Contribution to floral structure and palynological features of Turkish Periplocoideae and Asclepiadoideae taxa

The Apocynaceae are characterized by highly specialized flowers, including three derived structures, corona, gynostegium and pollinarium, which play an important role in explaining the evolutionary relationships among the major groups of the family. The floral morphology and microstructure of pollinaria in pollinia-bearing subfamilies, Periplocoideae and Asclepiadoideae, are not well understood. In the present study, we examined the detailed floral and pollen characters of six species from Türkiye-Asclepiadoideae: Araujia sericifera Brot., Cionura erecta (L.) Griseb., Cynachum acutum L. and Gomphocarpus fruticosus (L.) W.T.Aiton; Periplocoideae: Periploca gracilis Boiss. and P. graeca L.-using light and scanning electron microscopy for taxonomic insights.

Macro- and micromorphological observations were performed on both herbarium samples and fresh field specimens preserved in 70% ethanol. Floral and pollen descriptions were amended, and a dichotomous identification key was prepared for the studied taxa. Morphological and palynological traits were also analyzed numerically. Principal components analysis revealed that length of pedicel, sepal, petal and corona segment, pubescence of anther (hairy/glabrous), shape of stigma (unlobed/bifid), length of pistil, type of pollen aggregation (tetrad/pollinium), number of tetrad mass or pollinium per pollen translator (single/in pairs), pollen translator structure (receptacle + stalk + adhesive disc or corpusculum + caudicles), and pollen grain dimensions were diagnostically informative traits for distinguishing taxa at both subfamily and genus levels.

The dendrogram inferred from macro- and micromorphological floral and pollen traits generally agrees with the traditional subfamilial classification of the Periplocoideae and Asclepiadoideae. These findings offer insights into the evolution and diversification of pollinia-bearing Apocynaceae, and may reflect the role of key traits in reproductive strategies and ecological interactions.