Transpiration reduces the hydrostatic pressure in the xylem and water moves into the root and across the root cortex in response to the pressure gradient so produced. This effect was simulated by raising the pressure on the external medium surrounding a detopped root system. The flux of sap from the stem stump and the concentration of potassium in the sap were measured and the flux of potassium into xylem obtained as (sap flux X concentration of potassium). The application of a pressure of 2 atm. caused an approximately fourfold increase in potassium flux. This increase was independent of the presence of potassium in the external medium and was due, therefore, to an efflux of ions already stored in the root tissues. The effect was specifically due to hydrostatic pressure and not to the D.P.D. difference across the cortex, and was caused, in part at least, by an increase in the permeability of the tissues to iona. This is evident since the increased potassium flux occurred in response to pressure, even when the concentration of potassium in the xylem increased at the same time, thus precluding any ‘dilution effects‘. It was confirmed that metabolic inhibitors and low temperature reduce the sap flux but leave the concentration of the sap unchanged. Under a pressure gradient, however, there was an even greater porportionate reduction of sap flux by equivalent concentrations of inhibitor or low temperature and the concentration of potassium in the sap gradually increased. Adding calcium or magnesium ions to the medium caused an increase of potassium concentration in the sap, demonstrating exchange processes in the transpiration pathway. The difficulty of framing a hypothesis to cover all the facts is discussed and it is tentatively suggested that movement across the cortex to the xylem is a catenary process in which solute and water move independently at one stage and together as a solution at another stage.
Copyright (C) 1962, Society for Experimental Biology
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