A taxonomic revision of the Argyrolobium robustum and A. transvaalense groups (Fabaceae) in southern Africa

Previous studies on the genus Argyrolobium (Genisteae, Fabaceae) in southern Africa recognized a group of species defined by a suffrutescent habit, monomorphic flowers and leaves, and the plants not turning black when dried as ‘section A. transvaalense’. A formal rank was never published for this group. Here a taxonomic study of this group is conducted, including comprehensive descriptions of and a key to the species, nomenclature, typifications, diagnostic characters, and geographical distribution. Based on an assessment of morphological characters, two groups can be recognised; 1) the A. robustum group that comprises four species with a herbaceous habit with up to 55-flowered inflorescences, flowers drying black or turning black when damaged, keel petals lacking pockets, and staminal sheaths fully open above, and 2) the A. transvaalense group comprising five species with a suffruticose habit with up to 24-flowered inflorescences, flowers becoming discoloured (darker coloured or brown) when dried, keel petals with pockets except in A. megarhizum, and staminal sheaths variously fused into a closed tube.

Methods
Literature and herbarium material (including types) from JSTOR Global Plants (2022), AMD, B, BM, BOL, E, G, HBG, J, JE, K, L, M, MEL, MO, NBG (including SAM), NH, NU, PRE, REG, S, TCD, TUB, and UPS (herbarium acronyms according to Thiers 2023) were studied and used to delimit species following standard alpha-taxonomic methods. Specimens were arranged into morpho types based on overall morphological similarities and distributions. For each morpho type, three measurements of each vegetative and some reproductive (inflorescences, bracts, bracteoles, fruits and seeds) characters were taken from a minimum of five specimens, and where available, representative of the full range of variation. From these, one flower (where available) per specimen was rehydrated, dissected, and flower parts measured. Larger plant parts were measured with a ruler while micromorphological characters were photographed and measured using an Olympus DSX110 digital microscope with Olympus DSX software v. 3.1.9 (Olympus Corporation 2011-2019). Diagnostic characters were identified and used to describe taxa.

In addition, fieldwork was conducted in the Limpopo, Mpumalanga, and KwaZulu-Natal Provinces (South Africa) to observe plants in their natural habitat. Photos were taken using a Canon EOS 450D with a 100 mm Canon macro lens (Canon Inc.).

Specimens with sufficient locality information were georeferenced following the Natural Science Collections Facility protocol (Englebrecht 2021) and distribution maps generated with QGIS v. 3.22.7-Bialozieza software (© 1989, 1991 Free Software Foundation, Inc.).