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

Abstract

The hydrostatic pressure gradient across the root cortex induced by transpiration was simulated by raising the pressure on the medium surrounding the roots of detopped tomato (Lycopersicum esculentum) and castoroil plants (Ricinus communis).Applying a pressure of 2 atm. resulted in a doubling of the sodium flux from medium to xylern (table I). This flux depended on a maintained suply of Na in the medium (fig.1) indicating that there was a small but readily available storage and exchange capacity for Na in the root tissues. In the presence of metabolic inhaibitors the Na flux was considerablyu reduced and equalled the flux of mannitol (talble II) believed to be a passive mass flow. The Na flux under a pressure gradient appeared therefore to be metabollism-facilitated. Metabolic inhibition after the roots had been allowed to take up Na led to a large flux of sodium into the xylem (Fig.2). This was considered to come from a large unavailable store in the roots. Metabolism-facilitated processes also appeared to predominate in the flux of calcium to the xylem of Ricinus, buit not in the flux through tomato roots. The location of the centripetal metabolism-facilitated transfer and the available and unavailable stores is discussed in relation to the effects of pressure gradients.