Sabicea rubiginosa 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 : 134-138

6. Sabicea rubiginosa O. Lachenaud, Zemagho & Sonké , sp. nov.

( Fig. 2C–D , 11 , 12A–D ).

Holotypus: GABON. Moyen-Ogooué : Mabounié , 00°46'15"S 10°32'52"E, 31.X.2014, Lachenaud et al. 2105 ( BRLU!; isotype: BR [ BR0000024387101 ]!, G!, LBV!, MO!, P!, WAG!).

Habitu sarmentoso, ovario biloculare, inflorescentiis cymosis, foliis utrinque viridibus supra inter nervis glabrescentibus, corolla fauce pilis brevibus inflatis munita, fructibusque albogriseis cum granulis atris in mesocarpio S. proselyta similis , a quae differt calycis bracteisque atro-rubris (nec viridibus), seminibus striolatis (nec grosse reticulatis), calyce tubo intus glabro (nec villoso) et lobis extus villosis (nec puberulis), foliisque subtus cum indumento suberecto pilis brevibus albidis et pilis longioribus rufescentibus intermixtis (nec indumento omnino albido et appresso, raro inter nervis suberecto).

Sarmentose liana, 1 –2 m high, climbing or occasionally creeping on the ground; stems cylindrical, 2 –5 mm thick, densely covered with c. 1 mm long rufous appressed hairs, intermingled with shorter uncinate hairs. Stipules interpetiolar, dark reddish-brown, 10 –15 × 6 –11 mm, erect, ovate and ± longitudinally plicate, rounded at base, acute at apex, uniformly pubescent outside with same indumentum as the stems, glabrous inside. Leaves opposite, equal; petiole 0.8–4.8 cm long, with same indumentum as the stems; lamina 6.5–17 × 2.3–7.7 cm, elliptic, acute at base, gradually acuminate at apex, slightly coriaceous, discolorous; upper side dark green, glabrous except for the midrib (and sometimes the secondary veins) with same indumentum as the stems; lower side pale green with brown-red veins, densely pubescent with both soft rufous hairs, 1–1.5 mm long appressed to half-erect, denser on the veins, and shorter whitish erect hairs 0.2 – 0.5 mm long; secondary veins 9–15 pairs, rather strongly ascending, eucamptodromous; tertiary veins markedly raised on lower leaf surface, much stronger than quaternary veins, the latter densely reticulate, forming areolae <0.5 mm in diameter. Inflorescences axillary and paired at nodes, cymose or sometimes glomeruliform at anthesis, many-flowered, 1.2–6.5 cm long, densely covered with rufous hairs c. 1 mm long intermingled with shorter uncinate hairs; peduncle 0.3–3.5 cm long; primary ramifications (0–) 0.2–2.3 cm long. Bracts dark red, free, appressed-pubescent outside and towards the apex inside, all recaulescent except the basal pair; basal bracts elliptic, entire or rarely dentate towards the base, 4–11 × 1.3–3.5 mm, acute at apex; median bracts elliptic, 4.5–8 × 1–1.7 mm; upper bracts narrowly elliptic, 1–4.5 × 0.2–0.8 mm. Flowers 5-merous, heterostylous; pedicels c. 0.5 mm long, densely villose. Calyx dark red, the base sometimes green or whitish; tube 0.6–0.7 mm long, villose outside like the inflorescence, glabrous inside; lobes narrowly spathulate, 1.5– 3 × 0.5 – 1.2 mm, obtuse to subacute and curved outwards at apex, villose outside, puberulous inside except towards the base, alternating with minute colleters. Corolla pale green in bud, whitish at anthesis; tube cylindrical, 4 –4.5 × 1.5 mm, outside glabrous at base and sparsely appressed-pubescent at apex, inside with a ring of multicellular hairs around the upper third and short, inflated unicellular hairs at the throat, the intervening zone with very sparse hairs; lobes triangular, 1.5 –1.8 × 1.3–1.5 mm, patent, shortly appressed-hairy outside, glabrous inside; flower buds obtuse, the apex slightly enlarged and pentagonal. Stamens included and attached around the upper third of the corolla tube (long-styled flowers) or half-exserted and attached near the apex of the tube (short-styled flowers); anthers sessile, elliptic, c. 1 × 0.6 mm, glabrous. Ovary c. 1.5 mm long, densely villose, 2-locular. Disk cylindrical, c. 0.5 mm long, glabrous, slightly shorter than the calyx tube. Style c. 4.3 mm long and just reaching throat (long-styled flowers) or c. 3.5 mm long and included (short-styled flowers), bifid, with elliptic flattened stigmas c. 0.8 mm long, these shortly stipitate in long-styled flowers and sessile in short-styled flowers. Fruits whitish, ellipsoid, with persistent calyx, 5–10 × 4–8 mm in life, 4–10 × 3–9.5 mm when dry, villose, with pedicel 0.5–3.5 mm long; mesocarp very soft, whitish, c. 1.5 mm thick, with numerous darker granules. Seeds numerous, pale brown, polyedric, c. 0.8 × 0.5 mm, the surface finely striolate.

Etymology. – The epithet rubiginosa refers to the rustycoloured indumentum of the leaves and stems.

Distribution, ecology and phenology. – Sabicea rubiginosa is endemic to west-central Gabon, south to Rabi and north to the lower Ngounié river ( Fig. 10 ) and is locally very abundant in its range. It occurs in forest edges, especially along tracks, at low elevations (20–165 m).

Flowers have been recorded in January, March, May, July and October-November, probably most of the year; fruits from January to March, in June-July and in October-November.

Conservation status. – The extent of occurrence (EOO) of S. rubiginosa is estimated to be 4312 km ², and its area of occupancy (AOO) to be 88 km ², both values being within the limit for “Endangered” status under subcriteria B1 and B2. The species is endemic to west-central Gabon and is known from 30 specimens, representing five subpopulations, none of which is protected. One of these is situated in a forestry concession, which does not seem to represent a serious threat since the species occurs in forest edges and is likely to be favoured by selective logging. Two subpopulations occur in oil concessions, and one in a mining concession where an important exploitation project is scheduled; a decline in habitat extent and quality and number of individuals is therefore predicted. The five subpopulations represent five locations in the sense of IUCN (2012), and the species qualifies for “Endangered” [EN B1ab(iii,v)+2ab(iii,v)] using the IUCN Red List Categories and Criteria ( IUCN, 2012).

Notes. – Sabicea rubiginosa has previously been mistaken for S. proselyta (see that species), but differs by the indumentum of its lower leaf surface, the ornamentation of its seeds, the absence of hairs inside the calyx tube, and the dark red colour of its bracts and calyces ( Table 1 ; Fig. 12A, C ). The latter character is especially conspicuous in the field, and makes S. rubiginosa a remarkably attractive plant, which could certainly be grown for ornamental purposes. The typical subspecies of S. sanguinosa also has red bracts and calyces, but differs by the hirsute and whitish (not rufous) indumentum of its vegetative parts, its leaves that are markedly bullate in the fresh state, its calyx lobes acute at apex and its generally larger bracts ( Table 1 ). The three species, S. proselyta , S. rubiginosa and S. sanguinosa , appear to have separate ranges ( Fig. 10 ).

Paratypi. – GABON. Moyen-Ogooué : Mabounié , forest area south-west of camp , 6.V.2012, fl., Bidault 470 ( BRLU, P); zone de Mabounié , à 45 km au SW de Lambaréné, rive N de la rivière Ngounié , 16.XI.2013, fl., Bidault et al. 1329 ( BRLU); Concession froestière GWT , 5.VI.2012, fr., Boupoya & al. 727 ( BRLU); Ezanga, 1.II.1991, fr., Dibata 727 ( WAG); Mabounié, 13.X.2012, fl., Sonké & Ikabanga 5999 ( BR, BRLU, LBV, MO); ibid. loco, 13.X.2012, fr., Sonké & Ikabanga 6001 ( BR, LBV, MO); ibid. loco, 23.X.2012, fl., Sonké & Ikabanga 6132 ( BR, BRLU, LBV, MO); ibid. loco, 24.X.2012, fl., Sonké & Ikabanga 6140 ( BR, BRLU, LBV, MO); ibid. loco, 24.X.2012, fl., Sonké & Ikabanga 6141 ( BR, BRLU, LBV, MO); ibid. loco, 25.X.2012, fl. & fr., Sonké & Ikabanga 6153 ( BR, BRLU, LBV, MO); ibid. loco, 25.X.2012, fl., Sonké & Ikabanga 6155 ( BR, BRLU, LBV, MO); Mabounié , 1.V.2012, buds, Stévart et al. 4125 ( BRLU, LBV); ibid. loco, Ngounié River , 4.V.2012, fl., Stévart & Issembé 4176 ( BRLU, LBV); Mabounié mine , near Lambaréné , 9.II.2012, imm.fr., Stévart & Droissart 4206 ( BRLU, LBV, P). Ngounié : forêt sur la rive gauche (S) de l’Ikoy , à env. 10 km de l’embouchure avec la Ngounié, 26.X.2012, fl., Dauby et al. 2938 ( BRLU, LBV); concession Maurel & Prom , près du Lac Ezanga , 22.XI.2013, fl., Lachenaud et al. 1531 ( BR, BRLU, LBV, MO, P, WAG); Mabounié , le long de la rivière Ngounié , 12.X.2012, buds, Sonké & Ikabanga 5972 ( BR, BRLU, LBV, MO). Ogooué-Maritime : Rabi , 28.III.1990, fl. & fr., F.J. Breteler et al. 9576 ( BR, G, LBV, P, WAG); Rabi-Kounga , 8.VII.1998, fl. & imm.fr., F.J. Breteler et al. 14350 ( BR, LBV, WAG); Rabi-Kounga, S of Rabi , 1.I.1994, fl. & fr., Haegens & van der Burgt 222 ( BR, LBV, WAG); c. 15 km NW of Shell oil exploitation Rabi, 23.XI.1989, fl. & fr., J.J.F.E. de Wilde et al. 9688 ( LBV, WAG); Rabi ( parcelle Smithsonian ), 9.XI.2014, fl., D. Nguema et al. 2843 ( BRLU); Pechoud road southwards , 27.X.1990, fr., van Nek 128 ( BR, LBV, WAG); Rabi-E , near Pechaud camp , 4.XI.1990, fl. & fr., van Nek 190 ( BR, LBV, WAG); Rabi-Kounga , in between Shell-office and camp , 27.X.1992, fl. & fr., von Asmuth & Vosmeer 35 ( WAG); Rabi-Kounga , road to Echira , 5.XI.1992, buds, von Asmuth & Vosmeer 58 ( WAG); Rabi-Kounga , road to Echira , 5.XI.1992, fl., von Asmuth & Vosmeer 59 ( WAG); Rabi, 4½ km on road to Divangui, 4.III.1994, fr., Wieringa & Haegens 2389 ( LBV, WAG).

Figures

Fig. 2. – Seeds of Sabicea spp. viewed in electron microscopy. Sabicea proselyta (N. Hallé) Razafim. et al.: A. Entire seed; B. Detail of seed surface. Sabicea rubiginosa O. Lachenaud, Zemagho & Sonké: C. Entire seed; D. Detail of seed surface. Sabicea sanguinosa subsp. sanguinosa: E. Entire seed; F. Detail of seed surface. Sabicea segregata Hiern: G. Entire seed; H. Detail of seed surface. [A–B: van Valkenburg et al. 3140; C–D: Lachenaud et al. 2105; E–F: A. M. Louis & Reitsma 2004; G–H: J.J.F.E. de Wilde et al. 9485] [Photos: I. van der Beeten]

Fig. 11. – Sabicea rubiginosa O. Lachenaud, Zemagho & Sonké. A. Stem with flowers and fruits; B. Node with stipules, lateral view; C. Detail of lower leaf surface; D. Same, much enlarged, showing close reticulation; E. Bracts from lower node of inflorescence; F. Upper bract, inside; G. Flower bud, lateral view; H. Flower bud, seen from above; I. Short-styled flower; J. Longitudinal section of short-styled flower; K. Short-styled flower, seen from above; L. Long-styled flower; M. Longitudinal section of long-styled flower; N. Long-styled flower, seen from above; O. Longitudinal section of calyx and ovary; P. Fruit; Q. Cross section of fruit. [Lachenaud et al. 2105, BRLU] [Drawing: H. de Vries]

Fig. 12. – Sabicea rubiginosa O. Lachenaud, Zemagho & Sonké: A. Plant with flowers and fruits; B. Lower side of leaf; C. Inflorescence; D. Infructescence with mature (white) and immature fruits (green). Sabicea sanguinosa subsp. sanguinosa: E. Apex of flowering stem. Sabicea sanguinosa subsp. viridis O. Lachenaud, Zemagho & Sonké: F. Stem with flowers and fruits, note bullate leaves. [A: Sonké & Ikabanga 6155; B, D: Lachenaud et al. 2105; C: Bidault et al. 1329; F: Bidault et al. 1691] [Photos: A: B. Sonké; B, D: O. Lachenaud; C, F: E. Bidault; E: J.-P. Vande weghe]

Fig. 10. – Distribution map of Sabicea proselyta (N. Hallé) Razafim. et al. (circles), S. rubiginosa O. Lachenaud, Zemagho & Sonké (stars) and S. sanguinosa (N. Hallé) Razafim. et al. (triangles: subsp. sanguinosa; squares: subsp. viridis O. Lachenaud, Zemagho & Sonké).