34"S 11°08'02"E, 28.XI.2007, imm.fr., Boussengui-Nongo et al. 309 ( LBV).

REPUBLIC OF CONGO. Kouilou: env. de Dimonika , source Paris Sangha , 5.III.1980, ster., Cusset 875 ( P). Niari: Rte Komono–Mossendjo , 2 km du camp SIDETRA, 8.I.1989, imm.fr., Kimpouni 600 ( BRLU).

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. 3. – Sabicea couteronii O. Lachenaud, Zemagho & Sonké. A. Flowering stem; B. Node with stipules; C. Detail of upper leaf surface; D. Detail of lower leaf surface; E. Node with paired inflorescences; F. Lower bract, inside; G. Upper bract, inside; H. Flower bud; I. Flower, lateral view; J. Flower, seen from above; K. Longitudinal section of flower; L. Longitudinal section of ovary and calyx; M. Fruit; N. Cross section of fruit. [A–L: Sonké & Zemagho 6341, BR; M–N: Sonké & Zemagho 6334, BR] [Drawing: H. de Vries

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).