A mathematical analysis of water and solute transport across the root of Ricinus communis

Summary A mathematical analysis of the relationship between the flux of water, f _H2_O, and the flux of potassium f _K, to the xylem of exuding root systems of Ricinus communis, is presented. Previous analyses (Baker and Weatherley, 1969; Minchin and Baker, 1969) have indicated the presence of a water dependent and a water independent f _K both of which vary with the external concentration of potassium, Cm, supplied as potassium nitrate.The present analysis reveals that whereas at Cm values<1 mM both components of f _K contribute ions to the osmotically active solutions within the osmotic barrier, at Cm values>1 mM only the water dependent f _K is responsible for the osmotic work. This suggests that the ions are released within different regions of the stele. It is proposed that at cm values<1 mM both components are released from the inner stelar tissues whilst at higher Cm values the water dependent f _K is released from the outer stelar tissues. This requires that the solute permeability of the plasmalemma of the outer stelar tissues increases markedly at or about Cm values of 1 mM.It is postulated that the required separation of the two f _K components within the stelar symplasm at Cm values>1 mM is due to the water independent f _K being in a bound state, possibly being transported along a chain of binding sites whilst the water dependent f _K is in a free state within the aqueous phase of the cytoplasm.