Contribution to the Palynology and Its Taxonomic Relevance in Selected Taxa of Papilionoideae (Fabaceae) From Northeast India

The Papilionoideae is a taxonomically complex subfamily within the Fabaceae. Pollen characteristics are often used as additional diagnostic features in the taxonomy of Papilionoideae. Unfortunately, the pollen morphology of the Papilionoideae in India remains poorly understood. This study aims to bridge the palynological knowledge gap in the Papilionoideae taxa, focusing on Northeast India. In this investigation, light and scanning electron microscopy (SEM) were employed to examine the pollen morphology of 10 species belonging to nine genera of Papilionoideae: Aeschynomene americana, Cajanus scarabaeoides, Calopogonium mucunoides, Crotalaria calycina, Crotalaria spectabilis, Grona triflora, Lablab purpureus, Pueraria phaseoloides, Tephrosia candida, and Uraria crinita. The pollen grains were prepared using Wodehouse's method; photomicrographs were taken to describe and measure the pollen traits. Pollen traits such as size, aperture number, and shape are stable traits, while exine sculptures, P/E ratio, and gap between two adjacent colpi (GBC) traits can differentiate Papilionoideae taxa. Most of the species (nine species) had tricolporate pollen grains except Aeschynomene americana with syncolporate pollen. Medium-sized pollen was observed in most of the studied species (eight species), except with small-sized pollen grains (A. americana, C. calycina). Three distinct pollen shapes were identified among the studied species: oblate-spheroidal (six species), prolate-spheroidal (three species), and prolate in Uraria crinita. Different types of exine sculpture observed were reticulate, fossulate, fossulate-verrucate, perforate, foveolate-reticulate, foveolate-granulate, rugulate-perforate, and psilate-perforate. The first two principal component analysis (PCA) axis showed 60.3% of the total data variance. Polar axis (PA), equatorial diameter (ED), apocolpium, mesocolpium, and GBC, colpi breadth (ColpiB), P/E, and nexine (> 0.7) contribute significantly to the PCA. Further, a taxonomic key has been prepared based on the pollen morphological characters for correct species identification. This study strengthens and validates the subfamily's taxonomic status and will serve as a foundation for more phylogenetic and molecular research on Papilionoideae.