According to the previous section on flows of water and solutes (nutrients), there should be parallel cellular (cell-to-cell) and apoplastic components. However, the passive cell-to-cell component of solute flow should be small compared with that of water, because of the low permeability of membranes to solutes such as nutrient ions. Water, on the other hand, moves more rapidly by several orders of magnitude across membranes (in part, due to the existence of AQPs). In principle, the cell-to-cell or "protoplastic" component has to be split up into a transcellular (across membranes) and a symplastic (across plasmodesmata) component. However, to date, the latter two components cannot be separated experimentally and are, therefore, summarized as a cell-to-cell component. The cell-to-cell component can be determined from measurements with the cell pressure probe. These results have been compared with measurements at the root level, which allowed to judge about pathways, at least semiquantitatively. There is, to date, no technique to measure the contribution of the apoplastic water flow across the root cylinder or in other tissue. However, Zhu and Steudle (1991) compared hydraulic conductivities at the level of cortical cells of young corn seedlings (cell pressure probe) with those measured at the root level (root pressure probe). It turned out that values at the cell level were not much bigger than those measured at the level of entire roots. This indicated that, at least for hydroponically grown corn, there was a substantial "apoplastic bypass" of water.
When root pressure is established under conditions of zero transpiration or when a manometer is placed on top of an excised root (such as a root pressure probe), there will be an uptake of water along the cell-to-cell passage causing an increase of root pressure. This, in turn, will result in a backflow of water along the apoplastic passage tending to cause a root pressure smaller than expected from the osmotic pressure gradient between xylem and root medium, i.e., the overall reflection coefficient will be smaller than unity (Fig. 10.2). Eventually, a steady state will be attained where the opposing flows are equal. This results in a circulation flow of water across the root. To create additional extracellular bypasses, the system may be manipulated by puncturing the endodermis with needles as done with young corn roots (Steudle et al. 1993). As a response, the steady-state root pressure and the reflection coefficient were reduced, as one would expect in the presence of an additional bypass. On the other hand, solute permeability increased. In other experiments, the apoplastic path could be occluded using nanoparticles or causing salt precipitations in cell walls. These experiments were done with rice roots, where the outer part of the root with its well-developed exodermis provided a model system (Ranathunge et al. 2004, 2005a, b). As expected, blockage resulted in an increase of reflection coefficients and in a decrease of water permeability.
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