Apparent Free Space of Plant Roots

Abstract

To permit accurate measurement of the apparent free space (AFS) of plant roots by exodiffusion, the solution retained superficially by the roots was tagged with India ink, a colloid which does not permeate free space (FS). With this technique and with mannitol and chloride as test solutes, AFS values in reasonable agreement with those in the literature for a number of plant species were obtained. The AFS of pea and bean roots was affected by leaching the roots in distilled water, especially when salt solutions were subsequently used to measure AFS. This was attributed to the loss of exchangeable Ca from the roots and changes in cell wall structure. Observed rates of exo-diffusion of solutes from FS agreed well with theoretical diffusion from cylinders having the dimensions of bean roots. An accumulation of solutes in FS is believed to result from selective absorption of water by cortical and endodermal cells under the influence of rapid transpiration. A calculated doubling of solute concentration in FS could have resulted under observed conditions. However, experiments on the effect of transpiration on AFS values (AFS should be greater as solute concentration builds up in FS were inconclusive. The AFS method may be used to determine the onset of plasmolysis so that the osmotic pressure of root cells at incipient plasmolysis may be accurately measured.