Tansley Review No. 22 What becomes of the transpiration stream?

Radial transport of water in excised barley (Hordeum distichon, cv. Villa) roots was measured using a new method based on the pressure-probe technique. After attaching excised roots to the probe, root pressures of 0.9 to 2.9 bar were developed. They could be altered either by changing the root pressure artificially (with the aid of the probe) or by changing the osmotic pressure of the medium in order to induce water flows across the root. The hydraulic conductivity of the barley roots (per cm(2) of outer root surface) was obtained in different types of experiments (initial water flow, pressure relaxations, constant water flow) and was (0.3-4.3)·10(-7) cm s(-1) bar(-1). The hydraulic conductivity of the root was by an order of magnitude smaller than the hydraulic conductivity of the cell membranes of cortical and epidermal cells (0.8-2.2)·10(-6) cm s(-1) bar(-1). The half-times of water exchange of these cells was 1-21 s and two orders of magnitude smaller than that of entire excised roots (100-770 s). Their volumetric elastic modulus was 15-305 bar and increased with increasing turgor. Within the root cortex, turgor was independent of the position of the cell within a certain layer and turgor ranged between 3 and 5 bar. The large difference between the hydraulic conductivity of the root and that of the cell membranes indicates that there is substantial cell-to-cell (transcellular plus symplasmic) transport of water in the root. When it is assumed that 10-12 membrane layers (plasmalemma plus tonoplast) in the epidermis, cortex and endodermis form the hydraulic resistance to water flow, a value for the hydraulic conductivity of the root can be calculated which is similar to the measured value. This picture for water transport in the root contradicts current models which favour apoplasmic water transport in the cortex. Show more