Pollen morphology of Enkianthus (Ericaceae) and its taxonomic significance

Pollen morphology of ten Enkianthus species was examined using light (LM) and scanning electron microscopy (SEM) and previous infrageneric classification of Enkianthus was discussed in the light of the palynological characters obtained in this study. The pollen grains are monads, oblate spheroidal to prolate in shape and 3- to 5-colpor(oid) ate. Exine sculpture varies from granulate to coarsely rugulate-psilate with faint to distinct minute granules in SEM. Infraspecific geographical differences in palynological characters were found in E. deflexus. The species of Enkianthus are clearly differentiated into two pollen morphological groups that correspond to the sections Enkiantella + Meisteria and Andromedina + Enkianthus, respectively. The members of the sections Enkiantella and Meisteria have pollen grains typically with three apertures, prolate spheroidal to prolate, ratio of colpus length to polar axis (L/P) 0.69 - 0.84, relatively thinner exine and exine sculpturing tendency to granulate, character states that are probably plesiomorphic in this genus. The members of the sections Andromedina and Enkianthus have more derived pollen grains with four to five apertures, commonly oblate spheroidal, L/P 0.56 - 0.63, relatively thicker exine and exine sculpturing tendency to coarsely rugulate-psilate. It seems to be difficult to differentiate between sections of the same group on the basis of palynological characters only. Adding the palynological characters to the infrageneric framework of Anderberg indicates an evolutionary trend in exine sculpture within Enkianthus from finely verrucate-rugulate through coarsely rugulate to coarsely rugulate-psilate. A dichotomous key for Enkianthus was prepared based on the palynological characters.