Apoplastic mobility of sucrose in storage parenchyma of sugar beet

The apoplastic movement of sucrose through storage parenchyma discs (2.4 mm thick) from roots of sugar beet (Beta vulgaris var. altissima) was investigated in order to evaluate the suitability of the apoplast for transcellular sugar transport. The sucrose permeability of the discs (P = 5.7 × 10−8 cm s−1 at 25°C) was more than two orders of magnitude lower than that of an equally thick layer of unstirred water. This is due to the small volume fraction of free space (3.1%) and the decreased diffusion coefficient D of sucrose in the cell walls. The effective diffusion coefficient of the apoplast (6 to 9 × 10−7 cm2 s−1 at 25°C) was determined independently of the cross sectional area of free space by treating the time course of fluxes according to Fick's second law. The high diffusion resistance of the apoplast has to be considered in models of native parenchyma transport.