Abstract.A pressure-clamp technique was devised for the direct measurement of cell-to-cell and apoplasmic components of root hydraulic conductance; the experimental results were analyzed in terms of a theoretical model of water and solute flow, based on a composite membrane model of the root. When water is forced under a constant pressure into a cut root system, an exponential decay of flow is observed, until a constant value is attained; when pressure is released, a reverse water flow out of the root system is observed which shows a similar exponential behavour. The model assumes that the transient flow occurs through a cell-to-cell pathway and the observed decrease is the result of accumulation of solutes in front of the root semi-permeable membrane, whilst the steadystate component results from the movement of water through the parallel apoplasmic pathway. Root conductance components are estimated by fitting the model to experimental data. The technique was applied to the root systems of potted cherry (Prunus avium L.) seedlings; average apoplasmic conductance was 15.5 x 10 -9 m 3. s -1' MPa -t, with values ranging from 12.0• 10 -9 to 18.5 x 10-9 m3.s-l-MPa-1; average cell-to-cell conductance was 11.7x 10 -9m3.s -~.MPa -~, with values ranging from 8.5 x 10 .9 to 15.3 x 10 -9 m3.s-l.MPa -1. Cell-to-cell conductance amounted on average to 43% of total root conductance, with values between 41 and 45%. Leaf specific conductance (conductance per unit of leaf area supported) of the root systems ranged from 2.7x10 -s to 5.6xl0-Sm's-l'MPa -t, with an average of 3.7 x 10 -8 m.s-l.MPa -~. The newly developed Abbreviations and symbols: A L 0 = root hydraulic conductance; AaL~ = root apoplasmic conductance; ACCL~, C = root cell-to-cell conductance; C~(t) = concentration of solutes in apical root compartment at time t; Jv = flow of water through the root; ja = apoplasmic flow of water; Jv ~c = cell-to-cell flow of water; LSC = leaf specific conductance of the root system; P = root hydrostatic pressure; P,ppt = applied pressure; n~(t) = root osmotic pressure at time t; rim, osmotic pressure of rooting medium; cr = reflection coefficient of root membrane; z = time constant of cell-to-cell flow decay Correspondence to: F. Magnani; Fax: 44 (31) 662 0478; Tel.: 44 (31) 650 5437; E-mail: fmagnani@srv0.bio.ed.ac.uk technique allows the interaction of mass flow of water and of solutes to be explored in the roots of soil-grown plants.