Morphological and harmomegathic characters of pollen of Gnetum species (Gnetaceae) in Thailand: studies with light and scanning electron microscopy

摘要

The morphological plasticity that can occur in pollen of Gnetum due to harmomegathy demands a comparative overview of pollen characters in Gnetum when studying systematics, particularly when fossil pollen grains are taken into account. Using LM and SEM, we studied acetolyzed and dry and rehydrated non-acetolyzed pollen grains of eight Gnetum spp. in Thailand. They are subspheroidal, inaperturate, microechinate monads with bluntended microechini. In acetolyzed and non-acetolyzed pollen grains, the raised exine regions and the ridge-like folds appear to be characteristic of the dehydrated state, whereas a psilate surface between microechini is representative of the hydrated state. Although we were able to measure and record some variation in average values of certain features of all the pollen studied and in some characters of the dehydrated specimens, the general morphological similarity of all the examined Gnetum pollen makes it difficult to establish any systematic significance. Nonetheless, their continuous tectum supports the molecular phylogenetic information that suggests Asian Gnetum spp. are monophyletic. This study presents the first report of the pollen morphology of Gneturn microcarpum Blume and G. tenuifolium Ridl.