Osmotic water permeability of isolated protoplasts. Modifications during development

A transference chamber was developed to measure the osmotic water permeabil ity coefficient (P-os) in protoplasts 40 to 120 mu m in diameter. The proto plast was held by a micropipette and submitted to a steep osmotic gradient created in the transference chamber. P-os was derived from the changes in p rotoplast dimensions, as measured using a light microscope. Permeabilities were in the range 1 to 1000 mu m s(-1) for the various types of protoplasts tested. The precision for P-os was less than or equal to 40%, and within t his limit, no asymmetry in the water fluxes was observed. Measurements on p rotoplasts isolated from 2- to 5-d-old roots revealed a dramatic increase i n P-os during root development. A shift in P-os from 10 to 500 mu m s(-1) o ccurred within less than 48 h. This phenomenon was found in maize (Zea mays ), wheat (Tuiticum aestivum), and rape (Brassica napus) roots. These result s show that early developmental processes modify water-transport properties of the plasma membrane, and that the transference chamber is adapted to th e study of water-transport mechanisms in native membranes.