• Four new species are described.
• Polhillia canescens synonymized under P. connata.
• Molecular phylogenetic reconstruction confirms monophyly of the genus.
• New monotypic subgenus described.
• Updated Red Data List status for all 11 Polhillia species.
Polhillia is a small, but poorly known genus of legumes endemic to the Greater Cape Floristic Region (GCFR) of South Africa. The genus was described three decades ago to accommodate species erroneously placed in Argyrolobium and other Cape legume genera. Polhillia is the third most threatened plant genus in South Africa, hence priority for taxonomic study and current knowledge on distribution patterns are essential for conservation interventions. Eight species were known at the start of this study, all of which are threatened with extinction. Species boundaries were, however, unclear and many Renosterveld fragments, in particular in the Overberg, required further exploration in pursuit of the various Polhillia species. The phylogenetic relationships among species and monophyly of the genus, especially regarding the position of the morphological aberrant P. involucrata, had not been tested. This study was built upon a foundation of widespread field sampling and analysis of herbarium specimens. Having considered morphological, anatomical, palynological and molecular data, we present an updated taxonomy of the genus Polhillia. Morphological characters between species were often obscure and unique characters were uncommon, as such species are recognized based on unique combinations of characters. Pollen and leaf anatomical studies did not provide substantial taxonomically informative characters, although some interspecific differences were noted. Molecular phylogenetic reconstructions confirm Polhillia to be monophyletic and sister to Argyrolobium, and that Argyrolobium may not be monophyletic. The morphologically distinct species, P. involucrata, is strongly supported as sister to the rest of the genus. Internal phylogenetic resolution in Polhillia is weak in many clades, due to the low variability among DNA markers studied, and several polytomies are still present. Based on the morphological and molecular evidence, we place P. involucrata in the new monotypic subgenus Roseopolhillia. Four new species are described (P. fortunata B.du Preez, P. groenewaldii B.du Preez, P. stirtoniana B.du Preez, P. xairuensis B.du Preez), and a previously recognized species (P. canescens C.H.Stirt.) is placed in synonymy under P. connata (Harv.) C.H.Stirt. Red data list assessments are provided for all species to highlight their conservation importance. Maps and compound colour plates are provided for each species.
Keywords: Polhillia, Cape flora, Conservation, Legumes, New species, Phylogeny, Taxonomy
Polhillia C.H.Stirt. subgen. Polhillia
Type species: Polhillia obsoleta (Harv.) B.-E.van Wyk
(=Polhillia waltersii (C.H.Stirt.) C.H.Stirt.)
Shrubs erect or spreading, sprouting from rootstock after fire; stipules 2, deltoid, sericeous, veins not visible, connate around base, sheathing stem, often persistent; leaves subsessile or petiolate in P. brevicalyx, pubescent, conduplicate (or flattened in P. ignota); flowers single, paired or in clusters (4–5 in P. brevicalyx, P. ignota and P. fortunata), subsessile or pseudo-pedunculate up to 15 mm long; flowers yellow, 8–13 mm long; calyx yellow-green, often flushed red, villous or sericeous; standard broadly ovate, apex emarginate, pubescent abaxially; wing petals variable in shape and sculpturing, mostly glabrous or pilose along midrib; keel petals variable in shape and level of pubescence, usually restricted to the fore edge; pistil green, sericeous or villous, ovules 7–12, style sub-erect to erect; fruit indehiscent, oblong, laterally compressed between seeds, plicate in P. ignota, P. obsoleta, P. groenewaldii, and P. fortunata, sericeous to glabrescent; seeds ovate-reniform, smooth, uniformly coloured, olive green to dark brown (except for P. connata and P. pallens that have occasional mottling).
1. Polhillia brevicalyx (C.H.Stirt.) B.-E.van Wyk & A.L.Schutte
2. Polhillia connata (Harv.) C.H.Stirt.
3. Polhillia curtisiae C.H.Stirt. & Muasya
4. Polhillia fortunata B.du Preez
5. Polhillia groenewaldii B.du Preez
6. Polhillia ignota Boatwr.
7. Polhillia obsoleta (Harv.) B.-E.van Wyk
8. Polhillia pallens C.H.Stirt.
9. Polhillia stirtoniana B.du Preez
10. Polhillia xairuensis B.du Preez
Etymology: The generic and subgeneric epithet Polhillia honours the renowned botanist and legume systematist Dr. Roger M. Polhill from the Royal Botanical Gardens, Kew (Stirton. 1986a).
Polhillia C.H.Stirt. Subgen. Roseopolhillia B.du Preez, subgen. nov.
Type species: Polhillia involucrata (Thunb.) B.-E.van Wyk & A.L.Schutte,
Kew Bulletin 44: 3 (1989).
Shrubs erect, sprouting from rootstock after fire; stipules 2, deltoid-oblong, glabrous abaxially, villous adaxially with three prominent veins, connate around base, sheathing stem, often persistent; leaves digitately trifoliolate, shortly petiolate, glabrous or rarely sparsely hairy, conduplicate; inflorescence terminal, flowers in clusters of 2–5, subsessile, surrounded by enlarged stipules; flowers rose pink or rarely white, 10–12 mm long; calyx with trifid ventral lip, sparsely pilose, green, often flushed maroon; standard broadly elliptic to ovate, apex emarginate, pubescent only along abaxial midrib; wing petals broadly oblong with lamellate sculpturing along most part of dorsal half; keel petals lunate, pilose along ventral fore edge; stamens 10, monadelphous with single dorsal slit, filaments fused for most of their length, anthers alternating between long and short; pistil sericeous, variable in length, ovules 7, style sub-erect, stigma regular, capitate; fruit dehiscent, pale cream-brown, oblong, laterally compressed between seeds, sericeous; seeds ovate-squared, smooth, dark-brown to mottled pale-orange.
Species included: Polhillia involucrata (Thunb.) B.-E.van Wyk & A.L.Schutte.
Etymology: The subgeneric epithet Roseopolhillia was chosen to highlight the distinct and diagnostic rose pink coloured flowers unique to Polhillia species.
Polhillia fortunata B.du Preez
Etymology: The specific epithet is a direct translation from Latin meaning “lucky”. This epithet was selected as the discovery of this species was fortuitous. The area in which this species occurs was burnt in a massive fire for the first time in nearly 100 years in February 2018, allowing profusion of resprouting shrubs to be visible from the road in what was previously near-senescent Renosterveld vegetation.
Polhillia groenewaldii B.du Preez
Etymology: The specific epithet “groenewaldii” honours Mr Jannie Groenewald who first discovered this species and brought it to our attention. This honours the incredible work he has done in increasing our knowledge of new species and also new populations of threatened species in the Overberg region. This is also in recognition of the tremendous help he provided during fieldwork conducted for this study.
Polhillia stirtoniana B.du Preez
Etymology: The specific epithet honours Prof. Charles Howard Stirton for his extensive work on Cape legumes and his late wife, Dr Jana Stirton (Born Žantovská) who shared his love and passion for the diversity and conservation of the Overberg Renosterveld. They assisted in the formation of the Overberg Renosterveld Conservation Trust and establishment of the Haarwegskloof Private Nature Reserve and research centre. Prof. Stirton has significantly mentored the first author in his development as botanist.
Polhillia xairuensis B.du Preez
Etymology: The specific epithet is derived from the Hessequa Khoi name for the Suurbraak area, namely “!Xairu” which means ‘Beauty’ or ‘A place called Paradise’, referring to the beautiful nature of the area (Fourie, 2013). Pronunciation of the epithet can be with or without a click. Although much of the area has now been transformed for agriculture, the Langeberg Mountains above the town of Suurbraak remain scenic and lowland vegetation fragments hold beautiful floral gems such as this Polhillia species.
B. du Preez, L. L. Dreyer, C. H. Stirton and A. M. Muasya. 2021. A Monograph of the Genus Polhillia (Genisteae: Fabaceae). South African Journal of Botany. 138; 156-183. DOI: 10.1016/j.sajb.2020.12.022