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Water Dependent and Water Independent Fluxes of Potassium in Exuding Root Systems of Ricinus communis

F. R. Minchin and Dennis A. Baker
Planta
89. Bd., 3. H. (1969), pp. 212-223
Stable URL: http://www.jstor.org/stable/23368476
Page Count: 12
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Abstract

The flux of water, ${\mathrm{f}}_{{\mathrm{H}}_{2}\mathrm{O}}$, to the xylem of exuding root systems of Ricinus communis was controlled using a range of mannitol concentrations permitting the influence of this water flux on the potassium flux, fK, to be studied. The relationship between ${\mathrm{f}}_{{\mathrm{H}}_{2}\mathrm{O}}$ and fK thus obtained was investigated for a number of external concentrations of potassium, Cm, supplied as potassium nitrate. An analysis of these data indicated the presence of a water dependent and a water independent fK both of which varied with Cm. The water dependent fK shows a parabolic relationship with Cm for Cm values < 1 mM followed by a sharp inflection and decline at higher Cm values whereas the water independent fK shows an hyperbolic relationship over the same range of Cm values. Uptake of potassium by exuding root systems was measured and shown to be dependent on the solute potential of the medium. The uptake was also shown to exhibit a dual absorption isotherm the kinetics of which indicate a low Km system (system 1) and a high Km system (system 2). The Km value obtained for system 1 is very similar to that obtained for the water independent fK. It is postulated that the water independent fK is contributed by that portion of fK arriving in the stele via the cortical symplast and is directly dependent on Cm. The water dependent fK is contributed by those ions moved across the root in response to centripetal water movement through the cortical cell walls.

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