AbstractThe leaf economics spectrum (LES) characterizes a tradeoff between building a leaf for durability versus for energy capture and gas exchange, with allocation to leaf dry mass per projected surface area (LMA) being a key trait underlying this tradeoff. However, regardless of the biomass supporting the leaf, high rates of gas exchange are typically accomplished by small, densely packed stomata on the leaf surface, which is enabled by smaller genome sizes. Here, we investigate how variation in genome size‐cell size allometry interacts with variation in biomass allocation (i.e. LMA) to influence the maximum surface conductance to CO2 and the rate of resource turnover as measured by leaf water residence time. We sampled both evergreen and deciduous Rhododendron (Ericaceae) taxa from wild populations and botanical gardens, including naturally occurring putative hybrids and artificially generated hybrids. We measured genome size, anatomical traits related to cell sizes, and morphological traits related to water content and dry mass allocation. Consistent with the LES, higher LMA was associated with slower water residence times, and LMA was strongly associated with leaf thickness. Although anatomical and morphological traits varied orthogonally to each other, cell size had a pervasive impact on leaf functional anatomy: for a given leaf thickness, reducing cell size elevated the leaf surface conductance and shortened the mean water residence time. These analyses clarify how anatomical traits related to genome size‐cell size allometry can influence leaf function independently of morphological traits related to leaf longevity and durability.