Active and Passive Transport of the Major Nutrient Ions across the Root ofRicinus communis

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Abstract
Mesurements have been made of the electrical potential differnce between the exuding sap of detopped castor-oil plants and a nutrient solution bathing the roots. The initial concentrations of the major nutrient ions in the root medium were (m.equiv./I.): K 0.7, Ca 0.8, Mg 0.4, NO2 1.4, So4 0.4, and phosphate 0.2, when present, Na was at a level of 0.3 m.equiv./I. and Cl at 0.1 m.equiv./I. The concentration of each of these ions in the sap was of the order of 10 times that in the external solution, was —50 mV. Comparison of the measured potential difference with the Nernst potential calculated for each ion incicates that, under the experimental conditions used here, the influx of K, Na, Ca, and Mg is a passive process while the transport of No3, Cl, SO4, H2PO4 and HPO4 is in each case an active process against the electrochemical potential gradient. There is no evidence that a K—Na pump is working in these roots although, in the absence of estimates for flux ratios, an outward ‘pump’ is not ruled out for Na, or Ca and Mg. K is the only ion which appears to be at equilibrium concentration in the sap.