Petalidium sesfonteinense Swanepoel & E.Tripp, 2022

Swanepoel, Wessel & Manzitto-Tripp, Erin A., 2022, Petalidium sesfonteinense (Acanthaceae), a new species from the Kaokoveld, Namibia, Phytotaxa 549 (2), pp. 127-135 : 128-134

Petalidium sesfonteinense Swanepoel & E.Tripp , sp. nov. ( Figs. 1–3 )

Diagnosis:— A woody shrub up to 1 m tall, morphologically most similar to Petalidium kaokoense and P. variabile ; from P. kaokoense it differs in having an indumentum on vegetative parts of relatively slender dendritic trichomes (vs. relatively stout stalked-stellate trichomes interspersed with dendritic trichomes), bracts oblanceolate (v s. linear-oblanceolate or linear-lanceolate), bracteoles symmetrically elliptic or narrowly ovate with long multicellular glandular trichomes and venation weak or not prominent (vs. asymmetrically elliptic-oblong, long multicellular glandular trichomes absent, reticulation prominent), corolla with expanded portion of tube glabrous externally (vs. strigose), upper lobes connate for 20–30% of their length and obovate (vs. 50%, oblong), lobes variously coloured (white, pink, magenta, apricot, yellow, or cream) and in contrast with maroon throat and mouth (vs. lobes, throat, and mouth maroon), seeds cordate (vs. discoid or ovate); from P. variabile it differs in indumentum on young stems different from that on leaves (vs. similar), indumentum on leaves dendritic (vs. strigose), bracteoles usually with long multicellular trichomes (vs. absent), expanded portion of corolla tube glabrous externally (vs. with short appressed simple trichomes), all corolla lobes of same colour and shade (vs. front corolla lobe differently coloured or shaded than other lobes), capsule flattened ellipsoid or ovoid, sides rugose or smooth (vs. flattened ovoid, sides smooth).

Type: — NAMIBIA. Kunene Region: Eastern tributary to Ganamub River in mountainous area between Giribesvlakte and Hoanib River, 1913AB, 477 m, 14 June 2021, Swanepoel 568 (holotype WIND!; isotypes COLO!, PRE!, PRU!) .

Erect woody shrub to 1 m tall; all relatively young vegetative parts with dense indumentum of relatively slender white dendritic trichomes, glabrescent or nearly so. Stems: main stem up to 80 mm diam., bark fissured, creamcoloured or greyish white; bark on distal stems peeling in long, thin, narrow strips; young stems quadrangular with very short simple glandular and short to long eglandular trichomes, often multicellular, also bi-furcating and dendritic or long-stalked stellate trichomes, or trichomes absent; pale green with cystoliths linear or appearing circular. Leaves opposite and decussate, petiolate; laminae narrowly ovate, ovate or elliptic, rarely oblanceolate, 15–44 × 8–27 mm, with dense indumentum of dendritic trichomes, grey to greyish green, becoming green to yellowish green with age, cystoliths inconspicuous; apices acute, obtuse or rounded, sometimes minutely apiculate, bases attenuate, decurrent, rarely rounded, margins entire; midribs prominent both sides, lateral veins 4 to 6 each side, usually not prominent, petioles up to 15 mm long. Flowers in short dichasia, bracts foliaceous, oblanceolate, up to 15.0 × 2.2 mm, apex acute, sessile; pedicels (below bracteoles) 0.5–1.2 mm long; bracteoles symmetrically elliptic or narrowly ovate, coriaceous, forming prominent bulging on bracteole pair from which corolla limb emerges from one side, apex acute, sometimes slightly acuminate or asymmetric, pale green to yellowish green, reticulation slightly or not prominent, midrib straight, venation indistinct, pale green, ca. 10.6–13.3 × 5.4–6.8 mm, abaxially with dense glandular trichomes of various lengths, few to scattered long multicellular glandular trichomes, sometimes with very long multicellular sparsely branched eglandular simple or dendritic trichomes in addition, strigose towards base, sericous adaxially, margin lanate towards apex, cystoliths visible. Calyx 6.6–7.2 mm long including basal tube of ca. 2 mm, strigose in places on both surfaces; lobes 4, lanceolate, acute, anticous lobe indistinctly bifid, anticous and upper ca. 5.6–7.8 × 1.5–1.7 mm, folded lengthwise, laterals 5.6–7.0 × 0.9–1.1 mm. Corolla 20–24 mm long with lobes straightened, narrow unexpanded portion cylindrical, slightly widening towards throat, flattened, ca. 10 mm long, ca. 3.2 mm diam, expanded portion ca. 6 mm long, outside glabrous, inside puberulous on area immediately above insertion of filaments up to mouth, otherwise glabrous; inside of anticous portion of throat yellow, terminating in two narrowly triangular separate markings (nectar guides) on proximal portion of lobes; lobes white, pink, magenta, apricot, yellow or cream, expanded portion (throat and mouth) maroon, front lobe including nectar guides maroon dotted proximally, all lobes patent and with long, stiff, patent, white eglandular trichomes, upper lobes obovate, 5.8–8.2 × 3.5–4.3 mm, connate for 20–30% of their length, overlapping or not, apices rounded, retuse, lateral lobes oblong, 5.2–7.8 × 3.2–4.5 mm, apices retuse or truncate, front lobe broadly obovate, 5.5–8.8 × 6.2–7.2 mm, apex rounded, retuse, all lobe margins entire or irregularly denticulate to dentate; palate prominently transversely 3–7-ribbed. Filaments didynamous, inserted dorsally in throat, each long and short filament fused for ca. 1.4 mm at base, fused part prominent, adnate to tube, free parts tapering towards apex, flattened, with scattered short eglandular trichomes, long filament 4.3–5.7 mm long, short filament 2.2–3.3 mm long; filament curtain reduced ( Manktelow 2000); anthers 2-thecous, thecae oblong, subequal, 2.1–2.9 mm long with scattered short glandular and eglandular trichomes, apex rounded, with minute spurs at base. Gynoecium 12.6–15.6 mm long; ovary ovoid, laterally compressed, 1.5–2.0 × 1.5–1.7 × 1.0– 1.2 mm, situated on fleshy disc, glabrous, ovules elliptic, ca. 0.8 mm long; style filiform, 9.9–12.5 mm long, with scattered short eglandular trichomes, stigma lobes linear, slightly flattened, subequal, longer lobe 0.6–0.8 mm long, shorter lobe 0.5–0.7 mm long. Capsule flattened ellipsoid or ovoid, 6.2–6.5 × 3.3–3.6 × 1.4–2.0 mm, chestnut, glossy, sides rugose or smooth, glabrous. Seeds cordate, ca. 2.6 × 1.8 mm, densely covered with white hygroscopic trichomes.

Phenology:— Flowers have been recorded from March to June. Fruits have been recorded from April to October.

Distribution and habitat:— At present, Petalidium sesfonteinense is only known from the vicinity of Sesfontein and the surrounding mountainous area up to Tomakas in the west and Warmquelle in the east ( Fig. 3 ). It occurs on hillsides, along seasonally dry riverbeds and at the base of rocky outcrops at elevations of 340–1070 m a.s.l., about 75–125 km from the Atlantic Ocean. Average annual rainfall in the area is 100–150 mm ( Mendelsohn et al. 2002).

Conservation status:— Petalidium sesfonteinense is locally common and probably more widespread in suitable habitats than currently recorded. It is here considered not in immediate conservation danger, because it occurs in sparsely to unpopulated areas and does not seem to be utilised by humans. However, it is browsed by livestock (goats) of the local inhabitants (pers. obs.). The area of occupancy is estimated at <20000 km ² (2250 km ²) with less than 10 (8) subpopulations. However, since no decline in population size or numbers is known, it is here ranked as Least Concern (LC) ( IUCN 2012).

Etymology:— The specific epithet refers to Sesfontein, a village in northwestern Namibia near the type locality.

Notes:— Some of the morphological features to distinguish between Petalidium sesfonteinense , P. kaokoense , and P. variabile are provided in Table 1 .

Hitherto in herbaria, Petalidium sesfonteinense has mostly been confused with P. variabile , a species from which it differs in indumentum, leaf, inflorescence, and flower characters ( Table 1 & Fig. 4 ). Distribution of the two species, does not overlap: P. sesfonteinense occurs in northwestern Namibia in the vicinity of Sesfontein, and P. variabile from the Bergsig area 100 km to the south-southeast and southwards to west central Namibia and in northern Namibia near Otavi and Grootfontein. Records of P. variabile in Angola have not yet been vetted by the first author and might represent a different taxon. Petalidium sesfonteinense can also be confused with P. ohopohense , P. pilosi-bracteolatum , P. rossmannianum , and P.welwitschii , especially in herbarium specimens where fresh flowering material is not available. However, distribution of P. sesfonteinense does not overlap with these species, with P. pilosi-bracteolatum occurring to the south and the other three species to the east or the north of P. sesfonteinense . Other species of Petalidium occurring within the range of P. sesfonteinense are P. angustitibum Meyer (1967: 505) , P. coccineum , P. halimoides , and P. lanatum . All the mentioned species, including P. sesfonteinense , are from the group composed of plants with irregular, four-parted calyces ( Obermeijer 1936, Tripp et al. 2017). A key to P. sesfonteinense , P. kaokoense , P. variabile and other species of Petalidium within its distribution range is provided.

Indumentum is very useful in differentiating among the mentioned taxa and consists mainly of three types of trichomes: simple (unbranched), stellate, and dendritic. The simple trichomes are either single or multi-celled and gland-tipped or not. The stellate trichomes are either stalked or sessile with arms radiating from a common point. The dendritic trichomes are stalked with the arms spirally arranged, opposite and distichous, or opposite and decussate. Each taxon has a unique combination of trichomes (see Table 1 and key to species).

Tripp et al. (2017) conducted a phylogenomic study of Petalidium , sampling several species including Petalidium sesfonteinense (therein labelled as “ Petalidium variabile 16-20”), along with P. rossmannianum , P. welwitschii , and P. kaokoense (therein labelled with an arrow as “ Petalidium sp. 8 [ cf. variabile ]”). These four species together formed a strongly supported clade (Clades 7, 8, & 9 Tripp et al. 2017) that is sister to Clade 10, which contained Petalidium crispum Meuse ex Meyer (1961: 66) , P. coccineum , and P. bracteatum Obermeijer (1936: 161) . More sampling combined with next-generation sequencing is needed to further elucidate phylogenetic relatedness of this new species to other species of Petalidium , including the widespread P. variabile (Manzitto-Tripp et al. in prep.). In turn, this would help to clarify geographical limits of these closely related species.

Additional specimens examined (paratypes):— NAMIBIA, Kunene Region:—1813: North of Sesfontein, (– CD), 31 March 2004, Burke 04080 (WIND!); 42 km before Sesfontein from Puros, (–CD), 446 m, 18 May 2014, Klaassen EK2548 (WIND!); Hills 2 km north of Okarondokavi, (–CD), 742 m, 14 May 2022, Swanepoel 579 (WIND); Hills 2.5 km north of Okarondokavi, (–CD), 760 m, 14 May 2022, Swanepoel 580 (WIND); 6 km south of Okovikuta on road to Ganamub, (–CD), 694 m, 14 May 2022, Swanepoel 581 (WIND!).—1913: 33 km from Sesfontein turn-off to Hoanib River, (–AB), 347 m, 17 May 2014, Klaassen EK2542 (WIND!); 20 km Noord van Sesfontein, dolomiet berge, (–BA), 21 May 1976, Viljoen 313 (WIND!); ca. 3 km W of Sesfontein, (–BA), 600 m, 11 April 1985, Jacobsen & Moss K130 (WIND!); Dry water course in calcrete hills NW of Sesfontein, (–BA), 11 September 1995, Sullivan 350 (WIND!); 2 km NW of Sesfontein on D 3707, extremely dry slate slopes hosting on Ruellia , Justicia and Petalidium , (–BA), 26 March 2010, Tripp & Dexter 865–869 (PRE!, WIND!); Mountains 5 km southwest of Sesfontein, (–BA), 499 m, 12 May 2022, Swanepoel 578 (WIND!); Hoanib Poort 11 km east of Sesfontein on road to Warmquelle, (–BA), 609 m, 15 May 2022, Swanepoel 582 (WIND!); 6 km east of Ganamub on road to Sesfontein, (–BA), 631 m, 25 May 2022, Swanepoel 584 (WIND!); 9 km east-northeast of Warmquelle in poort on road to Beesvlakte, (–BB), 919 m, 12 May 2022, Swanepoel 576 (WIND!); 12 km east-northeast of Warmquelle in poort on road to Beesvlakte, (–BB), 1068 m, 12 May 2022, Swanepoel 577 (WIND!); 37 km from Khowarib on road to Palmwag, (–DB), 1006 m, 12 May 2022, Swanepoel 575 (WIND).

Figures

FIGURE 1. Petalidium sesfonteinense. A. Habit. Plant about 40 cm tall. B. Flower, shoots, and leaves; greyish leaves are still densely covered in trichomes; green leaves almost glabrous. Photographs: W. Swanepoel. Swanepoel 568.

FIGURE 2. Petalidium sesfonteinense. Morphology of flowers. A–D. Flowers in front view showing variation in corolla colour and lobe margins, with two yellow nectar guides. E. Corolla in lateral view. F. Corolla in dorsal view. G. Corolla in ventral view. Scale bar = 5 mm. Photographs: W. Swanepoel. Swanepoel 568 (E, F & G).

FIGURE 3. Known distribution of Petalidium sesfonteinense based on herbarium specimens.

FIGURE 4.Flowers of twospecies related toPetalidium sesfonteinense,for purposesof comparison.A.P.kaokoense.Notethe one-coloured lobes of similar shade with relatively long yellow nectar guides (Hartmann Mountains, Kaokoveld). B. P. variabile. Note the difference in shade between lobes and the relatively short yellow nectar guides (Okahandja, central Namibia). Photographs: W. Swanepoel.