Alternative methods for scaling leaf hydraulic conductance offer new insights into the structure-function relationships of sun and shade leaves

Hydraulic conductance (Kleaf) and morpho-anatomical parameters were measured in sun and shade Quercus ilex L. (Holm oak) leaves. Sun leaves had lower surface area (Aleaf) and volume (Vleaf) and higher specific mass (LMA) than shade leaves. Transpiration rate and Kleaf scaled by Aleaf (Kleaf_area) were two-fold higher in sun than in shade leaves. Kleaf_area was not correlated to vein density or stomatal density, that were found to be similar in the two leaf types. Values of Kleaf scaled by Vleaf or leaf dry weight (Kleaf_dw) were only 40% higher in sun than in shade leaves, suggesting that structural changes of Holm oak leaves acclimating to different light intensities are accomplished to enhance water transport to the unit evaporating leaf surface area, while maintaining more constant hydraulic supply to mesophyll cells and carbon costs of the water transport system. Sun leaves had higher Kleaf_dw and LMA than shade ones, indicating that high LMA resulted from resource allocation involved in both water transport and structural rigidity. Future studies of intra- and inter-specific variability of mass-based hydraulic efficiency might provide important insights into leaf hydraulics and carbon economy. Kleaf_dw might reveal to be an important driver of plant acclimation/adaptation to the environment.