Osmotic water permeability of plasma and vacuolar membranes in protoplasts I.: High osmotic water permeability in radish (Raphanus sativus) root cells as measured by a new method

Intra- and transcellular water movements in plants are regulated by the water permeability of the plasma membrane (PM) and vacuolar membrane (VM) in plant cells. In the present study, we investigated the osmotic water permeability of both PM (P-f1) and VM (P-f2), as well as the bulk osmotic water permeability of a protoplast (P-f(bulk)) isolated from radish (Raphanus sativus) roots. The values of P-f(bulk) and P-f2 were determined from the swelling/shrinking rate of protoplasts and isolated vacuoles under hypo- or hypertonic conditions. In order to minimize the effect of unstirred layer, we monitored dropping or rising protoplasts (vacuoles) in sorbitol solutions as they swelled or shrunk. P-f1 was calculated from P-f(bulk) and P-f2 by using the 'three-compartment model', which describes the theoretical relationship between P-f1, P-f2 and P-f(bulk) (Kuwagata and Murai-Hatano in J Plant Res, 2007). The time-dependent changes in the volume of protoplasts and isolated vacuoles fitted well to the theoretical curves, and solute permeation of PM and VM was able to be neglected for measuring the osmotic water permeability. High osmotic water permeability of more than 500 mu m s(-1), indicating high activity of aquaporins (water channels), was observed in both PM and VM in radish root cells. This method has the advantage that P-f1 and P-f2 can be measured accurately in individual higher plant cells.