Stylochaeton sekhukhuniense Struwig, S.J.Siebert & A.E.van Wyk, 2023

Struwig, Madeleen, Le Roux, Precious T., Siebert, Stefan J. & Van Wyk, Abraham E., 2023, Two new species of Stylochaeton (Araceae: Aroideae) from South Africa, Phytotaxa 620 (1), pp. 1-17 : 11-14

Stylochaeton sekhukhuniense Struwig, S.J.Siebert & A.E.van Wyk , sp. nov. ( Figs 4B & 7 )

In habit approaching forms of S. borumense with sagittate leaf blades, but differs in the spadix having male and female flowers contiguous and the filaments not thickened (vs. male and female flowers separated by a zone of few to 10 sterile flowers and filaments thickened towards apex). Morphologically most similar to S. glaucophyllum , differing in having scattered trichomes on petiole immediate distal to petiolar sheath (vs. glabrous); petiolar sheath distinctly ligulate (vs. not ligulate); leaf blade short, 44–180 mm (vs. longer, 112–332 mm) with basal and apical lobes elliptic (vs. linear), green (vs. greenish blue); spadix entirely enclosed by spathe-tube (vs. protruding somewhat from spathe-tube).

Type: — SOUTH AFRICA. Mpumalanga: Steelpoort, farm adjacent to Kennedy’s Vale where Dwars River breaks through hill, 2430 CC, 10 March 1998, Siebert 390 (holotype PRU!) .

Geophyte, with vertical rhizome, ca. 48 × 9 mm; roots stout, not tuberous. Cataphylls fibrous. Leaves 2–5; petioles 100–300 mm long, lower ¼ usually covered with transverse purplish and greenish white speckles or bands, upper ⅔ green; petiolar sheath up to 75 mm long, with transverse bands, apex distinctly ligulate, extend into two slender, curling elongations (ligulae) ca. 30 × 3 mm, apex obtuse, margin undulate; ¼ of petiole immediately distally to sheath often with scattered trichomes, distal ⅔ glabrous; blade sagittate, apical lobe 24–100 × 5–30 mm, elliptic, mucro up to 12 mm long, basal lobes 20–80 × 8–26 mm, elliptic, tips rounded; green, glabrous, margin entire or slightly repand, midvein and basal veins prominent. Inflorescence with peduncle ca. 34 × 7 mm; spathe ca. 111 mm long, glabrous, thickened, inside (ventrally) cream-coloured to pale buff, outside (dorsally) brownish olive-green, distinctly or vaguely longitudinally striped with creamish lines (veins), veins somewhat ribbed on outer surface; basal tube erect, ca. 29 mm long, upper part cylindrical, ca. 14 × 10 mm, lower part inflated, globose, ca. subterranean, ca. 15 × 15 mm; upper limb ca. 82 × 22 mm, oblong, acuminate, apex with cusp ca. 7 mm long. Spadix ca. 24 mm long, not projecting from spathe-tube; monoecious; staminate part cylindric, ca. 15 × 6 mm, many-flowered, contiguous with pistillate part, pistillate part hemispherical, ca. 9 × 11 mm, flowers in two rows. Staminate flowers with perigone cupular, rhomboid, thickened, connate; stamens 3, filaments filiform, ca. 1 mm long; anthers globose, basifixed, extrorse. Pistillate flowers with perigone cupular, contracted at the mouth, subregular, 4–6-sided, thickened, connate; stamens 0; ovary subglobose, 1- locular; style stout, ca. 3 mm long, exserted, stigma discoid, 1 mm broad. Fruit unknown, but infructescence suspected to be borne partly below-ground.

Phenology:— Inflorescences have been recorded in October and November (spring); infructescences not seen.

Distribution and habitat:— Stylochaeton sekhukhuniense is endemic to the Sekhukhuneland Center of Endemism, Limpopo province, South Africa ( Fig. 8 ). It grows in bushveld (savannah) on well-drained stony, clay soil along gentle to moderate slopes. It is found in open Sekhukhune Mountain Bushveld (mapping unit: SVcb 28) and Sekhukhune Plains Bushveld (SVcb 27) ( Mucina & Rutherford 2006). The dominant woody species are Dichrostachys cinerea ( Linnaeus 1753: 520) Wight & Walker-Arnott (1834: 271) , Searsia engleri ( Britten 1900: 316) Moffett (2007: 168) and Terminalia prunioides Lawson (1871: 415) . Co-occurring forbs include Justicia flava Vahl (1791: 15) , Sansevieria hyacinthoides ( Linnaeus 1753: 321) Druce (1914: 423) and Waltheria indica Linnaeus (1753: 673) .

Stylochaeton sekhukhuniense prefers the same soil conditions as S. glaucophyllum . However, its associated soil seems to be richer in S, but poorer in Ca ( Table 1 ). The serpentine soil effect is still maintained. Both species exhibited bio-accumulation of boron, whereas S. glaucophyllum accumulated 2× the total amount in the soil, S. sekhukhuniense accumulated at 4×. Boron is an essential element required for plant growth, and plants accumulate this element when levels in the soil are low ( Pereira et al. 2021).

Conservation status:— Stylochaeton sekhukhuniense is assessed as Endangered according to IUCN Red List criteria (2012) due to the extent of occurrence <5000 km 2 (i.e., 218 km 2), but with less than 10 km 2 (i.e., 3.4 km 2) area of occupancy, and the existence of seven known populations ( Siebert et al. 2002). Stylochaeton sekhukhuniense is, however, increasingly subjected to habitat degradation as a result of extensive mining activities and urban expansion in the Sekhukhuneland region.

Etymology:— The specific epithet “ sekhukhuniense ” refers to “Sekhukhuneland”, the geographical region to which the species is endemic. The region derives its name from that of the 19th century Bapedi king, Sekhukhune I [ca. 1814–1882] ( Raper et al. 2014). As English and Afrikaans names for the new species we propose the name “Sekhukhune bushveld arum” and sekhukhunebosveldvarkoor respectively.

Notes:— Stylochaeton sekhukhuniense is less well known than S. glaucophyllum and there is a need for more information on the former, notably its inflorescence.The species’ infructescence has also not yet been recorded, although based on similarities in inflorescence morphology, it is suspected to be rather similar to that of S. glaucophyllum and also borne below- or partly below-ground. The taxon is nevertheless immediately recognizable by the distinctly ligulate petiolar sheaths.

The apex of the cataphylls or petiolar sheaths of several species of Stylochaeton , e.g. S. crassispathum Bogner (1984: 77) , S. milneanum Mayo (1985: 47) , S. oligocarpum and S. puberulum Brown (1901: 188) are auriculated. The petiolar sheath of S. sekhukhuniense , however, extends into two slender elongations (ligulae) ca. 30 mm long. The leaf shape of S. sekhukhuniense is, as with S. glaucophyllum , very similar to the leaves of S. borumense , S. hypogeum and S. oligocarpum , with the most easily observed difference in the length of the basal and apical lobes. The basal lobes in these species are shorter (25–120 mm) than the apical lobe (63–200 mm), ca. 1:2, whereas in S. sekhukhuniense the basal lobes are almost the same length (20–80 mm) as the apical lobe (24–100 mm). The leaf blades of S. natalense ( Fig. 1 ) are larger (50–310 × 18–260 mm) and variable in shape (ovate to triangular to sagittate-hastate or hastate-cordate) while the blades of S. sekhukhuniense are smaller (44–180 × 5–30 mm) and sagittate ( Table 2 ).

This species was referred to as Stylochaeton sp. S 390 in Siebert et al. (2001) and Stylochaeton sp. S 672 in Siebert et al. (2002: 138).

Differences between Stylochaeton glaucophyllum and S. sekhukhuniense :— Until additional material of S. sekhukhuniense is sourced, the two new species are mainly differentiated based on their vegetative morphology. The apical and basal leaf blade lobes of S. glaucophyllum are linear in shape, whereas for S. sekhukhuniense these are elliptic. The leaf blade of S. glaucophyllum is longer (112–332 mm) than that of S. sekhukhuniense (44–180 mm). The leaf blades of S. glaucophyllum are greenish blue and leathery, whereas those of S. sekhukhuniense are green and soft. The petiolar sheath of S. sekhukhuniense extends into two slender, curling elongations (ligulae) which are absent in S. glaucophyllum . The reproductive morphology of the two species, as known at present, is very similar. The inflorescences may differ slightly in spathe colour, but at this stage very few of them were seen in S. sekhukhuniense , hence we are hesitant to claim any colour differences of diagnostic significance. What does seem to be a potentially useful taxonomic distinction, if confirmed to be species-specific by more material, is that in S. glaucophyllum the spadix protrudes a short distance from the spathe tube while in S. sekhukhuniense it is entirely enclosed within.

Additional specimens examined (paratypes):— SOUTH AFRICA. Limpopo: Steelpoort, Richmond, opposite Richmond turn-off (2430 CC), 10 December 1998, Siebert 672 ( PRU!) ; Burgersfort, banks of Steelpoort River (2430 CB), 11 November 2009, Siebert 4451 ( PUC!) .

Figures

FIGURE 1. Stylochaeton natalense, general morphology. A. Plants with ca. sagittate leaves. B. Inflorescence. C. Plants with ca. hastate leaves. D. Developing infructescence. Photographs: W. McCleland (A), R.G.C. Boon (B), G. Nichols (C) and A.E. van Wyk (D).

FIGURE 4. Stylochaeton glaucophyllum, morphology of infructescence. A. Plant with mature infructescence, the latter borne partly below-ground; some soil has been removed from around the infructescence to show the upper purplish surface visible above-ground and the lower whitish part that were below-ground. B. Infructescence removed from the soil; the bulk of it was below-ground, with only the area tinged greenish purple at the top exposed above-ground. C. Infructescence with purplish colouration indicating that most of the berries were borne above-ground. D. Infructescence that were mainly borne below-ground as indicated by the broad whitish lower part. Photographs:A.E. van Wyk (A, C & D) and S.J. Siebert (B).

FIGURE 7. Stylochaeton sekhukhuniense, morphology of leaves and inflorescence. A. Basal part of leaves showing the distinctly ligulate petiolar sheaths; note hairs on the part of the petiole above the sheath. B. Petiolar sheaths with curly ligulae, viewed from above. C. Inflorescence with strongly forward curving spathe. D. Inflorescences removed from plant and soil cleaned from basal parts; viewed from different angles. Photographs: A.E. van Wyk (A, C & D) and S.J. Siebert (B).

FIGURE 8. Topographical map showing the known distribution (black dots) of Stylochaeton sekhukhuniense. The insert shows a map of southern Africa with names of countries; the grey rectangle indicates the area depicted by the topographical map.