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Issue: Vol. 734 No. 1: 16 Dec. 2025
Type: Article
Published: 2025-12-16
DOI: 10.11646/phytotaxa.734.1.2
Page range: 9-19
Abstract views: 36
PDF downloaded: 23

Petalidium engoense (Acanthaceae), a new species from Angola and Namibia, with notes on phenotypic plasticity in the genus

Independent Researcher, P.O. Box 21168, Windhoek, Namibia; H.G.W.J. Schweickerdt Herbarium, Department of Plant and Soil Sciences, University of Pretoria, Pretoria, 0002 South Africa; Ongava Research Centre, Private Bag 12041, Windhoek, Namibia; Biodiversity Research Centre, Department of Agriculture and Natural Resources Sciences, Namibia University of Science and Technology, Private Bag 13388, Windhoek, Namibia
Ongava Research Centre, Private Bag 12041, Windhoek, Namibia
H.G.W.J. Schweickerdt Herbarium, Department of Plant and Soil Sciences, University of Pretoria, Pretoria, 0002 South Africa; South African National Biodiversity Institute, Private Bag X101, Pretoria, 0001 South Africa
endemism Engo River Engo Valley flora Hartmann Valley Iona National Park Kaokoveld Centre of Endemism Kunene Region Namib Desert Namibe Province Ruellieae taxonomy variation Eudicots

Abstract

Petalidium engoense, first collected in May 2025, is here described as a new species. It is a range-restricted species, only known from the western edge of the escarpment zone (Great Escarpment of southern Africa), in the hilly area bordering the Engo Valley (northwestern Namibia), and to the north of Espinheira (southwestern Angola), in the Kaokoveld Centre of Endemism. It grows amongst boulders on arid hillsides, and along an ephemeral riverbed. Diagnostic characters for P. engoense include the pale greyish green appearance of the plants, ashy-grey to white fissured bark on older stems, vegetative parts with a dense white indumentum of very small stellate (stellulate) and dendritic trichomes appearing tomentose, semi-succulent leaves, burgundy flowers borne in short few-flowered dichasia with inflorescence axis becoming spiny with age, and narrowly ovate, attenuate to acute, prominently reticulate bracteoles. The flowers of P. engoense are distinctive in having all lobes coloured burgundy, and the anterior lobe with two yellow ensiform nectar guides. A comparison of key morphological features distinguishing P. engoense from P. namibense, its closest relative in appearance, is provided. Based on IUCN Red List criteria, a provisional conservation assessment of Critically Endangered (CR) is recommended for the new species. The paper also summarises current knowledge of phenotypic plasticity within Petalidium in response to environmental variation. This contribution forms part of ongoing taxonomic studies on the genus.

References

  1. Atlas of Namibia Team. (2022) Atlas of Namibia: its land, water and life. Namibia Nature Foundation, Windhoek, 390 pp.
  2. Beentje, H. (2016) The Kew plant glossary: an illustrated dictionary of plant terms, 2nd ed. Kew Publishing, Kew, 184 pp.
  3. Coetzee, M.E. (2021) Geomorphological features of the Skeleton Coast National Park. Report prepared for the SCIONA Project, Windhoek, 76 pp.
  4. Craven, P. (2009) Phytogeographic study of the Kaokoveld Centre of Endemism. Ph.D. Thesis. University of Stellenbosch, Stellenbosch, 234 pp. [https://hdl.handle.net/10019.1/1325]
  5. Darbyshire, I., Kiel, C.A., Astroth, C.M., Dexter, K.G., Chase, F.M. & Tripp, E.A. (2020) Phylogenomic study of Monechma reveals two divergent plant lineages of ecological importance in the African savanna and succulent biomes. Diversity 12 (6): a237. [25 pp.] https://doi.org/10.3390/d12060237
  6. Dexter, K.G., Swanepoel, W., Loiseau, O., Darbyshire, I., Nanyeni, L., Gonçalves, F.M., Chase, F. & Manzitto-Tripp, E.A. (2023) High endemism of the genus Petalidium (Acanthaceae) in the highlands and escarpments of Angola and Namibia. Namibian Journal of Environment 8: 135–147. https://doi.org/10.64640/dr0jnh96
  7. Edwards, D. & Leistner, O.A. (1971) A degree reference system for citing biological records in southern Africa. Mitteilungen der Botanischen Staatssammlung München 10: 501–509. [https://www.biodiversitylibrary.org/page/15185301#page/509/mode/1up]
  8. Figueiredo, E. & Smith, G.F. (2008) Plants of Angola/Plantas de Angola. Strelitzia 22. South African National Biodiversity Institute, Pretoria, 279 pp. [https://www.sanbi.org/documents/plants-of-angola-strelitzia-22/]
  9. Germishuizen, G. & Meyer, N.L. (Eds.) (2003) Plants of southern Africa: an annotated checklist. Strelitzia 14. National Botanical Institute, Pretoria, 1231 pp.
  10. Goudie, A. & Viles, H. (2015) Landscapes and land forms of Namibia. Springer, Dordrecht, 173 pp. https://doi.org/10.1007/978-94-017-8020-9
  11. Hewson, H.J. (2019) Plant indumentum: a handbook of terminology, revised ed. Australian Biological Resources Study, Commonwealth Department of the Environment and Energy, Australia, 47 pp.
  12. IUCN. (2012) IUCN red list categories and criteria: Version 3.1. 2nd ed. Gland, Switzerland and Cambridge U.K., iv + 32 pp.
  13. Loiseau, O., Manzitto-Tripp, E.A., Swanepoel, W. & Dexter, K.G. (2023) Net diversification rates of the woody plant genus Petalidium (Acanthaceae) are highest in the ancient and arid Namib Desert. Frontiers in Ecology and Evolution 11: 1–10. https://doi.org/10.3389/fevo.2023.1193728
  14. Manktelow, M. (2000) The filament curtain: a structure important to systematics and pollination biology in the Acanthaceae. Botanical Journal of the Linnean Society 133 (2): 129–160. https://doi.org/10.1006/bojl.1999.0309
  15. Mendelsohn, J.M. & Huntley, B.J. (2023) Introducing the highlands and escarpments of Angola and Namibia. Namibian Journal of Environment 8: 7–22. https://doi.org/10.64640/espy7q72
  16. Meyer, P.G. (1961) Beiträge zur kenntnis der Acanthaceen Südwestafrikas (III). Mitteilungen der Botanischen Staatssammlung München 4: 59–72. [https://www.biodiversitylibrary.org/item/52384#page/747/mode/1up]
  17. Meyer, P.G. (1973) Neue und enig bekannte Acanthaceen aus dem Kaokoveld (Südwestafrika). Mitteilungen der Botanischen Staatssammlung München 11: 101–113. [https://www.biodiversitylibrary.org/item/51768#page/111/mode/1up]
  18. Mitchell, D., Henschel, J.R., Hetem, R.S., Wassenaar, T.D., Strauss, W.M., Hanrahan, S.A. & Seely, M.K. (2020) Fog and fauna of the Namib Desert: past and future. Ecosphere 11 (1): e02996. [40 pp.] https://doi.org/10.1002/ecs2.2996
  19. Moore, S. (1880) Enumeratio Acanthacearum Herbarii Welwitschiani Angolensis. Journal of Botany, British and Foreign 18: 225–233. [https://www.biodiversitylibrary.org/item/108761#page/236/mode/1up]
  20. Nees von Esenbeck, C.G. (1832) Acanthaceae India Orientalis. In: Wallich, N. (Ed.) Plantae Asiaticae rariores: or descriptions and figures of a select number of unpublished East Indian plants, Vol. 3. Treuttel & Würtz, London, pp. 70–117. [https://www.biodiversitylibrary.org/item/9716#page/172/mode/1up]
  21. Obermeijer, A.A. (1936) The South African species of Petalidium. Annals of the Transvaal Museum 18: 151–162.
  22. Seely, M. (2004) The Namib: natural history of an ancient desert. Desert Research Foundation of Namibia, Windhoek, 110 pp.
  23. Swanepoel, W. (2020) Petalidium kaokoense (Acanthaceae), a new species from Namibia. Phytotaxa 468 (3): 236–242. https://doi.org/10.11646/phytotaxa.468.3.1
  24. Swanepoel, W., Dexter, K.G., Adamo, M., Manzitto-Tripp, E.A. & Van Wyk, A.E. (2025) Petalidium saxatile (Acanthaceae), a new species from Namibia. Phytotaxa 716 (3): 161–174. https://doi.org/10.11646/phytotaxa.716.3.1
  25. Swanepoel, W., Dexter, K.G., Loiseau, O. & Van Wyk, A.E. (2023) Reinstatement of the name Petalidium ovatum (Acanthaceae), with an amplified description of the species. Phytotaxa 626 (3): 159–169. https://doi.org/10.11646/phytotaxa.626.3.2
  26. Swanepoel, W. & Manzitto-Tripp, E. (2022) Petalidium sesfonteinense (Acanthaceae), a new species from the Kaokoveld, Namibia. Phytotaxa 549 (2): 127–135. https://doi.org/10.11646/phytotaxa.549.2.1
  27. Swanepoel, W., Manzitto-Tripp, E.A., Dexter, K.G. & Van Wyk, A.E. (2024) Petalidium etendekaense (Acanthaceae), a new species from Namibia, with notes on the taxonomic identity of P. glutinosum. Phytotaxa 636 (1): 35–47. https://doi.org/10.11646/phytotaxa.636.1.3
  28. Swanepoel, W., Nanyeni, L. & Van Wyk, A.E. (2022) Petalidium mannheimerae (Acanthaceae), a new species from Namibia and South Africa, with notes on the taxonomic identity of P. parvifolium. Phytotaxa 561 (1): 1–13. https://doi.org/10.11646/phytotaxa.561.1.1
  29. Swanepoel, W. & Van Wyk, A.E. (2023a) Petalidium konkiepense (Acanthaceae), a new species from Namibia. Phytotaxa 585 (1): 29–38. https://doi.org/10.11646/phytotaxa.585.1.3
  30. Swanepoel, W. & Van Wyk, A.E. (2023b) Petalidium karasbergense (Acanthaceae), a new species from Namibia. Phytotaxa 609 (1): 1–9. https://doi.org/10.11646/phytotaxa.609.1.1
  31. Swanepoel, W. & Van Wyk, A.E. (2024) Petalidium namibense (Acanthaceae), a new species from Namibia. Phytotaxa 671 (2): 128–138. https://doi.org/10.11646/phytotaxa.671.2.2
  32. Swanepoel, W. & Van Wyk, A.E. (2025a) Petalidium hoarusibense (Acanthaceae), a new species from Namibia. Phytotaxa 681 (1): 1–10. https://doi.org/10.11646/phytotaxa.681.1.1
  33. Swanepoel, W. & Van Wyk, A.E. (2025b) Petalidium sebrabergense (Acanthaceae), a new species from Namibia. Phytotaxa 728 (1): 17–30. https://doi.org/10.11646/phytotaxa.728.1.2
  34. Thiers, B. (2025) Index Herbariorum: A global directory of public herbaria and associated staff. New York Botanical Garden’s Virtual Herbarium. Available from http://sweetgum.nybg.org/ih/ (accessed May 2025)
  35. Tripp, E.A. & Dexter, K.G. (2012) Taxonomic novelties in Namibian Ruellia (Acanthaceae). Systematic Botany 37 (4): 1023–1030. https://doi.org/10.1600/036364412X656509
  36. Tripp, E.A., Tsai, Y.E., Zhuang, Y. & Dexter, K.G. (2017) RADseq dataset with 90% missing data fully resolves recent radiation of Petalidium (Acanthaceae) in the ultra-arid deserts of Namibia. Ecology and Evolution 7 (19): 1–17. https://doi.org/10.1002/ece3.3274
  37. Van Wyk, A.E. & Smith, G.F. (2001) Regions of floristic endemism in southern Africa: a review with emphasis on succulents. Umdaus Press, Hatfield, Pretoria, 199 pp.

How to Cite

Swanepoel, W., Becker, R.W. & Wyk, A.E.V. (2025) Petalidium engoense (Acanthaceae), a new species from Angola and Namibia, with notes on phenotypic plasticity in the genus. Phytotaxa 734 (1): 9–19. https://doi.org/10.11646/phytotaxa.734.1.2