Pollen tetrad morphology of some members of Ericoideae and Vaccinioideae (Ericaceae) from temperate to sub-alpine forests of the eastern Himalayas, with systematic implications

Ericaceae represents one of the most diverse angiosperm lineages. Despite overlapping morphological characters hindering taxonomic resolution at family level, subfamilies Ericoideae and Vaccinioideae are relatively morphologically distinct. This study focuses on pollen morphology of these subfamilies for 28 taxa within an understudied region – the temperate to sub-alpine forests of the eastern Himalayas – and evaluates pollen morphology in relation to an existing systematic framework. Pollen tetrads were examined using light and scanning electron microscopy to document qualitative and quantitative features. The unweighted pair group method with arithmetic mean (UPGMA) and principal component analysis were performed to evaluate phenetic and multivariate relationships among taxa. Pollen of Ericoideae is characterised by tetrahedral tetrads with prominent viscin threads and a well-developed margo, whereas members of Vaccinioideae exhibit tetrahedral tetrads lacking viscin threads and margo. Exine ornamentation varies from fossulate or granulate in Ericoideae to predominantly psilate or tenuitate in Vaccinioideae. Cluster analysis (UPGMA) clearly separated the taxa into two major groups corresponding to subfamilial boundaries, while within-group variation reflected established sectional relationships, particularly within Rhododendron, where taxa were separated primarily on the basis of pollen sculpturing, aperture configuration, and exine features. Relative ectocolpal length and exine sculpture emerged as key diagnostic traits for distinguishing taxa at both intergeneric and infrageneric levels. These findings highlight the systematic significance of palynological features in addressing taxonomic ambiguities within these two subfamilies of eastern Himalayan Ericaceae.