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.