HYDRAULIC AND OSMOTIC PROPERTIES OF OAK ROOTS

Hydraulic and osmotic properties of root systems of 2.5-8-months-old o ak seedlings (Quercus robur and Q. petraea) were measured using the ro ot pressure probe. Root pressures of excised roots ranged between 0.05 and 0.15 MPa which was similar to values obtained for herbaceous spec ies, 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, D ifferences 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 h ad a hydraulic conductivity which was substantially higher, Nitrogen n utrition affected root growth of Q. robur more than that of Q. petraea , but did not affect root Lp(r) of either species. For Q. robur, Lp(r) decreased with root age (size) which is interpreted by an effect of s uberization during the development of fine roots, Root hydraulic condu ctance remained constant for both species. For Q. robur, this was due to the fact that the overall decrease in Lp(r) was compensated for by an increase in root surface area. Root reflection coefficients (sigma( sr)) were low and ranged between sigma(sr)=0.1 and 0.5 for solutes for which cell membranes exhibit reflection coefficients of virtually uni ty (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 10 d), solu te permeability increased due to ageing effects which, however, did no t cause a general leakiness of the roots as Lp(r) decreased. Hence, va lues were only used from measurements taken during the first day. Tran sport properties of oak roots are compared with those recently obtaine d for spruce (Rudinger et al., 1994). They are discussed in terms of a composite transport model of the root which explains low root sigma(s r) at low solute permeability and reasonable root Lp(r). The model pre dicts differences between osmotic and hydraulic water flow and differe nces in the transport properties of roots of herbs and trees as found.