Background/Question/Methods: Most carbon, water, and energy exchange between plants and the atmosphere occurs at the leaf surface.  Consequently, variation in leaf size has important implications for plant water use and primary productivity.  Recent research describing relationships between the size, chemistry and function of leaves suggests increasing leaf surface area (LA) has functionally negative consequences that result in a diminishing return on dry mass (M_D) investment in light interception.  However, these previous interpretations relied on indirect assessments of leaf gas exchange and photosynthetic metabolism. To explore these issues, we used maximum leaf hydraulic conductance (K_L) and leaf carbon isotope composition (δ¹³C)  in a coupled photosynthesis-leaf water balance model to test for the return on dry mass investment (i.e. carbon gain) in LA. 
Results/Conclusions:  Analysis of the leaves of C3 angiosperm species reveals that K_L scales isometrically with LA and no relationship between δ¹³C and LA.  The observed relationships between K_L and LA suggests that, all else being equal (e.g. soil water and nutrient availability),  the return on resource investment in LA is optimized for radiation interception in relation to the absolute humidity leaves commonly experience.