Osmotically Induced Changes in the Pressure-Flow Relationship of Maize Root Systems

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Abstract
An hypothesis is proposed that the observed non-linearity of flow v. applied pressure is due, not to a change in root resistance, but to an accumulation of osmotica outside the stele which changes the potential gradient driving water flow. Sap exudation rate from the detopped root system of well watered maize (Zea mays L.) plants that were sealed in a pressure chamber was a linear function of the applied air pressure when the applied pressure was greater than the numerical value of the osmotic potential of the solutions used to irrigate the plants. At applied pressures less than the numerical value of the solution osmotic potential, exudation rate was a curvilinear function of applied pressure. The exudation rate decreased at all pressures when the osmotic potential of the solution used to irrigate was decreased through the values 0, - 70, - 190 and - 380 kPa. The osmotic potential of the exudate remained between - 60 and - 70 kPa when both - 70 and - 380 kPa solutions were used.