Hydraulic and osmotic properties of root systems of 2.5-8-months-old oak seedlings (Quercus robur and Q. petraea) were measured using the root pressure probe. Root pressures of excised roots ranged between 0.05 and 0.15 MPa which was similar to values obtained for herbaceous species. Root hydraulic conductivity (Lp r ; per unit of root surface area) was much larger in the presence of hydrostatic than in the presence of osmotic pressure differences driving water flow across the roots. Differences were as large as a factor of 20 to 470. Roots of the young seedlings of Q. robur grew more rapidly than those of Q. petraea and had a hydraulic conductivity which was substantially higher. Nitrogen nutrition affected root growth of Q. robur more than that of Q. petraea, but did not affect root Lp T of either species. For O. robur, Lp T decreased with root age (size) which is interpreted by an effect of suberization during the development of fine roots. Root hydraulic conductance remained constant for both species. For Q. robur, this was due to the fact that the overall decrease in Lp T was compensated for by an increase in root surface area. Root reflection coefficients (er, r) were low and ranged between

To evaluate the possible role of solute transport during extension growth, water and solute relations of cortex cells of the growing hypocotyl of 5-day-old castor bean seedlings (Ricinus communis L.) were determined using the cell pressure probe.Because the osmotic pressure of individual cells (ir') was also determined, the water potential (4/) could be evaluated as well at the cell level. In the rapidly growing part of the hypocotyl of wellwatered plants, turgor increased from 0.37 megapascal in the outer to 1.04 megapascal in the inner cortex. Thus, there were steep gradients of turgor of up to 0.7 megapascal (7 bar) over a distance of only 470 micrometer. In the more basal and rather mature region, gradients were less pronounced. Because cell turgor = wr and #~0 across the cortex, there were also no gradients of i' across the tissue. Gradients of cell turgor and iri increased when the endosperm was removed from the cotyledons, allowing for a better water supply. They were reduced by increasing the osmotic pressure of the root medium or by cufting off the cotyledons or the entire hook. If the root was excised to interrupt the main source for water, effects became more pronounced. Gradients completely disappeared and turgor fell to 0.3 megapascal in all layers within 1.5 hours. When excised hypocotyls were infiltrated with 0.5 millimolar CaCI2 solution under pressure via the cut surface, gradients in turgor could be restored or even increased. When turgor was measured in individual cortical cells while pressurizing the xylem, rapid responses were recorded and changes of turgor exceeded that of applied pressure. Gradients could also be reestablished in excised hypocotyls by abrading the cuticle, allowing for a water supply from the wet environment. The steep gradients of turgor and osmotic pressure suggest a considerable supply of osmotic solutes from the phloem to the growing tissue. On the basis of a new theoretical approach, the data are discussed in terms of a coupling between water and solute flows and of a compartmentation of water and solutes, both of which affect water status and extension growth.

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(W.M., A.S., R.E.); and Timiryasev-lnstitute of Plant Physiology, Russian Academy of Sciences, 127 276 MOSCOW, Botanicheskaya Ulica, 35, Russia (A.B.M.) Stationary volume fluxes through living and denatured parenchyma slices of the potato (Solanum tuuberosum L.) storage organ were studied to estimate the hydraulic conductivity of the cell wall and to evaluate the significance of water transport through protoplasts, cell walls, and intercellular spaces. Slices were placed between liquid compartments, steady-state fluxes induced by pressure or concentration gradients of low-and high-molecular-mass osmotica were measured, and water transport pathways were distinguished on the basis of their difference in limiting pore size. The protoplasts were the dominating route for osmotically driven water transport through living slices, even in the case of a polymer osmoticum that is excluded from cell walls. The specific hydraulic conductivity of the cell wall matrix is too small to allow a significant contribution of the narrow cell wall bypass to water flow through the living tissue. This conclusion is based on (a) ultrafilter coefficients of denatured parenchyma slices, (b) the absence of a significant difference between ultrafilter coefficients of the living tissue slices for osmotica with low and high cell wall reflection coefficients, and (c) the absence of a significant interaction (solvent drag) between apoplasmic permeation of mannitol and the water flux caused by a concentration difference of excluded polyethylene glycol. Liquid-filled intercellular spaces were the dominating pathways for pressure-driven volume fluxes through the parenchyma tissue.In textbooks and reviews (Anderson, 1976; Lauchli, 1976;Pitman, 1977), the nonspecialized apoplast is usually considered more permeable to water than the protoplast or the symplast. However, severa1 authors (