Hydraulic architecture of leaf blades: where is the main resistance?

The hydraulic architecture of Laurus nobilis L. and Juglans regia L. leaves was studied using three different approaches: (1) hydraulic measurements of both intact leaves and of leaves subjected to treatments aimed at removing the extra-vascular resistance; (2) direct measurements of the vascular pressure with a pressure probe; and (3) modelling the hydraulic architecture of leaf venation system on the basis of measurements of vein densities and conductivities. The hydraulic resistance of leaves (R-leaf) either cut, boiled or frozen-thawed was reduced by about 60 and 85% with respect to control leaves for laurel and walnut, respectively. Direct pressure drop measurements suggested that 88% of the resistance resided outside the vascular system in walnut. Model simulations were in agreement with these results provided vein hydraulic conductance was 0.12-0.28 that of the conductance predicted by Poiseuille's law. The results suggest that R-leaf is dominated by substantial extra-vascular resistances and therefore contrast with the conclusions of recent studies dealing with the hydraulic architecture of the leaf. The present study confirms the 'classical' view of the hydraulic architecture of leaves as composed by a low-resistance component (the venation) and a high-resistance component (the mesophyll).