Solanum pseudospinosum C.H.Wright, Fl. Trop. Afr. [Oliver et al.] 4, 2: 220. 1906

Saerkinen, Tiina, Poczai, Peter, Barboza, Gloria E., Weerden, Gerard M. van der, Baden, Maria & Knapp, Sandra, 2018, A revision of the Old World Black Nightshades (Morelloid clade of Solanum L., Solanaceae), PhytoKeys 106, pp. 1-223 : 1

11. Solanum pseudospinosum C.H.Wright, Fl. Trop. Afr. [Oliver et al.] 4, 2: 220. 1906 Figures 34 , 35

Solanum pachyarthrotrichum Bitter, Repert. Spec. Nov. Regni Veg. 10: 542. 1912.

Solanum pseudospinosum Type. Cameroon. Sin loc., H. Deistel 631 (holotype: B, destroyed; no duplicates found).

Solanum hypopsilum Bitter, Repert. Spec. Nov. Regni Veg. 10: 543. 1912.

Solanum pseudospinosum Type. Cameroon. Sud-Ouest: Buea, 1000 m, 8 Mar 1898, H. Lehmbach 175 (holotype: B, destroyed; no duplicates found).

Solanum molliusculum Bitter, Repert. Spec. Nov. Regni Veg. 10: 546. 1912.

Solanum pseudospinosum Type. Cameroon. Sud-Ouest: Buea, canyon east of Mannsquelle, P. Preuss 740a (holotype: B, destroyed; no duplicates found).

Type.

Cameroon. Sud-Ouest: Mt Cameroon, Dec 1862, G. Mann 1938 (holotype: K [K000028649]; isotype: GH [00395064]) .

Description.

Annual or short-lived prostrate or sprawling perennial herbs to 0.5-1.5 m tall, subwoody and branching at base. Stems spreading, usually strongly ridged with pseudospinose dentate processes, green, older stems brown or grey, usually hollow; new growth densely pubescent with simple, spreading, uniseriate, translucent, mixed glandular and eglandular trichomes, these 7-10-celled, 1.0-1.5 mm long; older stems glabrescent. Sympodial units difoliate, the leaves not geminate. Leaves simple, 2.5-7 cm long, 1.3-4 cm wide, smaller at distal ends of branches, ovate, widest in the lower third, membranous, green, concolorous, smell not known; both surfaces evenly pubescent with simple uniseriate mostly eglandular trichomes like those on stem; major veins 4-5 pairs; base cuneate; margins entire or shallowly toothed in the basal half; apex acute to acuminate; petioles 0.5-2 cm long, pubescent with trichomes like the leaves. Inflorescences 1-2.5 cm long, opposite the leaves, unbranched, sub-umbelliform to racemose, with 2-5(-6) flowers clustered at the tip, pubescent with uniseriate trichomes like those of the stems; peduncle 0.3-1.5 cm long, straight; pedicels 0.7-0.8(-1) cm long, ca. 0.5 mm in diameter at the base, ca. 1 mm in diameter at the apex, slender, spreading, pubescent with simple uniseriate trichomes like the rest of the inflorescence, articulated at the base; pedicel scars clustered in the apical portion of the inflorescence rhachis, lowermost scars ca. 2 mm apart, the rest overlapping at the distal end. Buds ellipsoid, densely pubescent, the corolla at least 1/2 way exserted from the calyx tube until just before anthesis. Flowers 5-merous, all perfect. Calyx tube 1.5-2 mm long, conical, the lobes 1-1.5 mm long, 0.9-1.4 mm wide, elongate-deltate, rounded at the tips, densely pubescent with simple uniseriate trichomes like the rest of the inflorescence. Corolla 8-14 mm in diameter, white to pale violet ( “blue”) or white with purple venation, stellate, lobed nearly to the base, the lobes 4-6 mm long, 2-2.5 mm wide, spreading at anthesis, abaxially moderately pubescent with simple uniseriate 2-3-celled trichomes, these denser near the tips and margins, also papillate along the margins and tips, adaxially glabrous. Stamens equal; filament tube minute; free portion of the filaments 0.5-1 mm long, densely pubescent adaxially with tangled simple trichomes; anthers 2-2.5 mm long, ca. 1 mm wide, ellipsoid, yellow, poricidal at the tips, the pores lengthening to slits with age and drying. Ovary conical, glabrous; style 3-5 mm long, densely pubescent with tangled simple trichomes in the basal portion inside the anther tube, exserted ca. 2 mm beyond the anther cone, often elongating early and protruding from unopened buds; stigma capitate, the surfaces minutely papillose, colour in live plants not known. Fruit a globose berry, 4-6 mm in diameter, purple or black at maturity, the pericarp thin and matte; fruiting pedicels 0.9-1.2 cm long, ca. 0.75 mm in diameter at the base and at the apex, spreading, not falling with the fruit, remaining on the plant and persistent on older inflorescences; fruiting calyx not accrescent, the tube ca. 1 mm long, the lobes 1.5-2.5 mm long, appressed to the berry, spreading and hyaline, the tips slightly reflexed. Seeds ca. 20 per berry, ca. 1.5 mm long, ca. 1 mm wide, not markedly flattened, tear-drop shaped with a subapical hilum, pale beige, the surface minutely pitted and the lateral testal cell walls elongate and the seed appearing hairy, the testal cells pentagonal in outline. Stone cells 4-6(-10), often found near the pedicel junction at the base of the berry. Chromosome number: 2n =4x=48 ( Morton 1993).

Distribution

(Figure 36 ). Endemic to the escarpment running from the island of Bioko (Fernando Po) in Equatorial Guinea to Lake Oku in Cameroon; most collections we have seen are from the slopes of Mount Cameroon.

Ecology.

Grows at forest margins, in clearings and along roadsides; between 2,100 and 4,000 m elevation.

Common names.

Cameroon: afom.

Uses.

None recorded.

Preliminary conservation status

( IUCN 2016). Solanum pseudospinosum is a relatively narrow endemic with a relatively small AOO (48 km2, EN) and EOO (4,009 km2, EN; Table 7 ). Based on this narrow distribution and the fragmented high elevation habitat, it is given a preliminary conservation status of EN (Endangered). Most collections are from the protected area of Mount Cameroon and populations outside this national park will be vulnerable.

Discussion.

Solanum pseudospinosum is a high elevation species that, in our circumscription, only occurs along the so-called Cameroon line that is along the junction of the Congo and West African cratons ( Fitton 1987). We recognise the East African plants identified as S. pseudospinosum by Edmonds (2012) as high elevation, particularly pubescent populations of the more widespread S. tarderemotum . The key characters distinguishing the two species are the pedicel that remains on the plant after fruit drop in S. pseudospinosum , while those of S. tarderemotum drop with the berries and the prominent articulations at the bases of the pedicels in S. tarderemotum . Other differences of S. pseudospinosum , but less easy to see, are the elongate, spathulate calyx lobes in fruit (those of S. tarderemotum are less hyaline and usually pointed) and the few-flowered congested inflorescences (as opposed to elongate, rather evenly spaced flowers along the rhachis in S. tarderemotum ). Solanum pseudospinosum is also sympatric with wild populations of S. scabrum from which it can be distinguished by the dense spreading pubescence, the presence of stone cells in the fruit and the somewhat accrescent calyx lobes appressed to the berry.

The Cameroon line along which Solanum pseudospinosum occurs runs from the island of Bioko to the Tchabal Mbabo plateau and is of volcanic origin with activity beginning the Cretaceous period, but peaking in the Pliocene and continuing to the present ( Wright et al. 1985). The island of Bioko is Pliocene in age ( Wright et al. 1985) and has been connected and separated from the mainland several times throughout the Pleistocene. Smith et al. (2000) found only weak relationships between populations of two bird species along the Cameroon line, suggesting they evolved by allopatry. Plant diversity in this area is amongst the highest in tropical Africa ( Sosef et al. 2017), but despite having within it some well-sampled units (e.g. Mount Cameroon), the northern extent of the escarpment is very poorly sampled ( Sosef et al. 2017: 12, fig. 7). Better field knowledge of the distribution of S. pseudospinosum , coupled with better knowledge of its habitat preferences and genetic structure, will help unravel the evolutionary history of this narrowly endemic species.

Solanum pseudospinosum is a tetraploid species with unknown parentage ( Olet et al. 2015). The species has not been included in any previous crossing and molecular studies that have focused on understanding the parental origin of the hexaploid species S. nigrum and S. scabrum (e.g. Edmonds 1977, 1979a; Ganapathi and Rao 1985, 1986b, 1986c, 1987a, 1987b; Jacoby et al. 2003; Jacoby and Labuschagne 2006; van der Walt et al. 2008; Poczai and Hyvönen 2011). It could represent one of the tetraploid parents of the hexaploids and should be included in further studies focused on understanding the origin of the polyploids.

All of the species described by Bitter (1912a) and here recognised as synonyms of S. pseudospinosum were described from collections housed at B that were destroyed in World War II ( Vorontsova and Knapp 2010) and we have placed these in synonymy based on their very detailed descriptions. It is to be hoped that duplicates of these collections will eventually be found in herbaria not yet investigated or digitised. The differences between this species and that of C.H. Wright were very small, based on leaf size and on numbers of stone cells; we consider these minor differences insignificant.

Specimens examined. Cameroon. N slope of Cameroun Mountain, 7 Apr 1985, Thomas 4642 (MO); Nord-Ouest: Piste Acha-Abaw au Lac Oku, 40 km NE Bamenda, 5 Dec 1974, Letouzey 13444 (K); Sud-Ouest: Mt. Cameroon, above Buea, nr Hut 2, 1 Apr 1952, Boughey GC6933 (K); Mt. Cameroon, Johann-Albrechtshöhe, 30 Jan 1962, Breteler et al. MC39 (K); Mt. Cameroon, 9 Oct 1992, Cheek & Sidwell 3664 (K, SCA, WAG, YA); Mt. Cameroon, 4 Nov 1993, Cheek et al. 5352 (K); Mt. Cameroon, 17 Feb 1927, Dalziel 8335 (E, K); Mt. Cameroon, nr Hut 2, Jan 1967, Guile, 1560 (MO); Bamenda, Southern Cameroons, Bamenda Division, Bafut-Ngemba forest reserve, 23 Feb 1958, Hepper 2146 (K); Mt. Cameroon, beside Hut 2, 6 Apr 1937, Hutchinson & Metcalfe 54 (K); Mt. Cameroon, To no 3 Hut, Jan 1931, Maitland 1301 (K); Mt. Cameroon, Feb 1931, Maitland 1333 (K); Mt. Cameroon, Jan 1862, Mann 1321 (K); Mt. Cameroon, NW de Buea, 31 Mar 1981, Meijer 15435 (MO); Mt. Cameroon, Kamerun-Berg, oberhalb Buea, 22 Dec 1928, Mildbraed 10888 a (K); Mt. Cameroon, Hut 2, 21 Dec 1958, Morton 758 (K); Mt. Cameroon 2 mi W of Mann’s Spring, 29 Dec 1958, Morton K874 (K); Mt. Cameroon, around Mann’s Spring, 3 Dec 1996, Nning et al. 52 (K, MO, SCA, YA); Mt. Cameroon, Bokwango, 5 Oct 1992, Thomas 9348 (K).

Equatorial Guinea. Bioko: cumbre del pico Basilé, 3 Jul 1986, Fernández-Casas 10156B (K, MO); Carretera del pico Basilé, 10 Jul 1986, Fernández-Casas 10313 (BM, K); Bioko Norte: Pico Basilé, Distr. Barney, cruce de la Virgen de Bisila, 12 Dec 2007, Cabezas et al. 907 (MA); Cumbre del pico Basilé, 8 Oct 1986, do Carvalho 2544 (BM, H, K, MA, MO); Cumbre del pico Basilé, 3 Jul 1986, Fernández-Casas 10157 (BM, K, MO); Pico Basilé, cumbre del pico, 5 Feb 1989, Fernández-Casas 11184 (K, MA).

Figures

Figure 34. Solanum pseudospinosum A Flowering habit B Inflorescence C Bud D Flower at anthesis E Fruiting habit F Detail of adaxial leaf surface G Eglandular trichome H Glandular trichome I Infructescence J Seed (A, C, D Casas 10313; B Casas 10157; E, J Breteler et al. 39). Scale bar: 4 cm (A, E), 1 cm (B, I), 5 mm (C), 7 mm (D), 4 mm (F), 0.8 mm (G-H) and 2.5 mm (J). Drawing by L. Smith.

Figure 35. Solanum pseudospinosum A Infructescence B Fruiting pedicels remaining after fruits have dropped C Calyx in fruit remaining appressed (A-B W. Meijer 15435).

Figure 36. Distribution of Solanum pseudospinosum.