Hydraulic Parameters Measured in 1-Year-Old Twigs of some Mediterranean Species with Diffuse-Porous Wood: Changes in Hydraulic Conductivity and Their Possible Functional Significance

We report measurements of hydraulic conductivity of Vitis oinifera, Oleo europaea and Populus deltoides 1-year-old twigs. Single serial internodes were tested for the volume flow rate which was related to: (a) the xylem tissue cross-sectional area, (b) the vessel lumina cross-sectional area and (c) the leaf surface area supplied by a given stem section. From this, whole xylem hydraulic conductivity (Lx), vessel lumina hydraulic conductivity (Lxv) and leaf specific conductivity (LSC) were calculated. All the three parameters turned out to be linearly related to each other. This is kcause: (a) the leaf surface area (A1) was linearly related to xylem cross-sectional area (Ax and (b) the ratio of the vessel lumina cross-sectional area (Axv) to xylem cross-sectional area (Ax) was approximately constant along the twigs. Moreover, the hydraulic conductivity of twig segments where buds grow most actively (distal internodes in V. vinifera and proximal ones in O. europaea) was much lower than in the rest of the twigs. A possible role played by these ‘constricted’ twig regions is discussed.