The Effect of Hydrosatic Pressure Gradients on the Movement of Sodium and Calcium across the Root Cortex

The hydrostatic pressure gradient across the root cortex inducedby transpiration was simulated by raising the pressure on themedium surrounding the roots of detopped tomato (Lycopersicumesculentum) and castoroil plants (Ricinus communis).Applyinga pressure of 2 atm. resulted in a doubling of the sodium fluxfrom medium to xylern (table I). This flux depended on a maintainedsuply of Na in the medium (fig.1) indicating that there wasa small but readily available storage and exchange capacityfor Na in the root tissues. In the presence of metabolic inhaibitorsthe Na flux was considerablyu reduced and equalled the fluxof mannitol (talble II) believed to be a passive mass flow.The Na flux under a pressure gradient appeared therefore tobe metabollism-facilitated. Metabolic inhibition after the rootshad been allowed to take up Na led to a large flux of sodiuminto the xylem (Fig.2). This was considered to come from a largeunavailable store in the roots. Metabolism-facilitated processes also appeared to predominatein the flux of calcium to the xylem of Ricinus, buit not inthe flux through tomato roots. The location of the centripetal metabolism-facilitated transferand the available and unavailable stores is discussed in relationto the effects of pressure gradients.