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.

Parasitic plants form intimate contacts with host tissue in order to gain access to host solutes. There are a variety of cell types within the host which parasitic plants could access to extract solutes. Depending on the degree to which the parasite has embraced the parasitic lifestyle, the extent of solute flux and the pathways used to transfer solutes from host to parasite will vary. To date, a variety of experimental approaches argue for diversity in the mechanisms and the routes by which parasites accumulate host solutes. Contact between host and parasite ranges from direct lumen-to-lumen links between host and parasite xylem and continuity between the sieve elements of host and parasite, to the involvement of transfer cells between host and parasite. Progress has been slow since Solms-Laubach distinguished types of parasitic plants that fed from host phloem or xylem in 1867, but advances in clearly delineating the pathways that link host and parasite should now be possible using fluorescent proteins expressed and restricted to particular cell types of the host. This will initially necessitate using Arabidopsis, but should allow the types of connection, i.e. symplasmic or apoplasmic, to be determined and then the identification of parasite transporters responsible for solute flux.

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