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

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, L p , changing reflection coefficients, σ, 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, J max 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, J max 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.