Petalidium etendekaense Swanepoel, E.Tripp & A.E.van Wyk, 2024

Swanepoel, Wessel, Manzitto-Tripp, Erin A., Dexter, Kyle G. & Van Wyk, Abraham E., 2024, Petalidium etendekaense (Acanthaceae), a new species from Namibia, with notes on the taxonomic identity of P. glutinosum, Phytotaxa 636 (1), pp. 35-47 : 37-45

Petalidium etendekaense Swanepoel, E.Tripp & A.E.van Wyk , sp. nov. ( Figs 1–3 )

Diagnosis:— A woody shrub up to 1 m tall, morphologically most similar to Petalidium glutinosum and P. variabile , but differing from both in having indumentum on leaves tomentose-strigulose (vs. strigulose); corolla lobes ascending-spreading with respect to the corolla tube axis (vs. patent or upper lobes in line with tube to sub-patent), lobes all similarly coloured (vs. anterior and sometimes lateral lobes differently coloured or shaded than upper lobes); differing from P. glutinosum in absence of long, simple trichomes on bracteoles abaxially; differing from P. variabile in absence of short geniculate simple trichomes on bracteoles abaxially.

Type: — NAMIBIA. Kunene Region: Farm Driefontein 716, ca. 4 km south of old homestead, southern tributary to Springbok River , 2014 AC, 912 m, 4 April 2023, Swanepoel 624 (holotype WIND!; isotypes PRE!, PRU!) .

Erect woody shrub to 1 m tall. Stems: single main stem up to 100 mm long, 80 mm diam. or multi-stemmed from just above ground level, stems up to 30 mm diam., bark fissured, cream-coloured, greyish white or grey-black; bark on distal stems sometimes peeling in long, thin, narrow strips; young stems quadrangular to terete, indumentum as for leaves, glabrescent; green with cystoliths linear, inconspicuous. Leaves opposite and decussate; laminae ovate, elliptic, rarely suborbicular, 12–37 × 9–27 mm, apices acute, rounded or retuse, bases cuneate, rounded or truncate, not or slightly decurrent, flat, subconduplicate or often recurved towards apex and margins, margins entire or slightly undulate, midribs and lateral veins prominent especially adaxially, lateral veins 3–5 each side; grey to greyish green or green; cystoliths inconspicuous; indumentum initially of sparse, simple geniculate trichomes, few unevenly bifurcate or dendritic and patent, also with scattered stalked glandular trichomes of variable sizes, more robust ones multi-cellular, eventually with additional, dense, shorter simple geniculate trichomes and appearing tomentose-strigulose; petioles up to 9 mm long or leaves subsessile. Flowers in axillary dichasia, inflorescence main axis up to 100 mm long, secondary branches shorter, main axis and secondary branches becoming spiny with age, tapering, apices blunt to sharp; bracts foliaceous, oblanceolate or narrowly elliptic, up to 7.0 × 2.3 mm, apices acute, sessile, indumentum similar to that of leaves; pedicels (below bracteoles) 0.5–4.0 mm long; bracteoles symmetrically ovate or narrowly ovate, narrowly elliptic or lanceolate, coriaceous, forming prominent bulging on bracteole pair from which corolla limb emerges from one side, apices attenuate, pale green, midrib straight, venation reticulate, slightly prominent, inconspicuous, pale green, glutinous, slightly aromatic owing to the latter secretion, 11.8–16.1 × 4.2–8.2 mm, abaxially with scattered stalked glandular trichomes of various sizes, smallest ones subsessile, more robust ones multi-cellular, sometimes in addition sparsely puberulous in places, adaxially with stalked glandular trichomes, strigose towards apex and margins, margins lanate, cystoliths visible adaxially. Calyces 8.1–8.4 mm long including basal tube of 1.1–2.3 mm, adaxially strigulose with few small stalked glandular trichomes, abaxially puberulous with scattered stalked glandular trichomes of varying length; lobes 4, narrowly triangular, acute, 5.2–7.3 mm long, anticous lobe indistinctly bifid. Corollas 22–26 mm long with lobes straightened, narrow unexpanded portion cylindrical, sometimes slightly curved towards anterior side, slightly flattened laterally, 9.6–11.6 mm long, 2.6–3.5 mm diam., outside and inside cream-white, expanded portion at slight angle to anterior side of narrow portion, 5.4–7.1 mm long; outside dark brown to almost black, anterior side brown, herringbone pattern prominently transversely 6- or 7-ribbed, cream-brown, traces prominent, pink to carmine; inside dark burgundy, anterior side glabrous, yellow, terminating in two narrowly triangular separate markings (nectar guides) on proximal portion of anterior lobe; outside of expanded portion (including lobes) and anterior side of narrow portion puberulous distally, trichomes bulbous, inside puberulous on area immediately above insertion of filaments up to mouth, otherwise glabrous; lobes ascending-spreading, upper lobes ovate, 5.4–7.5 × 3.1–4.0 mm, connate for ca. one-third of their length, not or slightly overlapping, apices emarginate, truncate or rounded, lateral lobes oblongovate, 4.4–6.2 × 2.9–3.8 mm, apices truncate, obtuse or retuse, anterior lobe ovate, apex truncate, emarginate or retuse, 5.4–7.6 × 4.9–6.0 mm, all lobe margins entire or denticulate towards apex; lobes adaxially dark burgundy towards bases, carmine to pink towards apices, carmine abaxially; anterior lobe with long, stiff, patent, simple, white, eglandular trichomes towards base, other lobes glabrous or with few widely spaced similar trichomes. Stamens didynamous: filaments inserted dorsally in throat, fused portion 1.9–2.6 mm long, free portion tapering towards apex, slightly flattened, with few widely spaced short stalked glandular trichomes, long filaments 4.7–6.1 mm long, short filaments 2.8–3.7 mm long, outer filament decurrent for 4.6–5.8 mm (ca. halfway) towards base of tube, puberulous, trichomes bulbous; filament curtain (sensu Manktelow 2000) reduced; anthers 2-thecous, thecae elliptic-oblong with minute spurs at base, 2.2–2.5 mm long, mauve or pink, with scattered short stalked glandular trichomes. Gynoecium 17.3–18.4 mm long; ovary ovoid, laterally compressed, 1.5–2.1 × 1.5–1.7 × 1.0– 1.2 mm, situated on fleshy disc, 0.8–1.0 mm long, glabrous; style filiform, 13.5–15.5 mm long, with scattered short eglandular trichomes, stigma lobes linear, slightly flattened, subequal, longer lobe 0.5–1.0 mm long, shorter lobe 0.3–0.8 mm long. Capsule flattened ellipsoid or ovoid, 7.5–8.8 × 3.6–4.6 × 2.2–2.5 mm, chestnut, glossy, sides slightly rugose or smooth, glabrous. Seeds cordate, 4.1–4.3 × 3.0– 3.4 mm, surfaces and margins densely covered with white hygroscopic trichomes.

Phenology:— Flowers have been recorded from November to June and fruits from December to July.

Distribution and habitat:— At present, Petalidium etendekaense is known from the area to the northwest, west, and southwest of Bergsig, specifically on the farms Rooiplaat, Wêreldsend, Driefontein, and Krone. Plants have also been recorded further west, extending towards the Skeleton Coast National Park, reaching as far north as the Samanab River ( Fig. 4 ). The species is restricted to soils derived from basaltic rocks of the Etendeka Group ( Milner et al. 1994) and is the dominant woody dwarf shrub found on hillsides, along seasonally dry riverbeds and at the base of rocky outcrops. Its occurrence is documented at elevations of 380–1060 m a.s.l., about 25–85 km inland from the Atlantic Ocean. The average annual rainfall in the area is less than 100 mm ( Atlas of Namibia Team 2022).

Conservation status:— Petalidium etendekaense is likely more common and widespread in suitable habitats than currently documented. It is not considered to be in immediate conservation danger, as it is found in sparsely populated or uninhabited areas and does not seem to be utilised by humans. The estimated extent of occurrence is 2000 km ², with six subpopulations. Since there is no known decline in population size or numbers, it is classified as Least Concern (LC) according to the IUCN (2012) criteria.

Etymology:— The specific epithet “ etendekaense ” refers to the Etendeka Tableland landscape (or plateau), which was formed by lava flows (basalts of the Etendeka Group) ca. 132 million years ago due to sea floor spreading and the formation of the South Atlantic. This plateau is a component of a significant igneous province, and its counterpart in Brazil is represented by the Paraná basalts ( Goudie & Viles 2015). The name “Etendeka” is derived from the Otjiherero word for layered or stacked, in reference to the flat-topped mountains that define the landscape ( Detay & Detay 2017, Atlas of Namibia Team 2022). The entire known population of Petalidium etendekaense is located in this landscape, growing on soils derived from Etendeka basalt.

Notes:— Unless the indumentum of the bracteoles is investigated, it is difficult to differentiate among Petalidium etendekaense , P. glutinosum , and P. variabile in herbarium material. For example, the specimens Giess 3861 and Giess & Barnard 7960, both representing P. etendekaense , were mistakenly cited under P. variabile in the Prodromus einer Flora von Südwestafrika ( Meyer 1968). However, in the field, when in flower, P. etendekaense can be easily distinguished from the others by the combination of a dark burgundy corolla throat and proximal part of the lobes, contrasting with the carmine to pink distal part of the lobes ( Figs 5 & 6 ). Additionally, it has an ovate anterior lobe that is longer than wide and the lobes exhibit an ascending-spreading orientation. Table 1 provides some morphological features to differentiate P. glutinosum and P. variabile .

The distribution range of the new species slightly overlaps with that of P. glutinosum in the Springbok River to the west of Bergsig ( Fig. 4 ). Although P. etendekaense has been collected within ca. 15 km from P. glutinosum , they have not been recorded to occur sympatrically to date. Petalidium variabile occurs to the south of P. etendekaense , from south of the Huab River and Sorris-Sorris southwards ( Fig. 4 ).

All the mentioned taxa are from the group of plants with irregular, four-parted calyces ( Obermeijer 1936, Tripp et al. 2017). This is also the clade that shows the highest rates of net species diversification in Petalidium , and occupies the most arid areas within the geographic distribution of the entire genus ( Loiseau et al. 2023).

Additional specimens examined (paratypes):— NAMIBIA, Kunene Region:—1913: Samanab River , 10 km from dune field, (–CD), 385 m, 15 July 2019, Swanepoel 630 ( WIND!). —2013: Farm Wêreldend [Wêreldsend], etwa 1 meile w. des Hauses in kleinen Rivier, (–BB), 12 November 1961, Giess 3861 ( PRE!, WIND!) ; Farm Rooiplaat OU 710 , in Grob Geroellsenke , (–BB), 14 April 1964, Giess & Barnard 7960 ( WIND!) ; Gui-Tsawisib River, Road C39, opposite Wêreldsend Veterinary Gate , (–BB), 775 m, 4 April 2023, Swanepoel 626 ( WIND!) ; Tributary to Springbok River along track to Huab River , 2 km south of Wêreldsend Veterinary Gate, (–BD), 766 m, 4April 2023, Swanepoel 627 ( WIND!) ; 1 km south of Ugibputs, along track to Huab, River , (–BD), 720 m, 4 April 2023, Swanepoel 628 ( WIND!) ; 6.4 km south of Ugibputs, along track to Huab River, (– DB), 741 m, 4 April 2023, Swanepoel 629 ( WIND). —2014: Huab drainage, farm Krone 721, NNE old farmhouse, (– AC), 560 m, 28 March 1996, Ward 13731 ( WIND!) ; 8.6 km west of Bergsig towards Torra Bay on C39, (–AC), 28 March 2010, Tripp & Dexter 876 ( US!) ; Farm Driefontein 716, ridge above southern tributary to Springbok River , ca. 5 km southeast of old homestead, (–AC), 963 m, 4 April 2023, Swanepoel 625 ( WIND!) .

Figures

FIGURE 1. Petalidium etendekaense. Habitat and habit. A. Several plants (foreground and greyish dwarf shrubs in the background) growing on the gently sloping ground between rocky hills in stony soil derived from Etendeka Group basalt. B. Mature plant (ca. 600 mm high) with very short main stem and greyish appearance. Photographs by W. Swanepoel.

FIGURE 2. Petalidium etendekaense. Habitat and habit. A. Plant (ca. 350 mm high) with relatively long single main stem, growing among rocks of Etendeka Group basalt. B. Part of fissured greyish white bark on woody stem (ca. 60 mm in diam.) at base of mature plant. C. Plants growing in abundance (right front and low greyish shrublets in background) among rocks of Etendeka Group basalt on a hill slope. Photographs by W. Swanepoel.

FIGURE 3. Petalidium etendekaense. Morphology of flowers and leaves. A, B, C, D. Flowers in front view, each from a different plant to show variation; note all corolla lobes of a flower being similarly coloured. E. Flower and bracteole in lateral view; note absence of long, simple trichomes on bracteoles. F. Flower with bracteoles, viewed from below. G. Shoot showing leaves with dense white indumentum and persistent bracteoles of spent flowers, some enclosing developing fruit. Scale bar = 3 mm. Photographs by W. Swanepoel.

FIGURE 4. Known distribution of Petalidium etendekaense (large black dots), P. variabile (small black dots), and P. glutinosum (open circles): overlap between P. variabile and P. glutinosum depicted as an open circle with central dot. Based on herbarium specimens in Herbs. PRE and WIND.

FIGURE 5. Petalidium glutinosum (= P. pilosibracteolatum), flower and leaf morphology. A. Shoot showing leaves with dense white indumentum and a flower in front view. B. Leafy shoot showing flowers (one faded) and bracteoles in lateral view; note long, simple trichomes on bracteoles. C, D. Flowers from different plants in front view; front corolla lobe being differently coloured than the others. E. Flower and bracteoles in lateral view; note long, simple trichomes on bracteoles. Scale bar = 3 mm. Photographs by W. Swanepoel.

FIGURE 6. Petalidium variabile, flower and leaf morphology. A. Shoot showing leaf with dense white indumentum and open flower. B, C. Flowers of different plants in front view; note front corolla lobe being differently coloured than the others. Scale bar = 3 mm. Photographs by W. Swanepoel.