Petalidium karasbergense Swanepoel & A.E.van Wyk, 2023

Swanepoel, Wessel & Van Wyk, Abraham E., 2023, Petalidium karasbergense (Acanthaceae), a new species from Namibia, Phytotaxa 609 (1), pp. 1-9 : 2-8

Petalidium karasbergense Swanepoel & A.E.van Wyk , sp. nov. ( Figs 1 & 2 )

Diagnosis: —A woody dwarf shrub up to 1 m tall, morphologically most similar to Petalidium parvifolium , differing in having the distal stems and lateral branchlets tapering to a blunt or spinescent apex (vs. cylindric); indumentum on vegetative parts strigulose (vs. scattered sessile glandular trichomes); young growth not glutinous (vs. covered with a glutinous secretion [glossy]); leaf lamina subconduplicate towards recurved apex (vs. flat or subreduplicate, apex slightly recurved); bracteoles free or almost free from base (vs. distinctly connate towards base), indumentum (abaxially) on bracteoles strigulose (vs. scattered sessile glands, towards base sometimes with very short stalked glandular trichomes in addition).

Type: — NAMIBIA. || Kharas Region: Groot Karasberge [Great Karas Mountains], Farm Tsaraxaibis 275, ca. 2 km north of Smôrenswind homestead along Road D612, opposite reservoir, 2719AD, 985 m, 24 August 2022, Swanepoel 616 (holotype WIND!; isotypes PRE!, PRU!) .

Woody, dwarf shrub up to 1 m tall. Stems multi-stemmed from just below or above ground level from a thick rootstock or main stem, up to 150 mm in diam., bark rough and fissured, grey, grey-white or grey-black; distal stems and lateral branchlets tapering to a blunt or spinescent apex, some bearing brachyblasts with tightly arranged phyllopodia (persistent petiolar bases, ca. 0.5–1.0 mm long), bark smooth or longitudinally fissured, cream or white, lateral branchlets usually decussate; young stems strigulose, glabrescent. Leaves opposite and decussate on new shoots, fascicled on older stems, lateral branchlets and brachyblasts, subsessile, petiole up to 1.6 mm long; lamina oblanceolate to broadly oblanceolate or narrowly obovate, flat, often subconduplicate towards apex, up to 19 × 7 mm, dark green, strigulose with trichomes pointing backwards, glabrescent, apex truncate, emarginate or acute, recurved, margin entire, often tinged maroon, midrib and 1–3 principal lateral veins slightly prominent adaxially, midrib conspicuous abaxially, yellow-green, darker in herbarium material, cystoliths linear, inconspicuous. Flowers solitary, axillary; bracts absent; pedicels (below bracteoles) up to 4 mm long; bracteoles separate from base or connate ≤ 1 mm, elliptic or narrowly ovate, symmetrical, flat, membranaceous, apex attenuate, acute or obtuse, base cuneate or rounded, cream-brown, venation reticulate, prominent adaxially, conspicuous, green or green-brown, strigulose both sides and on margins, glabrescent, 10.5–11.5 × 5.0– 6.2 mm, cystoliths not visible. Calyx ca. 6 mm long including basal tube of ca. 2 mm long, glabrous except for few short-stalked glandular and simple trichomes towards lobe apices; lobes 5, regular, narrowly triangular, acute, unequal, 3.5–4.0 mm long. Corolla with narrow unexpanded portion of tube cylindrical, slightly narrowing towards expanded part, laterally slightly flattened, glabrous except for few short-stalked glandular trichomes, ca. 27 mm long with lobes straightened, limb ca. 21 mm diam., narrow portion ca. 10 mm long, ca. 2.9 mm diam., expanded portion ca. 10 mm long, inside of narrow portion puberulous distally, inside of expanded portion cinnamon, puberulous proximally, anterior part with few long stiff white simple trichomes; lobes patent, obovate, apices widely retuse, margins entire, upper lobes free, overlapping, ca. 7.8 × 7.0 mm, lateral lobes ca.7.1 × 7.1 mm, front lobe ca. 8.0 × 7.5 mm, lobes white or cream-white, occasionally with a faint lilac tinge, two narrowly triangular nectar guides on front lobe pale yellow with purple patches, two narrowly triangular markings towards base of lateral lobes purple; palate outside cinnamon towards base of expanded portion, prominently transversely ca. 10-ribbed, especially on inside. Filaments 4, didynamous, inserted dorsally in throat, fused portion of filaments ca. 1.6 mm long, free portion tapering towards apex, slightly flattened, sparingly puberulous towards base, long filaments ca. 8.8 mm long, short filaments ca. 6.2 mm long, outer filament trace decurrent to base of tube, puberulous; filament curtain phaulopsoid ( Manktelow 2000); anthers 2-thecous, thecae oblong, equal, ca. 2.5 mm long with few simple trichomes. Gynoecium ca. 19.7 mm long; ovary ovoid, laterally compressed, 2.0 × 1.3 mm, inserted on a fleshy disc, glabrous; style filiform, ca. 15.8 mm long, puberulous towards base, stigma lobes linear, unequal, longer lobe ca. 1.2 mm long, shorter lobe ca. 0.5 mm long. Capsule not seen.

Phenology: —Flowers and developing fruit have been recorded in January, March, April, and August.

Distribution and habitat: —At present, Petalidium karasbergense is only known from the Groot Karasberge and environs where it occurs in small colonies on arid stony slopes, 260–310 km from the Atlantic Ocean at elevations of 800–900 m ( Fig. 3 ). Plants grow in Karas Dwarf Shrubland on shallow soil among sandstone rocks ( Fig. 1 ) of the sedimentary Nama Group ( Mendelsohn et al. 2002). Average annual rainfall in the area is 100–200 mm ( Mendelsohn et al. 2002).

Conservation status: — Petalidium karasbergense is known from only one location with four small subpopulations, all of which are on commercial farmland. It is locally common at the type locality with about 70 plants seen and all in good condition. Although a brief search at various other localities with seemingly suitable habitat did not reveal any plants, it is probably more widespread than currently known. Due to the small, estimated population size of less than 1000 mature individuals, a conservation status of Vulnerable VU D1 is proposed for Petalidium karasbergense ( IUCN 2012) .

Etymology: —The specific epithet refers to the Groot Karasberge in southeastern Namibia, the mountain complex to which the new species is confined.

Notes: —In addition to P. parvifolium (see “Diagnosis” above), the new species can be confused with P. lucens and P. mannheimerae Swanepoel et al. (2022: 5) due to similarities in the habit, leaves and flowers. However, the indumentum on vegetative parts of P. karasbergense is strigulose (vs. dense short simple or stellate trichomes, usually with isolated dendritic trichomes in addition [ P. lucens ]; puberulent, usually with robust, multi-cellular stalked glandular or eglandular trichomes in addition [ P. mannheimerae ]). The leaves of P. karasbergense are recurved at the apex and not succulent (vs. apex not recurved [ P. lucens ] and semi-succulent [ P. mannheimerae ]). The corolla lobes of P. karasbergense are predominantly white or cream-white and superficially similar to those of P. mannheimerae (vs. mauve [ P. lucens ]). The distribution ranges of P. lucens and P. karasbergense marginally overlap ( Fig. 3 ), although at no locality has the two species been found to grow sympatrically. Petalidium mannheimerae is endemic and P. lucens near-endemic to the Gariep Centre of Endemism in Namibia and South Africa ( Van Wyk & Smith 2001, Swanepoel et al. 2022). Some of the morphological features to distinguish between Petalidium karasbergense , P. parvifolium , and P. lucens are provided in Table 1 ; also see Fig. 4 .

All the mentioned species are from the infrageneric group composed of plants with regular, five-parted calyces ( Obermeijer 1936, Tripp et al. 2017).

Additional specimens examined (paratypes): — NAMIBIA, || Kharas Region: Keetmanshoop District, Aob , Great Karas Mountains [Groot Karasberge], 2719 AA , 18 April 1924, Dinter 5150 & 5154 ( PRE!); Karasburg, Numdis , in rante, 2719 AD , January 1975, Auret 5617 ( PRE!); Karasburg District , along the main road from Karasburg to Ariamsvlei, 2819AB, 811 m , 6 March 2008, Hasheela HHa0020 ( PRE!, WIND!) .

Figures

FIGURE 1. Petalidium karasbergense, habitat and habit. A. Plant (ca. 50 cm high) with open flower. B. Old, stunted plant showing thick, woody stem (ca. 8 cm in diameter). C. Mature plant (ca. 80 cm high) in habitat; surrounded by sandstone rocks of the Nama Group. Photographs by W. Swanepoel.

FIGURE 2. Petalidium karasbergense, morphology of flowers, stems, and leaves. A. Flower in front view. B. Flower with bracteoles in lateral view. C. Flower viewed from below; note the bracteoles that are not connate at the base, hence the calyx is clearly visible over the whole of its length. D. Branchlet showing an open flower, persistent bracteoles, and leafy side shoots (some tapering to a spinescent tip). E. Enlarged portion of Auret 5617 in Herb. PRE showing three brachyblasts, the longest one with several phyllopodia, the latter each ca. 1 mm long. Photographs by W. Swanepoel (A–D) and A.E. van Wyk (E).

FIGURE 3. Known distribution of Petalidium karasbergense (black dots; ●) based on specimens in Herbs WIND and PRE.Also depicted are the distribution ranges of P. parvifolium (A. blue), P. lucens (B. orange), and P. mannheimerae (C. green). Note that as a result of further field work the ranges of P. parvifolium and P. lucens as depicted in Swanepoel et al. (2022) have been slightly adjusted on the present map.

FIGURE 4. Petalidium parvifolium, flower and leaf morphology. A. Branchlet showing glossy, non-succulent leaves; blade ± flat and margins without long, robust, multi-cellular trichomes and isolated, robust, stalked glandular trichomes (present in P. mannheimerae). B. Flower in front view showing nectar guides. C. Flower in lateral view showing white bracteoles with conspicuous venation. Photographs by W. Swanepoel. Republished from Swanepoel et al. (2022).