Taxonomic Identification and Characterization of African Nightshades (Solanum L. Section Solanum)

Abstract

African nightshades play an important role in meeting the nutritional needs of rural households, and are reported as being particularly rich in protein, vitamin A, iron and calcium. Nightshades are among three top priority African indigenous vegetables identified for improvement and promotion through research. A major constraint facing this objective is the scantiness of taxonomic and nomenclatural knowledge on African nightshades resulting in extensive synonymy and confusion. As a consequence, the toxic species are difficult to discriminate from those with high nutritional value. It is also difficult to identify species with good agronomic traits for genetic enhancement. This study was conducted to identify, characterize, and delimit African nightshade species. Fifty accessions of Solanum section Solanum from eastern, southern and western Africa were raised in a greenhouse at the Botanical and Experimental Garden, Radboud University, Nijmegen, the Netherlands. A descriptor list with 48 vegetative and reproductive characters was developed and used to characterize flowering and fruiting plants. Counting of chromosome was done on root squash preparations from one weekold seedlings, aided by digital enhancement of microscopic images. Nine species were represented in the study material, including two diploids: Solanum americanum, and Solanum chenopodioides; five tetraploids: Solanum retroflexum, Solanum villosum, Solanum florulentum, Solanum grossidentatum and Solanum tarderemotum; and two hexaploids: Solanum nigrum and Solanum scabrum. Most of the section Solanum species were distinguishable and easily identified. The exception was S. florulentum and S. tarderemotum which were identified tentatively and assigned respective names, but are difficult to differentiate and require further studies. The S. florulentum/tarderemotum group has three distinguishable variants and further studies are needed to determine the taxonomic status of each as a separate species, subspecies or genotypic/phenotypic variants. Furthermore, S. retroflexum, S. villosum and S. scabrum each had a high degree of within-species variation, and further studies are recommended to determine whether the variations within each constitute subspecies.