HYDRAULIC AND OSMOTIC PROPERTIES OF SPRUCE ROOTS

Hydraulic and osmotic properties of roots of 2-year-old Norway spruce seedlings (Picea abies (L.) Karst) were investigated using different t echniques (steady flow, pressure probe, and stop flow technique). Root pressures were measured using the root pressure probe. Compared to ro ots of herbaceous plants or deciduous trees, excised root systems of s pruce did not develop appreciable root pressure (-0.001 to 0.004 MPa o r -10 to 40 cm of water column). When hydrostatic pressure gradients w ere used to drive water flows across the roots, hydraulic conductiviti es (L(pt)) were determined in two types of experiments: (i) root press ure relaxations (using the root pressure probe) and (ii) steady flow e xperiments (pneumatic pressures applied to the root system or xylem or partial vacuum applied to the xylem). Root L(pr) ranged between 0.2 a nd 8 x 10(-8) m s(-1) MPa(-1) (on average) depending on the conditions . In steady flow experiments, L(pr) depended on the pressure applied ( or on the flow across the roots) and equalled (0.19+/-0.12) to (1.2+/- 1.7)x10(-8) m s(-1) MPa(-1) at pressures between 0.2 and 0.4 MPa and ( 1.5+/-1.3)x10(-8) m s(-1) MPa(-1) at applied pressures between 0.8 and 1.0 MPa. When pressures or vacuum were applied to the xylem, L(pr) va lues were similar. The hydraulic conductivity measured during pressure relaxations (transient water flows) was similar to that obtained at h igh pressures (and water flows). Although there was a considerable sca tter in the data, there was a tendency of the hydraulic conductivity o f the roots to decrease with increasing size of the root system. When osmotic gradients were used to drive water flows, L(pr) values obtaine d with the root pressure probe were much smaller than those measured i n the presence of hydrostatic gradients. On average, a root L(pr)=0.01 7 x 10(-8) was found for osmotic and L(pr)=6.4 x 10(-8) m s(-1) MPa(-1 ) in corresponding hydrostatic experiments, i.e. the two values differ ed by a factor which was as large as 380. The same hydraulic conductiv ity as that obtained in osmotic experiments using the pressure probe w as obtained by the 'stop flow technique'. In this technique, the sucti on created by an osmoticum applied to the root was balanced by a vacuu m applied to the xylem. L(pr) values of root systems did not change si gnificantly when measured for up to 5 d. In osmotic experiments with d ifferent solutes (Na2SO4, K2SO4, Ca(NO3)(2), mannitol), no passive upt ake of solutes could be detected, i.e. the solute permeability was ver y low which was different from earlier findings on roots of herbs. Ref lection coefficients of spruce roots (sigma(sr)) were low for solutes for which plant cell membranes exhibit values of virtually unity to (s igma(sr) = 0.18-0.28 for Na2SO4, K2SO4, Ca(NO3)(2), and mannitol). On average, reflection coefficients of spruce roots were smaller by about a factor of two than those obtained in the past for roots of herbaceo us species. To test for an influence of active nutrient transport and osmoregulatory processes on the absolute value of the root reflection coefficients measured, model calculations were performed. The results showed that huge changes in active pumping rates (changes of several h undred per cent) would be required to account for the same changes in root pressure when assuming a root sigma(sr) = 1 and that responses we re completely controlled by active processes, The large differences in hydraulic conductivity between osmotic and hydrostatic water flow and low reflection coefficients (at a low solute permeability) are explai ned by the composite transport model of the root recently introduced. The model also explains the non-linear relation between water flow and driving forces, i.e. the variable hydraulic resistance of roots which has often been reported in the literature. It is concluded that both the low root sigma(sr) and low rates of active solute uptake cause par t of the low root pressures observed in conifers.