| Abstract: | Transpiration reduces the hydrostatic pressure in the xylemand water moves into the root and across the root cortex inresponse to the pressure gradient so produced. This effect wassimulated by raising the pressure on the external medium surroundinga detopped root system. The flux of sap from the stem stumpand the concentration of potassium in the sap were measuredand the flux of potassium into xylem obtained as (sap flux Xconcentration of potassium). The application of a pressure of 2 atm. caused an approximatelyfourfold increase in potassium flux. This increase was independentof the presence of potassium in the external medium and wasdue, therefore, to an efflux of ions already stored in the roottissues. The effect was specifically due to hydrostatic pressureand not to the D.P.D. difference across the cortex, and wascaused, in part at least, by an increase in the permeabilityof the tissues to iona. This is evident since the increasedpotassium flux occurred in response to pressure, even when theconcentration of potassium in the xylem increased at the sametime, thus precluding any ‘dilution effects‘. It was confirmed that metabolic inhibitors and low temperaturereduce the sap flux but leave the concentration of the sap unchanged.Under a pressure gradient, however, there was an even greaterporportionate reduction of sap flux by equivalent concentrationsof inhibitor or low temperature and the concentration of potassiumin the sap gradually increased. Adding calcium or magnesium ions to the medium caused an increaseof potassium concentration in the sap, demonstrating exchangeprocesses in the transpiration pathway. The difficulty of framing a hypothesis to cover all the factsis discussed and it is tentatively suggested that movement acrossthe cortex to the xylem is a catenary process in which soluteand water move independently at one stage and together as asolution at another stage. |
