Determinations of dye diffusivities in the cell-wall apoplast of roots by a rapid method

A method is described of measuring dye diffusivity through the cell-wall apoplast of roots, using sections of fresh material cut by hand under paraffin oil. Root pieces of Vicia faba and Glycine max were immersed in a solution of sulphorhodamine G for short periods, and the advance of the dye through the cell-wall apoplast was measured, either in a number of sections cut after various periods of diffusion, or by photographing the same section at intervals while diffusion into it from the surface was observed under the micros-cope. Fick's second law was used to estimate the diffusivity. Diffusivities, expressed relative to the diffusivity in water (D-aq) were in the range D-aq/16 to D-aq/55 for Vicia roots. The values for Glycine roots showed a wide scatter, with a modal value of D-aq/1700. Comparison of this rapid method with the much more laborious stabilization of the dye by freeze-substitution confirmed the values of measured diffusivities in Vicia.