Axial and Radial Hydraulic Resistance to Roots of Maize (Zea mays L.).

A root pressure probe was employed to measure hydraulic properties of primary roots of maize (Zea mays L.). The hydraulic conductivity (Lp(r)) of intact root segments was determined by applying gradients of hydrostatic and osmotic pressure across the root cylinder. In hydrostatic experiments, Lp(r) was constant along the segment except for an apical zone of approximately 20 millimeters in length which was hydraulically isolated due to a high axial resistance. In osmotic experiments, Lp(r) decreased toward the base of the roots. Lp(r) (osmotic) was significantly smaller than Lp(r) (hydrostatic). At various distances from the root tip, the axial hydraulic resistance per unit root length (R(x)) was measured either by perfusing excised root segments or was estimated according to Poiseuille's law from cross-sections. The calculated R(x) was smaller than the measured R(x) by a factor of 2 to 5. Axial resistance varied with the distance from the apex due to the differentiation of early metaxylem vessels. Except for the apical 20 millimeters, radial water movement was limiting water uptake into the root. This is important for the evaluation of Lp(r) of roots from root pressure relaxations. Stationary water uptake into the roots was modeled using measured values of axial and radial hydraulic resistances in order to work out profiles of axial water flow and xylem water potentials.