Water Transport in Onion (Allium cepa L.) Roots (Changes of Axial and Radial Hydraulic Conductivities during Root Development)

Abstract

The hydraulic architecture of developing onion (Allium cepa L. cv Calypso) roots grown hydroponically was determined by measuring axial and radial hydraulic conductivities (equal to inverse of specific hydraulic resistances). In the roots, Casparian bands and suberin lamellae develop in the endodermis and exodermis (equal to hypodermis). Using the root pressure probe, changes of hydraulic conductivities along the developing roots were analyzed with high resolution. Axial hydraulic conductivity (L,) was also calculated from stained cross-sections according to Poiseuille's law.Near the base and the tip of the roots, measured and calculated 1, values were similar. However, at distances between 200 and 300 mm from the apex, measured values of L, were smaller by more than 1 order of magnitude than those calculated, probably because of remaining cross walls between xylem vessel members. During development of root xylem, L. increased by 3 orders of magnitude. In the apical 30 mm (tip region), axial resistance limited water transport, whereas in basal parts radial resistances (low radial hydraulic conductivity, Lp,) controlled the uptake. Because of the high axial hydraulic resistance in the tip region, this zone appeared to be "hydraulically isolated from the rest of the root. Changes of the Lp. of the roots were determined by measuring the hydraulic conductance of roots of different length and referring these data to unit surface area. At distances between 30 and 150 mm from the root tip, Lp, was fairly constant (1.4 X lO-' m s-' MPa-'). In more basal root zones, Lp, was considerably smaller and varied between roots. The low contribution of basal zones t o the overall water uptake indicated an influence of the exodermal Casparian bands and/or suberin lamellae in the endodermis or exodermis, which develop at distances larger than 50 to 60 mm from the root tip.

Natural Variation of Root Hydraulics in Arabidopsis Grown in Normal and Salt-Stressed Conditions

“…For instance, suberization of apoplastic barriers has classically been associated with root maturation and reduced water uptake capacity (Melchior and Steudle, 1993;Zimmermann and Steudle, 1998). Although Arabidopsis accessions showed significant variation in their root anatomy, we did not observe any clear link between root suberization and the hydraulic conductivity of the aquaporin-independent path.…”

“…Water transport across living tissues can also involve the apoplast (Steudle and Peterson, 1998), although the molecular bases and genetic control of this path are somewhat less understood. Yet, it was recently shown that suberin deposition in specialized root cell layers (exodermis and endodermis), which may affect root water transport properties, can vary between genotypes (Baxter et al, 2009) and physiological contexts (Melchior and Steudle, 1993;Zimmermann et al, 2000;Vandeleur et al, 2009). In addition, fully differentiated xylem vessels usually provide, with respect to radial hydraulic conductance, a nonlimiting axial conductance.…”

Soil-Tree-Atmosphere Water Relations

“…One can observe that stem resistance also contributes to the overall resistance of the canopyatmosphere system. However, in resistance studies by Melchior and Steudle (1993), it was found that resistance to water flow was usually negligible where the xylem had already matured.…”

“…Thornley and Johnson (1990) explained how the resistance from root surface to the plant vary with the number of roots per unit area in a vertically-structured soil profile. Melchior and Steudle (1993) stated that R l can contribute substantially to the overall hydraulic resistance or even can be the limiting component. Passioura (1972), Newman (1976) and Meyer and Ritchie (1980) used a modified Poiseuille-Hagen equation to calculate R l from root xylem dimensions:…”

Transport of Water and Solutes across Maize Roots Modified by Puncturing the Endodermis (Further Evidence for the Composite Transport Model of the Root)

“…3) of pressure relaxations. The use of a proper value of PE had some impact on the evaluation of k, , (TsI2"') (Birner and Steudle, 1993;Melchior and Steudle, 1993).…”