A palynological review of Oxalis (Oxalidaceae) in southern Africa

Abstract

The genus Oxalis includes about 800 species, with centres of diversity in both South America and southern Africa. There are an estimated 210 Oxalis species within southern Africa, with the highest species diversity found in the Western Cape. Although it is a diverse and interesting genus with marked horticultural potential, the taxonomy of southern African members is badly neglected. The disputable revision by Salter ( 1944) constitutes the most comprehensive treatment to date. The main objective of the present study was to determine the taxonomic significance of palynology among the southern African members of Oxalis. To this end LM, SEM and TEM techniques were employed. Preliminary AFM results were also evaluated. Oxalis was found to be palynologically diverse, resulting in the identification of four main pollen types (A, B, C & D) and 19 subtypes within the genus. Detailed structural and sculptural characters of the exine proved useful in this regard. Keys to pollen types and subtypes are provided, supplemented by full LM, SEM and TEM descriptions of each type. The palynological classification proposed in the present study was compared to the taxonomic classification proposed by Salter (1944). Palynological groupings were found to be congruent with some of the sections in Salter's system, while palynology reiterated the need for a taxonomic re-assessment of others. The different pollen types proved to be significant at different levels of the taxonomic hierarchy within the genus. Pollen types A and D are important at sectional and infrasectional levels, while pollen type B is meaningful at the specific level. Pollen type C is diverse, with significance at various levels. An evolutionary discussion, considering morphological diversity, biogeography, means of speciation and pollination led to a more detailed discussion of the evolution of pollen types. A hypothesis is set forward according to which pollen type C constitutes the ancestral type, with pollen types A, Band D representing derived states. Pollen type Dis considered as the most advanced type in the genus. The phylogenetic implications of the results of the present study are also briefly considered. It is concluded that palynology can greatly assist in the construction of an improved classification of Oxalis in southern Africa.