Hydraulic and osmotic properties of oak roots

Abstract

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,; 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, of either species. For Q. robur, Lpr 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 Lpr was compensated for by an increase in root surface area. Root reflection coefficients (σsr) were low and ranged between σsr=0.1 and 0.5 for solutes for which cell membranes exhibit reflection coefficients of virtually unity (salts, sugars etc.). Solute permeability was small and was usually not measurable with the technique. When root systems were attached to the root pressure probe for longer periods of time (up to 10d), solute permeability increased due to ageing effects which, however, did not cause a general leakiness of the roots as Lpr decreased. Hence, values were only used from measurements taken during the first day. Transport properties of oak roots are compared with those recently obtained for spruce (Rüdinger et al., 1994). They are discussed in terms of a composite transport model of the root which explains low root σsr at low solute permeability and reasonable root Lpr. The model predicts differences between osmotic and hydraulic water flow and differences in the transport properties of roots of herbs and trees as found.