Hydrostatic and osmotic hydraulic conductivities and reflection coefficients of onion (Allium cepa L.) roots

Abstract

Axial hydraulic conductivity of segments of onion (Allium cepa L.) roots was measured using the root pressure probe and calculated from stained cross sections according to Poiseuille’s law. The contribution of the longitudinal path to overall hydraulic resistance of the roots was negligible except for the apical 30 mm. Therefore, total hydraulic conductivity represented a good approximation of the radial hydraulic conductivity (Lp_r) when expressed on the basis of the ‘effective surface area’. Hydrostatic Lp_r (measured using a gradient in hydrostatic pressure) was fairly constant in the apical root zones (30 to 150 mm from the tip), but was lower in more basal zones (at distances > 150 mm from the tip). Thus, basal root parts contributed to a smaller extent to total water uptake of the roots. Lp_r was also measured by applying gradients in osmotic pressure. The osmotic hydraulic conductivity was smaller than the hydrostatic by one to two orders of magnitude and reflection coefficients of the roots for nutrient salts were significantly smaller than unity (0.18–0.88), indicating that roots did not behave like ‘ideal osmometers’ as commonly assumed. A composite transport model is discussed to account for the non-semipermeability of roots. The essentially nonselective apoplasmic path in parallel with the highly selective cellular path could explain both low reflection coefficients for nutrient salts and the dependency of water flow on the nature of the driving force.