The Radial Exchange of Labelled Water in Maize Roots

Abstract

Experiments have been performed on the efflux kinetics of labelled water in isolated maize roots. The purpose of this study was to establish the identity of the rate-limiting step in this radial exchange process. Although the efflux curve could be mathematically described by the solution of an appropriate cylindrical diffusion problem, other experimental evidence was incompatible with this model. It was considered that the efflux kinetics of labelled water resulted from the char acteristics of membrane penetration in a heterogeneous multicellular preparation like the root. On this basis we have computed from the efflux curve that the 'typical' membrane permeability of root cells to water was about 4-5 X ioJcm sec-1. Similar efflux studies were performed on 'dead' roots prepared by boiling the normal excised roots for a short time. These experiments indicated that the solution, obtained from a cylindrical diffusion analysis, represented a satisfactory description of the exchange of labelled water in 'dead' roots. A common feature of both efflux curves for normal and 'dead' roots was that each deviates significantly from the theoretical diffusion curves for large values of time. The precise origin of these deviations has not been determined but they do reflect some property of the root tissue.