RADIAL EXCHANGE OF LABELLED WATER IN ISOLATED STELES OF MAIZE ROOTS

The kinetics of the efflux of labelled water from isolated steles of maize roots have been studied.Analysis of the efflux kinetics indicated that the rate-limiting step was possibly permeation across cell 'membranes'. On this basis we have computed from the efflux curve that the 'typical' membrane permeability of stelar cells to water is about 1.8×10-5 cm s-1; thus, the cells in the stele are about four times more permeable to water than the cortical cells. The water permeability of the stelar cells, however, may have been substantially altered by the extraction of steles from the cortical sleeves. Similar efflux studies were performed on 'dead' steles prepared by boiling the normal steles for a short time. These experiments demonstrated that diffusion within the tissue and external 'unstirred layer' was rate-controlling the initial stage of the efflux from 'dead' steles.A common feature of both efflux curves for normal and 'dead' steles was that each deviated from theoretical cylindrical diffusion curves for large values of time. Certain experimental evidence suggested that this deviation resulted from the existence of a slowly exchanging unidentified compartment occupying at least 8 per cent of the stelar volume. © 1969 Oxford University Press.