Measurement of apoplasmic and cell-to-cell components of root hydraulic conductance by a pressure-clamp technique

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 steady-state 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 × 10⁻⁹m³· s⁻¹· MPa⁻¹, with values ranging from 12.0 × 10⁻⁹ to 18.5 × 10⁻⁹m³· s⁻¹· MPa⁻¹; average cell-to-cell conductance was 11.7 × 10⁹ m³· s⁻¹· MPa⁻¹, with values ranging from 8.5 × 10⁻⁹ to 15.3 × 10⁻⁹ m³ · s⁻¹·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 2.7 × 10⁻⁸ to 5.6 × 10⁻⁸ m· s⁻¹·MPa⁻¹, with an average of 3.7 × 10⁻⁸ m · s⁻¹·MPa⁻¹. The newly developed technique allows the interaction of mass flow of water and of solutes to be explored in the roots of soil-grown plants.