Transpiration Induces Radial Turgor Pressure Gradients in Wheat and Maize Roots

Abstract

Contrary to the conventional view, the flow of water and solutes across the symplastic pathway through the plasmodesmata cannot be inwardly directed under transpiring conditions.

Trans‐root potential, xylem pressure, and root cortical membrane potential of ‘low‐salt’ maize plants as influenced by nitrate and ammonium

“…This observation supported the hypothesis of a symplastic pathway of low resistance in intact, transpiring plants. However, the discrepancy between the existence of stationary (hydrostatic and osmotic) pressure gradients within the cortex observed previously (Zimmermann et al 1992;Rygol et al 1993) and ion transport through the symplast as well as the contribution of the xylem parenchyma to the TRP and the response of the TRP upon addition of nitrate remain to be resolved. Further progress can be expected if the above measurements are combined with the determination of (presumably existing) concentration gradients of nitrate, ammonium and other ions between the ambient medium, cortical cells and the xylem sap.…”

“…The situation may, however, be fundamentally different for roots of intact transpiring plants. In contrast to excised roots, steep, inwardly directed gradients of both turgor pressure and osmotic pressure exist in transpiring plants that collapse upon excision of the roots (Zimmermann et al 1992;Rygol et al 1993). Evidence has also been presented for the existence of membrane potential gradients in the root of intact plants; these data were obtained by advancing a microelectrode radially through the root tissue (Cortes 1997).…”

Simultaneous recording of xylem pressure and trans‐root potential in roots of intact glycophytes using a novel xylem pressure probe technique

“…Solvent drag effects can account for many of these phenomena provided that the radial reflection coefficients of the root are fairly low (Zimmermann et al 1992;Rygol et al 1993). Values of the radial reflection coefficients lower than σ r = 1 were suggested by work on excised roots (Steudle 1989(Steudle , 1992) and low-transpiring plants (Zhu et al 1995).…”

“…The first approach suffers from two shortcomings. In excised roots the turgor and intracellular osmotic pressure gradients collapse (see above, Rygol et al 1993) indicating that the behaviour of excised root systems may not always be a reliable guide to that of the intact plant (Clarkson 1993). Electrical double layers (Amin 1982) and longitudinal potentials (Fensom 1957(Fensom , 1962 which may be formed in the lumen of the vessels (due to xylem loading, water ascent and/or ion accumulation in the leaf vessels, e.g.…”

“…the so-called trans-root potential, provide a way to elucidate the contribution of electrical forces to solute and water transport through the root (Etherton & Higinbotham 1960;Bowling & Spanswick 1964;Shone 1969;Davis & Higinbotham 1969;Läuchli, Spurr & Epstein 1971;Cortes 1992;Rygol et al 1993;Clarkson 1993;Marschner 1995). Reliable data on these forces and their coupling to transpiration-induced changes in xylem pressure, and to turgor pressure of the hydraulically coupled root cells (Andrews 1976;Stahlberg & Cosgrove 1995;Schneider, Zhu & Zimmermann 1997a), are also urgently needed to explain the generation of radial turgor and osmotic pressure gradients through the root cortex of transpiring plants (Rygol & Zimmermann 1990).Solvent drag effects can account for many of these phenomena provided that the radial reflection coefficients of the root are fairly low (Zimmermann et al 1992;Rygol et al 1993). Values of the radial reflection coefficients lower than σ r = 1 were suggested by work on excised roots (Steudle 1989(Steudle , 1992 and low-transpiring plants (Zhu et al 1995).…”