Comparative leaf micromorphology of Drypetes and Putranjiva (Putranjivaceae) and its taxonomic significance

Putranjivaceae are a pantropically distributed but poorly known glucosinulate-producing family of three genera (Drypetes, Putranjiva and Sibangea), previously included in Euphorbiaceae subfamily Phyllanthoideae. Characters of the leaf epidermis were previously suggested as being of infrafamilial taxonomic relevance, but epidermal features of the three genera have not so far been compared in the light of the phylogenetic relationships recovered by molecular data. Therefore, we compared the leaf micromorphology of 44 (out of c. 200) Drypetes spp. and two (out of three) Putranjiva spp. using light microscopy. Our findings revealed that there are more stomata types than previously reported in both genera. We also present a new record of stomatal complexes and epidermal cell characteristics. The cell wall ornamentation character on both surfaces of the epidermis is useful for distinguishing D. aframensis subsp. aframensis. Based on a combination of the studied characters, doubtful species of Drypetes and Putranjiva are recognizable as belonging to the two genera, and this will require taxonomic and nomenclatural adjustments. Other taxonomically useful characters are trichome distribution and types and leaf areole shape and tertiary veins branching patterns that appear to be relatively constant across the two genera. Oil droplets located within the cell lumen in Putranjiva differentiates it from Drypetes, where they inwardly streak the epidermal walls among other features. Thus, our study provides evidence that the investigated leaf micromorphological characters are useful for distinguishing the species of the two genera, and they corroborate the existing infrageneric classification of Drypetes that was based on exomorphology.