APOPLASTIC ION CONCENTRATION OF INTACT LEAVES OF FIELD BEAN (VICIA-FABA) AS INFLUENCED BY AMMONIUM AND NITRATE NUTRITION

Ln order to differentiate between free and exchangeable apoplastic cat ions, intact leaves of Vicia faba were vacuum infiltrated with solutio ns differing in cation exchange strength (water and 50 mol . m(-3) BaC l2). Apoplastic washing fluids were subsequently obtained by careful c entrifugation. While for the bivalent cations Ca2+ and Mg2+ it was pos sible to differentiate between a free and bound fraction this was not the case with K+. Concentration of free apoplastic Ca2+ and K+ varied between 0.3-0.8 mol . m(-3) and 1.3-4.5 mol . m(-3), respectively. Tot al apoplastic Ca2+ accounted for up to 40% of total leaf Ca2+ content while just 1-8% of total leaf Mg2+ and K+ content were found in this c ompartment. Surprisingly, the N-form revealed little influence on apop lastic ion concentration. Independently from the form of N-nutrition, NH4+ was not found in the leaf apoplast while NO3- concentration was r ather low (0.01-0.03 mol . m(-3)). The pH of the intercellular washing fluid (pH 6.1) remained unaffected by the N-form.