SummaryLeaf hydraulic conductance (K leaf ) quantifies the capacity of a leaf to transport liquid water and is a major constraint on light-saturated stomatal conductance (g s ) and photosynthetic rate (A max ). Few studies have tested the plasticity of K leaf and anatomy across growth light environments. These provided conflicting results.The Hawaiian lobeliads are an excellent system to examine plasticity, given the striking diversity in the light regimes they occupy, and their correspondingly wide range of A max , allowing maximal carbon gain for success in given environments. We measured K leaf , A max , g s and leaf anatomical and structural traits, focusing on six species of lobeliads grown in a common garden under two irradiances (300/800 lmol photons m À2 s À1). We tested hypotheses for light-induced plasticity in each trait based on expectations from optimality.K leaf , A max , and g s differed strongly among species. Sun/shade plasticity was observed in K leaf , A max, and numerous traits relating to lamina and xylem anatomy, venation, and composition, but g s was not plastic with growth irradiance. Species native to higher irradiance showed greater hydraulic plasticity.Our results demonstrate that a wide set of leaf hydraulic, stomatal, photosynthetic, anatomical, and structural traits tend to shift together during plasticity and adaptation to diverse light regimes, optimizing performance from low to high irradiance.