Pauridiantha gracilipes O.Lachenaud & Ntore, 2019

Salvator NTORE & Olivier LACHENAUD, 2019, Two new species and a new combination in the genus Pauridiantha Hook. f. (Rubiaceae) from tropical Africa, Adansonia 41 (4), pp. 29-40 : 35-38

Pauridiantha gracilipes O.Lachenaud & Ntore , sp. nov.

( Figs 1C, D ; 3 )

Stipulis breviter triangularibus quam ramis angustioribus, ramis gracilibus glabris vel subglabris, foliis domatiis pubescentibus exceptis glaberrimis subtus dense et conspicue reticulatis, seminibus sublaevibus (nec prominente reticulatis ut in omnibus alteribus congeneribus) P. liebrechtsianae (De Wild. & T.Durand) Ntore & Dessein affinis, sed pedicellis fructiferis 12-18 mm longis (nec 2-8 mm), ovario basi 2-loculare et apice 4-loculare (nec basi 3-4-loculare et apice 6-8-loculare), inflorescentiis 2(-3)-floris et umbelliformibus (nec 3-9-floris et saepe racemosis) caulibusque nodis inferioribus saepe spinosis (nec inermibus) differt.

TYPUS. — Gabon. Ogooué-Maritime, Région du Lac Alombié, ± 7 km au nord de Mpaga , 0°50’32”S, 9°27’19”E, 17.X.2014, Lachenaud, Ikabanga & Lafferty 1981 (holo-, BRLU!; iso-, BR [ BR0000024391191 !]; G!; LBV!; MO!; P!; WAG!) .

PARATYPI. — Gabon. Préfecture de Mpaga , à environ 7 km au nord du Lac Alombié, 0°49’59”S, 9°26’09”E, 15.X.2014, Bidault & Lissambou 1726 ( BR [ BR0000016176461 !]; BRLU!; LBV n.v.) ; ibid., 0°47’29”S, 9°29’34”E, 16.X.2014, Bidault, Stévart, Nguema & Lissambou 1735 ( BR [ BR0000016173255 !]; BRLU!; LBV n.v.); Préfecture de Mpaga, nord du Lac Alombié , 0°49’06”S, 9°27’22”E, 25.X.2014, Zébazé, Barbier, Niangadouma & Stévart 419 ( BR [ BR0000016173910 !]; BRLU!; LBV n.v.) .

ETYMOLOGY. — The specific name refers to the particularly long and slender fruiting pedicels, a diagnostic character of the species.

DISTRIBUTION. — Lower Guinea subcentre of endemism. Endemic to the lower Ogooué basin in Gabon ( Fig. 4 ), where it is only known from a small area north of Lake Alombié; locally abundant in its highly restricted range.

HABITAT. — Restricted to seasonally flooded swamp forests dominated by Ctenolophon englerianus Mildbr. , Dactyladenia eketensis (De Wild.) Prance & F.White and Anthostema aubryanum Baill. , where it is locally dominant in the undergrowth and forms dense thickets. This peculiar habitat is described by Vandeweghe & Stévart (2017: 207-209) as “forêt inondable monodominante” (although, in fact, Ctenolophon is not always strictly monodominant) and, from an analysis of satellite images, is estimated to cover around 50 km ² in a restricted area north of Lake Alombié. Ctenolophon is believed to have been a key species for the maintenance of forest micro-refugia in the Ogooué delta during dry periods, due to its unusual ability to grow both on well drained and seasonally flooded soils ( Vandeweghe & Stévart 2017: 159, 177).

PHENOLOGY. — Fruits (not fully mature) in October, corresponding to the onset of the rainy season.

PRELIMINARY CONSERVATION ASSESSMENT. — Critically Endangered [CR B1ab(iii)+B2ab(iii)]. The extent of occurrence (EOO) of Pauridiantha gracilipes O.Lachenaud & Ntore , sp. nov. is estimated to be 8.252 km 2, and its area of occupancy (AOO) to be 8 km ², both values being within the limit for Critically Endangered status under subcriteria B1 and B2. The species is endemic to the coastal part of the Ogooué delta in Gabon, and occurs in Ctenolophon englerianus seasonally flooded forests, a highly localized habitat, which is estimated to cover around 50 km ² in the delta ( Vandeweghe & Stévart 2017: 207-209). It is known from four specimens representing four subpopulations, all situated outside protected areas (although its presence in the adjacent Wonga-Wongué National Park is highly probable). Though its habitat currently suffers little from human activities, forest exploitation has impacted it in the past and may do so again in the future; a decline in the quality of the habitat may therefore be expected. The four subpopulations represent one ‘location’ according to IUCN (2012), and the species is evaluated as Critically Endangered according to the conditions B1ab(iii)+B2ab(iii).

AFFINITIES. — Pauridiantha gracilipes O.Lachenaud & Ntore , sp. nov. appears to be most closely related to P. liebrechtsiana (De Wild. & T.Durand) Ntore & Dessein. The two species share nearly glabrous vegetative parts, stipules narrower than the twigs, leaves with hairy domatia and densely reticulate tertiary veins, and, most importantly, a nearly smooth seed surface (in all other species of the genus the margins of the exotestal cells are markedly raised). It is also related, though more distantly, to P. letestuana (N.Hallé) Ntore & Dessein and P. smetsiana Ntore & Dessein. The differences between these four species are shown in Table 1 . Remarkably, three of them are endemic to west-central Gabon; only P. liebrechtsiana is more widespread, occurring in seasonally flooded forests both in the Congo and the Ogooué basins, its westernmost known locality being Lake Gomé in Gabon, c. 120 km northeast of the range of P. gracilipes O.Lachenaud & Ntore , sp. nov.

DESCRIPTION

Lianescent shrub

To 3-4 m tall, with drooping branches bearing divaricate, ± horizontal flowering twigs; older stems sometimes bearing opposite spines, 0.8-1.7 cm long, formed by the persistent base of twigs; twigs cylindrical, 0.7-1.5 mm thick, glabrous or with sparse minute appressed hairs towards the apex.

Stipules

Tardily caducous, narrowly triangular, 1.3-2 × 0.5-0.7 mm, ± thickened, narrower than the twig and slightly diverging from it, sparsely appressed-puberulous.

Leaves

Petiole. 0.4-0.7 cm long, canaliculate, shortly pubescent on the upper side.

Leaf-blade. Elliptic, 5.5-10.8 × 2.1-4.1 cm, acute to rounded at base, narrowly acuminate at apex with acumen 1-2 cm long, papyraceous, glabrous except the underside of the nerves very sparsely appressed-puberulous, dark green above and pale green below, drying reddish on both sides; midrib concave above; secondary veins 5-7 pairs, strongly curved and brochidodromous, forming loops 1.5-2 mm from the margin; tertiary veins reticulate, conspicuous and dark green in life on the lower side of the leaf, forming a dense network with areolae 0.7-1.5 mm in diameter.

Domatia. Present in the main axils but not very conspicuous, crypt-like, ± pubescent.

Inflorescences

Axillary, solitary in each axil, in contracted cymes, (2-)3-flowered, c. 0.8 cm long in the flowering stage and 1.8-2.5 cm long in fruit, sparsely appressed-puberulous.

Peduncle. 0.2-0.6 cm long, with a pair of prophylls c. 0.5 mm long in the upper half.

Floral bracts. Absent.

Flowers

5-merous.

Pedicel. 3-5 mm long, sparsely appressed-puberulous.

Calyx. Pale green, cupuliform, c. 0.7 mm long, divided for 1/3 to 1/2 of its length, the lobes broadly triangular and acute at apex, puberulous on both sides.

Corolla (only one seen, dry). With tube 1.5 × 1.5 mm, inflated near the base, glabrous outside and densely hairy in the throat, and lobes triangular, 1.5 × 1 mm, reflexed, glabrous on both sides.

Floral bud. Not seen.

Stamens and style. Not seen, apparently included.

Disk. C. 0.5 mm high, hemispherical with 5 lateral grooves and 10 apical fossulae (formed by the imprints of the filaments and anthers), whitish, papillose.

Ovary. 2-locular below, 4-locular by false septa in the upper part, sparsely appressed-puberulous.

Fruits

Dark green (not fully mature), subglobose, 5.5-7.5 × 6.5- 8.5 mm in life, 4-5 × 5.3-7 mm when dry, smooth, glabrous, crowned with persistent calyx slightly enlarged in diameter (2.3-4.5 mm).

Pedicels. Drooping and strongly accrescent in the fruiting stage, 12-18 mm long.

Seeds

Numerous, ellipsoid, c. 0.7 × 0.6 mm.

Exotesta cells

Polygonal without raised margins, forming a very faint reticulum; radial walls pitted, not thickened; inner tangential wall broadly pitted, not thickened.

REMARKS

This very peculiar species was discovered in 2014 during the botanical exploration of the Lower Ogooué, and had apparently never been collected before. Although the flowering material is incomplete (only one dried flower was found, with both the style and stamens apparently included), the plant is so distinctive that we have no hesitation in describing it. Its remarkably long fruiting pedicels are unique in the genus, as is the presence of spines on the older stems, which are formed by the persistent base of broken twigs (these spines are only seen on the older individuals). The lianescent habit, with drooping branches somewhat reminiscent of Rubus L. spp., is further unusual, most other Pauridiantha species being erect shrubs or trees, although a few, such as P. pyramidata (K.Krause) Bremek. , can be more or less sarmentose.

Figures

FIG. 1. — Seeds of Pauridiantha chlorantha (K.Schum.) Ntore & O.Lachenaud, comb. nov., from Jannerup & Mhoro 129 (A, B) and P. gracilipes O.Lachenaud & Ntore, sp. nov., from Lachenaud et al. 1981 (C, D) in electron microscopy: A, C, entire seed; B, detail of exotesta cells with outer tangential wall removed, showing thickened radial walls with small pits, and large pits in the inner tangential walls; D, detail of exotesta cells with outer tangential wall removed, showing thin radial walls with small pits, and large pits in the inner tangential walls. Scale bars: A, C, 100 µm; B, D, 10 µm.

FIG. 3. — Pauridiantha gracilipes O.Lachenaud & Ntore,sp.nov.: A, fruiting stem; B, portion of old leafless stem, with spines formed by the persistent basal part of old twigs;C, detail of lower leaf surface,showing reticulate venation;D, detail of lower surface showing a domatia;E, node with paired inflorescences and stipule;F, calyx and disk,seen from above; G, flower; H, infructescence with one fruit and two undeveloped persistent ovaries; I, cross-section of fruit,basal part; J, cross-section of fruit, apical part. All details from Lachenaud et al. 1981. Scale bars: A, 4 cm; B, 2 cm; C, E, I, J, 5 mm; D, 1 mm; F, G, 2 mm; H, 1 cm. Illustration: Antonio Fernandez.

FIG. 4. — Distribution of Pauridiantha gracilipes O.Lachenaud & Ntore, sp. nov. () and P. principensis Ntore & O.Lachenaud, sp. nov. ().