Abstract
Summary In order to differentiate between free and exchangeable apoplastic cations, intact leaves of Vicia faba were vacuum infiltrated with solutions differing in cation exchange strength (water and 50 mol · m -3 BaCl 2 ). Apoplastic washing fluids were subsequently obtained by careful centrifugation. While for the bivalent cations Ca 2+ and Mg 2+ it was possible to differentiate between a free and bound fraction this was not the case with K + . Concentration of free apoplastic Ca 2+ and K + varied between 0.3–0.8 mol· m -3 and 1.3–4.5 mol · m -3 , respectively. Total apoplastic Ca 2+ accounted for up to 40 % of total leaf Ca 2+ content while just 1–8 % of total leaf Mg 2+ and K + content were found in this compartment. Surprisingly, the N-form revealed little influence on apoplastic ion concentration. Independently from the form of N-nutrition, NH 4 + was not found in the leaf apoplast while NO 3 - concentration was rather low (0.01–0.03 mol · m -3 ). The pH of the intercellular washing fluid (PH 6.1) remained unaffected by the N-form.
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