Heteromorphy in pollen grains of the tropical and subtropical selected Fabaceae species

Pollen grains have long fascinated biologists who used their significant intra and inter-specific diversity as a marker to infer profiles of past and present vegetation and environment. Our study addresses the question of the diversity in pollen morphology at the intra and inter-specific level: how different are pollen grains of the same species sampled from the tropical and subtropical regions of China and Pakistan. Such differences are expected and are well known to palynologists, but at the same time technically challenging to quantify. We used both light microscopy (LM) and scanning electron microscopy (SEM), to explore the intra and inter-specific pollen variability and its taxonomic relevance in selected Fabaceae taxa. Pollen features were described in terms of size, shape, apertures and exine sculpturing and were then subjected to correlation and principal component analysis (PCA). A high morphological disparity and phenotypic plasticity were found in Leguminaceae species. A weak intra-specific variation was found in the exine sculpturing while it was high when considering the polar axis and equatorial diameter, colpus length and width, shape and P/E ratio. Variation in environmental factors explained a significant portion of the naturally occurring variation in pollen size. Relatively large size pollen was found in the warmer subtropical region, which leads to the conclusion that most of the pollen traits of Fabaceae species are plastic in nature. This intra-specific variability in pollen futures could indicate that plants are trying to adapt their pollen morphology to environmental conditions such as maximum temperature, and humidity.