Water entry into detached root systems saturates with increasing externally applied pressure; a result inconsistent with models of simple passive diffusion

Abstract

The most widely accepted model of radial water entry from the soil into the xylem of roots is based on principles of ordinary passive diffusion. However, long-standing problems with this model remain unresolved, which concern variable intrinsic properties of conductivity, Lp, changing reflection coefficients, sigma, and inaccurate resolution of osmotic differentials between the soil and xylem. Our study re-examined pressure flow relationships in isolated roots of tomato (Lycopersicon esculentum Mill. cv. Montfavet), pea (Pisum sativum cv. Baccara) and soybean (Glycine max L. Merryl cv. Essor) manipulated in a pressure chamber. In addition to problems previously recognized with the simple passive diffusion model, a new conflict, flow saturation, was observed at high pressures. Experiments revealed that the plateau in flow, Jmax seen at high pressures followed natural rhythms diurnally and developmentally, and was not due to root damage or unnatural flow restriction. Near the end of the photoperiod, Jmax closely correlated with root dry mass. The above inconsistencies between observations in pressure-flow kinetics and ordinary passive diffusion indicate that either the current model should be adjusted or a new model should be proposed.

Hypoxia-Responsive Root Hydraulic Conductivity Influences Soybean Cultivar-Specific Waterlogging Tolerance

“…The formula for each regression line is as follows: Treatment: y = a × x + b; where a = Lp and b = constant. The Lp value is defined by the slope of the linear phase of the curve (Emery and Salon 2002) [23]. The values indicated by half tone symbols were not used for calculating the regression line because they were not included in the linear relationship between the loading pressure and flow ratio per total root length.…”

Hydraulic conductance and K+ transport into the xylem depend on radial volume flow, rather than on xylem pressure, in roots of intact, transpiring maize seedlings

“…Studies on root function had to rely entirely on techniques that were massive‐invasive, casting doubt on the physiological relevance of the data obtained by that means. Our current knowledge is frequently based on work performed on excised roots, despite the fact that root excision will strongly affect root physiology and functioning (Bloom & Caldwell, 1988; Emery & Salon, 2002; but see also Anderson & Allen, 1970). In particular, methods for studying radial volume flow as well as the composition of the xylem sap frequently required pressurization of the root system, leading to an infiltration of air‐filled spaces (De Boer & Prins, 1984).…”

Plant Hydraulic Conductivity: The Aquaporins Contribution

“…However the flow rate detected by this method hardly represents the natural status of transpiring plants (Emery and Salon, 2002). Using the hydrostatic pressure chamber to force xylem sap out from decapitated plants, it is difficult to know exactly how high the applied pressure should be, because different values of over pressure can result in different xylem water fluxes (Else et al, 1995).…”