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

Mesurements have been made of the electrical potential differncebetween the exuding sap of detopped castor-oil plants and anutrient solution bathing the roots. The initial concentrationsof 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 at0.1 m.equiv./I. The concentration of each of these ions in thesap was of the order of 10 times that in the external solution,was —50 mV. Comparison of the measured potential differencewith the Nernst potential calculated for each ion incicatesthat, under the experimental conditions used here, the influxof K, Na, Ca, and Mg is a passive process while the transportof No₃, Cl, SO₄, H₂PO₄ and HPO₄ is in each case an active processagainst the electrochemical potential gradient. There is no evidence that a K—Na pump is working in theseroots although, in the absence of estimates for flux ratios,an outward ‘pump’ is not ruled out for Na, or Caand Mg. K is the only ion which appears to be at equilibriumconcentration in the sap.