Taxonomic notes on the Rhynchosia densiflora group (Phaseoleae, Fabaceae) in South Africa and its segregation from Rhynchosia section Arcyphyllum

Abstract

Background: Rhynchosia section Arcyphyllum is one of the five sections of Rhynchosia as currently circumscribed. Previous studies in South Africa placed two species of Rhynchosia in this section. Some authors treated the species as a group rather than a section, to avoid phytogeographical confusion because the section is based on the North American generic name Arcyphyllum.

Objectives: To formally remove the South African taxa from section Arcyphyllum and to provide diagnostic features for these taxa, a key to the subspecies, distribution maps and an illustration of their morphological features.

Methods: Observations were made on herbarium specimens housed at NH, NU and PRE. Several field trips were undertaken in search of Rhynchosia connata. Morphological and anatomical features were studied and measurements of characters recorded.

Results: In South Africa, the section was until now represented by two species, Rhynchosia densiflora (subsp. chrysadenia) and R. connata. These were separated primarily on stem indumentum, stipule shape, petiole length, leaflet shape and apices. However, this study revealed that there are no clear discontinuities between the two taxa apart from the lobes of the uppermost calyx lip, which are connate more than halfway in R. connata and up to halfway in R. densiflora subsp. chrysadenia. Therefore, R. connata is here relegated to varietal level as R. densiflora subsp. chrysadenia var. connata (Baker f.) Jaca & Moteetee.

Conclusions: Two varieties of Rhynchosia densiflora subsp. chrysadenia occur in South Africa and are treated as part of the R. densiflora group instead of section Arcyphyllum.