Model for iso-osmotic water flow in plant roots

The different interpretations of iso-osmotic water flow in plant roots are reviewed and a model based on the symplasm theory is proposed to explain this phenomenon according to the thermodynamics of irreversible processes. Analysis of data from the literature with the model lead to the following conclusions: (1) different reflection coefficients (σ) at the symplasm boundary membranes are sufficient to account for the inwardly directed iso-osmotic water flow observed in maize roots; (2) the difference between the hydraulic conductivities (Lp) and the difference between σLp at the boundary membranes can either increase or decrease iso-osmotic water flow depending on external conditions (i.e. hydrostatic pressure, concentration of permeable and impermeable solutes); (3) the model is consistent with anatomical and physiological data from the literature and it also explains how metabolism is involved in iso-osmotic water flow. The probable importance of iso-osmotic water flow for whole plants is briefly discussed.