Genome size, the amount of DNA contained in the nucleus, is a biodiversity trait that displays astonishing diversity in land plants and influences various plant traits from the subcellular to the organismal levels, as well as community, ecosystem, and evolutionary processes. Correlations between genome size variation and nuclear volume, duration of the cell cycle, minimum cell volume, cell packing densities, leaf vein density, and photosynthetic capacity, has been revealed in many studies. However, most studies linking genome size variation to phenotypic traits have focused on broad taxonomic diversity with little fine-scale phylogenetic sampling. Here, we sampled ~120 Rhododendron accessions of over 50 taxa, including both temperate and tropical species from the wild and in botanical gardens, along with leaf morphological and anatomical traits. We found multiple occurrences of naturally occurring interploidy hybrids among Rhododendron sect. Pentanthera. Additionally, genome size was correlated with guard cell size, stomatal density, epidermal cell area, leaf vein density. However, genome size and cell- and tissue-level anatomical traits were largely unrelated to leaf morphological traits, such as leaf water content, leaf mass per area, and leaf thickness. While both anatomical and morphological traits can influence maximum photosynthetic rates, our results show that anatomical and morphological traits are largely orthogonal to each other. Thus, while changes in ploidy and genome size can be important drivers of lineage diversification and can have direct impacts on leaf anatomical and photosynthetic function, how cells and tissues are arranged, and the allocation of biomass and water can also influence leaf construction costs independent of genome size variation.