Dastpak, A.; Williams, M.; Perkins, S.; Perkins, J.; Horn, C.; Thompson, P.; Ryan, C.; Medeiros, J.; An, Y.-D.; Jiang, G.-F.; Simonin, K.; Roddy, A.

Background and Aims: Despite the predominance of scaling photosynthetic metabolism by two-dimensional leaf surface area, leaves are three-dimensional structures composed of multiple tissues that directly and indirectly influence photosynthetic metabolism. The structure of leaf surfaces for CO2 diffusion and light transmission and the internal volume of tissues that process energy and matter work together to control rates of resource acquisition and turnover. Here we investigate the influence of cell size and packing density on resource acquisition as measured by surface conductance to CO2 and water vapor and on resource turnover as measured by leaf water turnover time. Methods: We sampled wild and cultivated congeneric species in the genus Rhododendron (Ericaceae) and measured genome size, anatomical traits related to cell sizes and packing densities, and morphological traits related to water content and dry mass allocation. Results: Among Rhododendron, anatomical traits related to cell size and morphological traits related to water content and dry mass investment varied largely orthogonally to each other, allowing for many combinations of leaf traits to exist. However, there was a strong, negative relationship between the leaf water residence time ({tau}) and the maximum leaf surface conductance per leaf volume (gmax,vol), both of which are influenced by cell size and cell packing densities. Conclusions: Despite leaf function being controlled by many potential combinations of leaf cell- and tissue-level traits, cell size has a pervasive effect on leaf function. Small cells allow for higher diffusion of CO2 and water vapor per unit leaf volume (gmax,vol) even at constant leaf thickness, but small cells also result in shorter leaf water residence times ({tau}). The strong tradeoff between gmax,vol and ({tau}) illuminates how genome size-cell size allometry influences the fast-slow continuum of plant carbon and water economy.