Palynology of the family Apocynaceae in Thailand

Abstract

Acetolysed pollen grains from eighty-five species belonging to thirty-six genera of the family Apocynaceae in Thailand were studied on morphology by means of light and scanning electron microscopes. Twenty-three pollen types are recognized based on dispersal unit, polarity, aperture, ornamentation, size as well as shape of pollen grains. Pollen grains are mostly monads, isopolar to subisopolar, subspheroidal, small (14-19 µm) to very large (100-106.33 µm), 3-4-zonocolporate or 3-4-zonoporate, psilate to perforate ornamentation. Although it seems that pollen morphological evidence of Thai Apocynaceae is not congruent with the current morphological and molecular classification for delimitation of existing taxonomic groups, it does provide some valuable information. Palynologically, Rauvolfioideae is heterogeneous subfamily while Apocynoideae, on the other hand, is much more homogeneous. In the context of identification, because the Apocynaceae is rather eurypalynous family, it is feasible to utilize pollen morphology to identify different taxonomic groups of Thai Apocynaceae, even species, based mainly on the aperture, shape and size. Pollen morphological changes within this family are mainly in the context of reduction of apertural area, colporate to porate , as occurred in the Rauvolfioideae to Apocynoideae, respectively. Furthermore, this palynological trend can be found in the change of colporate basal tribes of Rauvolfioideae to diporate Alyxieae. The palynological data of Thai Apocynaceae support the view that Asclepiadaceae is not distinct family and it should be considered as a part of Apocynaceae with distinct characteristics